Proc. Natl. Acad. Sc:. USA Vol. 74, No. 7, pp. 2825-2829, July 1977
Biochemistry
Sugar uptake into brush border vesicles from normal human kidney (glucose/membranes/proximal tubule transport)
R. JAMES TURNER AND M. SILVERMAN* Department of Medicine, University of Toronto, Toronto, Ontario M5S 1A8, Canada
Communicated by Charles H. Best, May 9, 1977
Uptake studies of simple sugars were performed on a membrane fraction containing osmotically active vesicles prepared from normal human kidney cortex. The uptake of D-glucose was found to be sodium-dependent and phlorizin-sensitive. The specificity of the D-glucose transport mechanism is such that it is shared by a-methyl-D-glucoside, D-galactose, and 5-thio-D-glucose, while 2 deoxy-D-glucose, 3-0methyl-D-glucose, D-mannose, and D-fructose show little, if any, affinity. Measurement of the sodium-dependent component of the initial 1-glucose uptake as a function of glucose concentration resulted in a curvilinear Scatchard plot, indicating the possibility of cooperative effects, or alternatively, the existence of two (or more) sodium-dependent 1-glucose transporters. In the case of two transporters, we estimate that Km 0.3 mM and Vmax f 2.5 nmol/min per mg of protein for the "high-affinity Km 8 nmol/min 6 mM Vmax transporter," mg of protein for the "low-affinity transporter." These specificity and kinetic properties strongly suggest that the sodium-dependent D-glucose transport mechanism characterized in our studies is localized to the brush border of the proximal tubule. ABSTRACT
-
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It is now well established that transport mechanisms for many substrates differ at the luminal and antiluminal surfaces of the renal proximal tubular epithelium (1). In the intact organ, however, it is difficult to study the relative contributions of the two opposing plasma membrane surfaces to the overall net transepithelial transport process. With the development of membrane purification procedures it has been possible to obtain relatively pure fractions of both brush border membrane (BBM) and antiluminal membrane from kidney (2) and intestinal (3) epithelia. These techniques have provided a means for investigating the properties of the BBM and antiluminal membrane in considerable detail in the absence of complicating factors such as metabolism. In particular, by preparing closed vesicles from these membrane fractions it has been possible to study the uptake characteristics of many transported substrates. To date, the majority of the transport studies on renal membrane vesicles have been carried out in the rat (4), rabbit (5), or dog (ref. 6 and unpublished work). To improve our understanding of inherited and acquired transport defects in man, it is essential to have data available on normal human tissue as well. In this paper we report the results from a series of experiments characterizing the uptake of simple sugars into vesicles prepared from normal human renal BBM. METHODS AND MATERIALS Preparation of Brush Border Vesicles. Fresh normal kidney cortex was obtained from nephrectomy specimens at the time of surgery for hypernephroma. In several instances kidneys that Abbreviations: BBM, brush border membrane; Tris/Hepes, 1 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (Hepes) titrated with Tris to pH 7.4. * To whom reprint requests should be addressed at: Medical Sciences Building, Room 7226, University of Toronto, Toronto, Ontario M5S 1A8, Canada. 2825
had undergone perfusion on a Beltzer apparatus for 19-36 hr were also used (no appropriate recipient being available). It is of interest that kidney membranes from both sources gave similar results for sugar transport studies. The membrane preparation technique used in this study is essentially identical to the one we have developed in the dog (6) for obtaining vesiculated brush border membrane fragments. In the case of human tissue, however, the healthy cortex is carefully dissected away from the medulla and tumor tissue rather than being removed by scraping as in the dog. The finely minced cortex is suspended in buffer IM (10 mM triethanolamine-HCl, pH 7.6, containing 250 mM sucrose), homogenized in a tight-fitting glass-Teflon homogenizer, and passed successively through coarse fiberglass screen and nylon mesh to remove unruptured tissue fragments and glomeruli. A "crude membrane fraction" is then prepared by a series of differential centrifugations. This "crude membrane fraction" is suspended in buffer IM and frozen at -20°. A "final vesicle fraction" is prepared on the day of the uptake experiment by a series of centrifugation and homogenization steps incorporating a modification of the purification method of Schmitz et al.
(7).
Criteria of Purity. The fractions are routinely assayed for enzymes known from other species to be localized to brush border membranes, basal lateral membranes, and mitochondria; these are alkaline phosphatase (8), Na+- and K+-dependent ATPase (9), and succinate dehydrogenase (10), respectively. Protein determinations are made by the Lowry method (11) after precipitation with 10% trichloroacetic acid, using bovine serum albumin as a standard. All preparations are monitored by phase microscopy. In addition, several low-power thin sections stained with uranyl acetate and osmium tetroxide have been examined under the electron microscope. Typical membrane vesicle profiles from 0.1 to 0.8 ,m in diameter corresponding to the dimensions of individual microvilli are seen. Some minor microsomal and lysosomal contaminants are also readily identified. Transport Studies. The results of two types of experiments are presented in this paper: "timed uptake studies" in which the uptake of radiolabeled sugars is measured as a function of time, and "initial uptake studies" in which the initial rate of uptake is estimated from a 10- or 15-sec sample. All experiments reported here were carried out at 370 with the exception of those presented in Figs. 2 and 3, which were performed at 250. Unless otherwise noted the "final vesicle fraction" is suspended in buffer A [1 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (Hepes) titrated to pH 7.4 with Tris and containing 100 mM D-mannitol] in the case of the "timed uptake studies" and in buffer A/300 (1 mM Tris/Hepes, pH 7.4, containing 300 mM D-mannitol) in the case of the "initial uptake studies." The protocols are as follows: (a) Timed Uptake Studies: A 50-,ul aliquot of the "final vesicle fraction," typically containing
2826
Biochemistry: Turner and Silverman
10 mg of protein per ml, is incubated in a small test tube at 370 (25°) for 2 min. At time zero 200 Al of incubation medium also at 370 (250) is added to the membranes. The incubation medium contains 1 mM L-[3H]glucose and 1 mM D-[14C]glucose as well as 1 mM Tris/Hepes, pH 7.4, 100 mM D-mannitol, and other additions as noted. The usual specific activities of the 14C and 3H-labeled sugars are 5.0 Ci/mol and 20.0 Ci/mol, respectively. Uptake is measured by removing 20-AI aliquots at specific times and diluting them in 1 ml of ice-cold "stop solution" containing 10 mM Tris/Hepes, pH 7.4, 150 mM NaCl, and 0.2 mM phlorizin. The diluted samples are then rapidly filtered through a Millipore filter (HAWP 0.45 .m) and washed once with 3 ml of ice-cold "stop solution." The filters, which retain the vesiculated membrane fragments, are dissolved in Bray's solution and their radioactivities are measured. (b) Initial Uptake Studies: These experiments have been carried out in the same way described above with the following exceptions: the final vesicle fraction typically contains 2-4 mg of protein per ml; activities for 14C- and 3H-labeled sugars are usually 2 ,uCi and 10 ,Ci/ml of incubation medium, respectively; a single 100-Al sample is taken; the stop solution contains 250 mM NaCI. All experiments were carried out in triplicate. The errors indicated are the SEMs. Experiments carried out using the above protocols with no membranes present indicate no significant binding or retention of radioactive sugars by the Millipore filters. Materials. All materials used were of the highest chemical grade available. Unlabeled L-glucose, D-glucose, a-methylD-glucoside, D-galactose, D-mannose, 2-deoxy-D-glucose, and 3-O-methyl-D-glucose were purchased from Sigma Chemical Co. (St. Louis, MO). Hepes, Tris, and phlorizin were also from Sigma. D-Fructose was from Nutritional Biochemical Corp. (Cleveland, OH), 5-thio-D-glucose from Aldrich Chemical Co. (Milwaukee, WI), and phloretin from K and K Laboratories, Inc. (Plainview, NY). D-Mannitol was a product of the Fisher Chemical Co. (Montreal, PQ). All radioactively labeled materials were purchased from New England Nuclear Corp. (Boston, MA). RESULTS Enzymatic characterization of the membranes Recoveries and enrichments were found to vary somewhat from preparation to preparation, presumably as a result of the condition of the tissue used. We have not yet been able to gather statistics on a large number of samples; however, measured relative to the initial homogenate, the "final vesicle fraction" typically shows a 10-fold enrichment in alkaline phosphatase activity (the BBM marker), a 4-fold enrichment in Na+- and K+-dependent ATPase activity (the antiluminal membrane marker), and a 3-fold reduction in succinate dehydrogenase activity (the mitochondrial marker). Our yield is roughly 2 mg of vesicles per g of healthy cortex. Generally speaking these results are very similar to those we obtain in the dog (6). Binding, metabolism, and uptake It is crucial to the interpretation of our results to be able to distinguish membrane transport effects from those associated with binding or metabolism. When the osmolarity of the "extravesicular" incubation medium is increased by the addition of sucrose we find that the amounts of both D- and L-glucose retained by the vesicles decrease as linear functions of the inverse osmotic pressure. Furthermore, when these data are extrapolated to higher osmotic pressures, the stereospecific
Proc. Natl. Acad. Sci. USA 74 (1977) P
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FIG. 1. The timed uptake of 1 mM D- and L-glucose at 37°. (Upper) The uptake of D-glucose is shown in the presence of an initial 100 mM NaCi gradient (0), in the presence of 100 mM NaCi in equilibrium inside and outside the vesicles (A), and in the presence of a 100 mM NaCl gradient and 100 gM phlorizin (-). The uptake of L-glucose (X) is the same in each case. (Lower) The uptake of 1 mM D-glucose (0) and 1 mM L-glucose (X) in the presence of an initial 100 mM KC1 gradient. The incubation medium was buffer A containing sufficient labeled D- and L-glucose, phlorizin, and NaCl or KC1 to give the final concentrations indicated.
component of D-glucose uptake (i.e., D-glucose uptake minus L-glucose uptake) can be seen to decrease to zero as the osmotic pressure goes to infinity. These results clearly show that we are measuring stereospecific uptake into an osmotically active space rather than binding to the membranes. When the vesicular radioactivity was extracted with water and submitted to thin-layer chromatography using the solvent system 1-butanol/ethanol/water (50:32:18, vol/vol) no significant component of activity outside the glucose spot could be detected. Time-dependence of glucose uptake Sodium Dependence. The effect of sodium on D-glucose uptake is illustrated in Fig. 1 upper. The upper curve shows the uptake of D-glucose in the presence of an initial 100 mM sodium chloride gradient between the external medium and the intravesicular contents. The characteristic temporary overshoot of the intravesicular D-glucose concentration over its equilibrium value has also been observed in brush border vesicle preparations from rat (4), rabbit (5), and dog (6) kidney as well as in intestinal BBM (12). The existence of the overshoot can ultimately be traced to the sodium dependence of the D-glucose transporter in the BBM (see Fig. 1 lower). It is thought that D-glucose and sodium combine with a transport molecule at the exterior surface of the BBM and are simultaneously carried across the membrane. Thus, under the experimental conditions described above, Dglucose continues to move into the vesicular space against its concentration gradient as long as the electrochemical sodium gradient can provide the energy required. As the sodium gradient dissipates so does the D-glucose overshoot. As shown in Fig. 1 upper, preincubation of the membranes with 100 mM NaCI to remove the Na+ gradient eliminates the overshoot. The above interpretation of Fig. 1 is further supported by the results of other authors who have studied the effects of anion perme-
Biochemistry:
Turner and Silverman
Proc. Natl. Acad. Sci. USA 74 (1977)
2827
Table 1. Comparison of sodium dependence and phlorizin sensitivity for six monosaccharides and monosaccharide derivatives Initial sugar uptake* at 370, nmol/mg per 10 sec 100 mM Na+ gradient Na+ gradient + 10AM phlorizin 100 mM
Sugar
4
8
12
16
20
Incubation time, min
FIG. 2. The uptake of 1 mM D- and L-glucose at 250 in membrane vesicles preincubated with 100 mM KC1. The uptake of D-glucose is shown in the presence of a 100 mM NaCl gradient for normal membranes (A-A) and for membranes preincubated with 8 Mg of valinomycin per mg of protein (---- --). The uptake of L-glucose (X-X) is the same in each case. The vesicles were prepared in buffer A containing 100 mM KCl with or without valinomycin. The incubation medium was such that the initial extravesicular concentrations were 1 mM Tris/Hepes, 100 mM D-mannitol, 1 mM D- and L-glucose, 100 mM NaCl, and 9.1 mM KC1. In this experiment 200 Ml of incubation medium was added to 20 Ml of the final vesicle fraction.
ability on the overshoot phenomenon (4, 13). The uptake of L-glucose is unaffected by sodium in all cases. Phlorizin Inhibition. The uptake of D-glucose in the presence of 100MuM phlorizin and a 100 mM NaCl gradient is also shown in Fig. 1 upper. The overshoot has been completely eliminated by this competitive inhibitor of glucose transport. Again the uptake of L-glucose is not affected. Effect of Valinomycin. In the experiment illustrated in Fig. 2, the final vesicle fraction was preincubated with 100 mM KCL The timed uptake curves for D-glucose in the presence of an initial 100 mM NaCl gradient were then obtained in the absence of valinomycin and in the presence of 8Mg of valinomycin per mg of protein. Because the uptake of L-glucose is identical in the presence and absence of valinomycin it is clear that the larger D-glucose uptake in the absence of valinomycin is not a result of increased vesiculation. The same experiment when carried out for the dog (6) showed similar qualitative features and indicated that the uptake in the absence of valinomycin does indeed continue to fall slowly with time, reaching the level with valinomycin after several hours. Thus, the broad overshoot with slow return to equilibrium observed with KCI loading seems to indicate that efflux is impeded from these vesicles and that this effect is removed by valinomycin. In fact, inspection of Fig. 3 suggests that valinomycin may enhance influx as well. Valinomycin is known to provide an electrogenic shunt for K+ across the membrane. The dramatic effects of its presence on D-glucose transport in this experiment strongly imply that the transport event must also be electrogenic. Although our results can be shown to be consistent with the assumption that glucose and sodium are cotransported electrogenically as discussed earlier, further experiments employing other ionophores under a variety of initial conditions are necessary to further clarify these results. Uptake of Other Sugars. Table 1 compares the sodium dependence and phlorizin sensitivity of the initial velocity of uptake of six pyranoside derivatives. The experiments were carried out under the following conditions: (i) in the absence of sodium, (ii) in the presence of an initial 100 mM sodium chloride gradient, and (ii) in the presence of 10MgM phlorizin as well as a 100 mM sodium chloride gradient. Inspection of Table 1 reveals that D-glucose, a-methyl-D-glucoside, and
D-Glucose a-Methyl-D-glucoside D-Galactose D-Mannose 2-Deoxy-D-glucose 3-0-Methyl-D-
glucose
No Na+ 0.21 ± 0.02
3.00 I 0.09
0.79
0.13 ± 0.02 0.07 + 0.04 0.13 I 0.02 0.27 ± 0.03
2.32 4 0.12 0.90 ± 0.03 0.79 4 0.08 0.50 : 0.03
0.42 4 0.04 0.30 4 0.04 0.44 I 0.06 0.49 i 0.01
0.20 ± 0.04
0.43 + 0.01
0.36
:
:
0.02
0.01
* The corresponding uptake of L-glucose has been subtracted in each case to correct for nonstereospecific effects.
D-galactose clearly fall into one group. They show a marked sodium dependence, with their initial uptakes enhanced by over an order of magnitude in the presence of a NaCl gradient. They also exhibit considerable sensitivity to inhibition by 10 gM phlorizin, with approximately 75% reduction in initial uptake. On the other hand, 2-deoxy-D-glucose and 3-0-methyl-Dglucose show relatively little sodium dependence or phlorizin sensitivity. D-Mannose seems to lie between these two groups. The specificity characteristics of sugar transport in human BBM were further studied by determining the inhibitory potency of various substrates on the initial uptake of 1 mM Dglucose. The results are shown in Table 2 and are expressed as Table 2. Inhibitory effects of various substances on the initial (10 sec) uptake of 1 mM D-glucose* at 370 Inhibitor
a-Methyl-D-glucoside D-Galactose 5-Thio-D-glucose 2-Deoxy-D-glucose
3-O-Methyl-D-glucose D-Mannose D-Fructose
L-Glucose Phlorizin
Phloretin
Anti-BBM antibody
Concentration, mM 5 25 50 25 50 25 50 25 50 25 50 25 50 25 50 0.001 0.01 0.01 0.10 0.06 mg/mg of protein
Relative uptaket 0.45 I 0.01 0.59 A 0.01 0.51 i 0.02 0.61 4 0.05 0.57 I 0.01 0.94 4- 0.02 0.81 + 0.03 0.83 + 0.05 0.74 0.02 1.01 + 0.05 1.03.+ 0.08 0.92 + 0.05 0.95 I 0.03 0.90 ± 0.04 1.00 0.67 ± 0.02 0.26 i 0.04 0.84 + 0.04 0.74+0.04 1.02 + 0.03
* The corresponding uptake of L-glucose has been subtracted in each case to correct for nonstereospecific effects. t Uptake is given relative to the uptake in the presence of 50 mM Lglucose.
2828
Biochemistry: Turner and Silverman
Proc. Natl. Acad. Sci. USA 74 (1977)
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FIG. 3. A Scatchard plot of the initial stereospecific uptake of D-glucose as a function of glucose concentration. Uptake is shown in the presence of a 100 mM NaCl gradient (E), in the presence of a 100 mM NaCl gradient and 1 ,M phlorizin (A), and in the presence of a 100 mM KCI gradient (@). The incubation medium was buffer A containing sufficient D- and L-glucose, phlorizin, and NaCl or KCI to give the final concentrations indicated. The inset shows the difference between D-glucose uptake in the presence of NaCl and that in the presence of KCl (0).
a fraction of the D-glucose uptake in the presence of 50 mM L-glucose. Inspection of Table 2 (and 1) suggests that a-methyl-D-glucoside, D-galactose, and 5-thio-D-glucose share the D-glucose transporter at the BBM in human kidney (in order of decreasing affinity). In contrast, 2-deoxy-D-glucose, 3-O-methyl-D-glucose, D-mannose, and D-fructose seem to have little, if any, affinity for the transporter. Phlorizin gives more than an order of magnitude greater inhibition of D-glucose uptake than its aglycone, phloretin. Finally, an anti-BBM antibody raised against intact purified BBM from human kidney (14) has no detectable inhibitory capability with respect to D-glucose uptake.
Quantitative characterization of the D-glucose transporter The initial uptake of D-glucose as a function of concentration is illustrated in Fig. 3 in the form of a Scatchard plot. This experiment was carried out at 250 rather than 370 in order to ensure a better estimate of the initial velocity of uptake. The three curves represent uptake in the presence of an initial 100 mM NaCl gradient, in the presence of a sodium gradient and 1 AiM phlorizin, and in the presence of an initial 100 mM KCI gradient. The sodium-independent uptake seems to represent a single transporter site with a Km - 36 mM and a Vml 74 nmol/min per mg of protein. The characterization of the sodium-dependent uptake can be found by subtracting the sodium-independent uptake at each concentration from that found in the presence of sodium. These results are shown in the inset of Fig. 3. This plot is definitely curvilinear, indicating either cooperative effects or the existence of two (or more) sodium-dependent transporters in our preparation. A curvilinear Scatchard plot is also seen at 370 with the human and at 250 in our experiments with the dog. Assuming that the inset of Fig. 3 represents two independent sites, we estimate that the "high-affinity transporter" has a Km 0.3 mM and a Vmax 2.5 nmol/min per -
-
-
FIG. 4. The stereospecific sodium-dependent uptake of D-glucose as a function of concentration in the absence (o) and presence (&) of 1 AM phlorizin is plotted according to Lineweaver and Burk. The points represent the same experiment shown in Fig. 3.
mg of protein and that the "low-affinity transporter" has a Km 6 mM and a Vmax 8 nmol/min per mg of protein. Fig. 4 is a Lineweaver-Burk plot of the same data shown in the inset of Fig. 3. The inhibitory effect of 1 AtM phlorizin has also been included. The competitive nature of phlorizin inhibition of the "high-affinity transporter" is obvious. The KI for phlorizin calculated from the graph is 0.8 AtM. Fig. 5 documents the Na+ dependency of D-glucose uptake into BBM vesicles at 37°. The apparent Km for Na+ is estimated tobe l 114mM. -
DISCUSSION We have investigated D-glucose transport in a plasma membrane vesicle preparation from human kidney cortex. Our results are qualitatively similar to our own findings in the dog (6) and to those of others in the rat (4) and rabbit (5). We see two components of stereospecific D-glucose uptake: a sodiumindependent component with a relatively low affinity (Km36 mM) for D-glucose and a sodium-dependent, phlorizinsensitive component which, as our results indicate, may represent several transport systems. If we extrapolate from other mammalian renal tissue, the enzymatic marker profile of our final vesicle fraction reveals a mixture of brush border (alkaline phosphatase) and antiluminal (Na+- and K+-dependent ATPase) membranes, with a .'a) 0.8 0
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O 0.6C
-r E 0.4 CL
x
T
° 0.2-
0.02
0.04
0.06
1/[Na],
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0.10
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FIG. 5. The stereospecific sodium-dependent initial (10 sec) uptake of 1 mM D-glucose as a function of initial extravesicular sodium concentration at 37° plotted according to Lineweaver and Burk. The extravesicular NaCl concentration was varied by replacing it iso-osomotically with KC1.
Biochemistry:
Turner and Silverman
minor contamination by mitochondria, lysosomes, and probably microsomes. Restricting ourselves to the available data on renal sugar transport in humans, we would be hard put to identify precisely which type of membrane was responsible for the above observed sugar transport mechanisms. However, comparison of the characteristics of the sodium-dependent component of D-glucose uptake in human tissue, as defined by our experiments, with results from more highly purified brush border membranes from rat, rabbit, and dog indicates striking similarities with respect to (i) Km, (ii) Na+-dependent electrogenic transport, (iii) competitive inhibition by phlorizin, and (iv) specificity of transport and inhibition. Moreover, the transport of D-glucose at the antiluminal membrane appears to be sodium-independent as we have demonstrated in the dog (6). Thus, we feel that there is compelling evidence that the sodium-dependent D-glucose uptake we observe is principally due to BBM. Obviously we cannot comment on the sidedness of the human BBM vesicles; however, preliminary studies in our laboratory suggest that our dog BBM vesicles are almost all right-side-out. It is of interest that in the human as well as the dog (6) the initial uptake data can be interpreted to indicate the existence of two (or more) Na+-dependent D-glucose transporters in the BBM. Alternatively, these observations could be the result of cooperative effects between subunits of the same transporter. Obviously more work is needed to clarify this matter. Our results provide an important rationale for extrapolating findings, such as those on model Fanconi states, across species barriers. Moreover, it is hoped that the present documentation of specificity and kinetic characteristics of the BBM glucose transporter in normal humans will contribute to a secure basis for future studies on pathological sugar transport mechanisms in human beings.
Proc. Natl. Acad. Sci. USA 74 (1977)
2829
The authors thank R. Rutherford and S. McGugan for excellent
technical and secretarial assistance. This work was supported by the Medical Research Council of Canada (MT4590). M.S. is a Medical Research Council Scholar. The costs of publication of this article were defrayed in part by the
payment of page charges from funds made available to support the research which is the subject of the article. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact.
1. Silverman, M. (1976) Biochim. Biophys. Acta 457,303-351. 2. Heidrich, H.-G., Kinne, R., Kinne-Saffran, E. & Hannig, K. (1972) J. Cell Biol. 54, 232-245. 3. Forstner, G. G., Sabesin, S. M. & Isselbacher, K. J. (1968) Biochem. J. 106,381-390. 4. Kinne, R., Murer, H., Kinne-Saffran, E., Thees, M. & Sachs, G. (1975) J. Membr. Biol. 21, 375-395. 5. Beck, J. C. & Sacktor, B. (1975) J. Biol. Chem. 250, 86748680. 6. Turner, R. J. & Silverman, M. (1977) J. Supramol. Struct. Suppl. 1,141. 7. Schmitz, J., Preiser, H., Maestraaci, D., Ghosh, B. K., Cerda, J. J. & Crane, R. K. (1973) Biochim. Biophys. Acta 323,98-112. 8. Bessey, 0. A., Lowry, 0. H. & Brock, M. J. (1946) J. Biol. Chem. 164, 321-329. 9. Post, R. A. & Sen, A. K. (1967) in Methods in Enzymology, eds. Estabrook, R. W. & Pullman, M. E. (Academic Press, New York), Vol. 10, pp. 762-769. 10. Pennington, R. J. (1961) Biochem. J. 80,649-654. 11. Lowry, 0. H., Rosebrough, N. J., Farr, A. L. & Randall, R. J.
(1951) J. Biol. Chem. 193, 265-275.
12. Hopfer, U., Sigrist-Nelson, K. & Murer, H. (1975) Ann. N.Y. Acad. Sci. 264,414-427. 13. Murer, H. & Hopfer, U. (1974) Proc. Natl. Acad. Sci. USA 71, 484-488. 14. Chant, S. & Silverman, M. (1977) Kidney Int., in press.
Biochemistry
Sugar uptake into brush border vesicles from normal human kidney (glucose/membranes/proximal tubule transport)
R. JAMES TURNER AND M. SILVERMAN* Department of Medicine, University of Toronto, Toronto, Ontario M5S 1A8, Canada
Communicated by Charles H. Best, May 9, 1977
Uptake studies of simple sugars were performed on a membrane fraction containing osmotically active vesicles prepared from normal human kidney cortex. The uptake of D-glucose was found to be sodium-dependent and phlorizin-sensitive. The specificity of the D-glucose transport mechanism is such that it is shared by a-methyl-D-glucoside, D-galactose, and 5-thio-D-glucose, while 2 deoxy-D-glucose, 3-0methyl-D-glucose, D-mannose, and D-fructose show little, if any, affinity. Measurement of the sodium-dependent component of the initial 1-glucose uptake as a function of glucose concentration resulted in a curvilinear Scatchard plot, indicating the possibility of cooperative effects, or alternatively, the existence of two (or more) sodium-dependent 1-glucose transporters. In the case of two transporters, we estimate that Km 0.3 mM and Vmax f 2.5 nmol/min per mg of protein for the "high-affinity Km 8 nmol/min 6 mM Vmax transporter," mg of protein for the "low-affinity transporter." These specificity and kinetic properties strongly suggest that the sodium-dependent D-glucose transport mechanism characterized in our studies is localized to the brush border of the proximal tubule. ABSTRACT
-
and
and
-
per
It is now well established that transport mechanisms for many substrates differ at the luminal and antiluminal surfaces of the renal proximal tubular epithelium (1). In the intact organ, however, it is difficult to study the relative contributions of the two opposing plasma membrane surfaces to the overall net transepithelial transport process. With the development of membrane purification procedures it has been possible to obtain relatively pure fractions of both brush border membrane (BBM) and antiluminal membrane from kidney (2) and intestinal (3) epithelia. These techniques have provided a means for investigating the properties of the BBM and antiluminal membrane in considerable detail in the absence of complicating factors such as metabolism. In particular, by preparing closed vesicles from these membrane fractions it has been possible to study the uptake characteristics of many transported substrates. To date, the majority of the transport studies on renal membrane vesicles have been carried out in the rat (4), rabbit (5), or dog (ref. 6 and unpublished work). To improve our understanding of inherited and acquired transport defects in man, it is essential to have data available on normal human tissue as well. In this paper we report the results from a series of experiments characterizing the uptake of simple sugars into vesicles prepared from normal human renal BBM. METHODS AND MATERIALS Preparation of Brush Border Vesicles. Fresh normal kidney cortex was obtained from nephrectomy specimens at the time of surgery for hypernephroma. In several instances kidneys that Abbreviations: BBM, brush border membrane; Tris/Hepes, 1 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (Hepes) titrated with Tris to pH 7.4. * To whom reprint requests should be addressed at: Medical Sciences Building, Room 7226, University of Toronto, Toronto, Ontario M5S 1A8, Canada. 2825
had undergone perfusion on a Beltzer apparatus for 19-36 hr were also used (no appropriate recipient being available). It is of interest that kidney membranes from both sources gave similar results for sugar transport studies. The membrane preparation technique used in this study is essentially identical to the one we have developed in the dog (6) for obtaining vesiculated brush border membrane fragments. In the case of human tissue, however, the healthy cortex is carefully dissected away from the medulla and tumor tissue rather than being removed by scraping as in the dog. The finely minced cortex is suspended in buffer IM (10 mM triethanolamine-HCl, pH 7.6, containing 250 mM sucrose), homogenized in a tight-fitting glass-Teflon homogenizer, and passed successively through coarse fiberglass screen and nylon mesh to remove unruptured tissue fragments and glomeruli. A "crude membrane fraction" is then prepared by a series of differential centrifugations. This "crude membrane fraction" is suspended in buffer IM and frozen at -20°. A "final vesicle fraction" is prepared on the day of the uptake experiment by a series of centrifugation and homogenization steps incorporating a modification of the purification method of Schmitz et al.
(7).
Criteria of Purity. The fractions are routinely assayed for enzymes known from other species to be localized to brush border membranes, basal lateral membranes, and mitochondria; these are alkaline phosphatase (8), Na+- and K+-dependent ATPase (9), and succinate dehydrogenase (10), respectively. Protein determinations are made by the Lowry method (11) after precipitation with 10% trichloroacetic acid, using bovine serum albumin as a standard. All preparations are monitored by phase microscopy. In addition, several low-power thin sections stained with uranyl acetate and osmium tetroxide have been examined under the electron microscope. Typical membrane vesicle profiles from 0.1 to 0.8 ,m in diameter corresponding to the dimensions of individual microvilli are seen. Some minor microsomal and lysosomal contaminants are also readily identified. Transport Studies. The results of two types of experiments are presented in this paper: "timed uptake studies" in which the uptake of radiolabeled sugars is measured as a function of time, and "initial uptake studies" in which the initial rate of uptake is estimated from a 10- or 15-sec sample. All experiments reported here were carried out at 370 with the exception of those presented in Figs. 2 and 3, which were performed at 250. Unless otherwise noted the "final vesicle fraction" is suspended in buffer A [1 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (Hepes) titrated to pH 7.4 with Tris and containing 100 mM D-mannitol] in the case of the "timed uptake studies" and in buffer A/300 (1 mM Tris/Hepes, pH 7.4, containing 300 mM D-mannitol) in the case of the "initial uptake studies." The protocols are as follows: (a) Timed Uptake Studies: A 50-,ul aliquot of the "final vesicle fraction," typically containing
2826
Biochemistry: Turner and Silverman
10 mg of protein per ml, is incubated in a small test tube at 370 (25°) for 2 min. At time zero 200 Al of incubation medium also at 370 (250) is added to the membranes. The incubation medium contains 1 mM L-[3H]glucose and 1 mM D-[14C]glucose as well as 1 mM Tris/Hepes, pH 7.4, 100 mM D-mannitol, and other additions as noted. The usual specific activities of the 14C and 3H-labeled sugars are 5.0 Ci/mol and 20.0 Ci/mol, respectively. Uptake is measured by removing 20-AI aliquots at specific times and diluting them in 1 ml of ice-cold "stop solution" containing 10 mM Tris/Hepes, pH 7.4, 150 mM NaCl, and 0.2 mM phlorizin. The diluted samples are then rapidly filtered through a Millipore filter (HAWP 0.45 .m) and washed once with 3 ml of ice-cold "stop solution." The filters, which retain the vesiculated membrane fragments, are dissolved in Bray's solution and their radioactivities are measured. (b) Initial Uptake Studies: These experiments have been carried out in the same way described above with the following exceptions: the final vesicle fraction typically contains 2-4 mg of protein per ml; activities for 14C- and 3H-labeled sugars are usually 2 ,uCi and 10 ,Ci/ml of incubation medium, respectively; a single 100-Al sample is taken; the stop solution contains 250 mM NaCI. All experiments were carried out in triplicate. The errors indicated are the SEMs. Experiments carried out using the above protocols with no membranes present indicate no significant binding or retention of radioactive sugars by the Millipore filters. Materials. All materials used were of the highest chemical grade available. Unlabeled L-glucose, D-glucose, a-methylD-glucoside, D-galactose, D-mannose, 2-deoxy-D-glucose, and 3-O-methyl-D-glucose were purchased from Sigma Chemical Co. (St. Louis, MO). Hepes, Tris, and phlorizin were also from Sigma. D-Fructose was from Nutritional Biochemical Corp. (Cleveland, OH), 5-thio-D-glucose from Aldrich Chemical Co. (Milwaukee, WI), and phloretin from K and K Laboratories, Inc. (Plainview, NY). D-Mannitol was a product of the Fisher Chemical Co. (Montreal, PQ). All radioactively labeled materials were purchased from New England Nuclear Corp. (Boston, MA). RESULTS Enzymatic characterization of the membranes Recoveries and enrichments were found to vary somewhat from preparation to preparation, presumably as a result of the condition of the tissue used. We have not yet been able to gather statistics on a large number of samples; however, measured relative to the initial homogenate, the "final vesicle fraction" typically shows a 10-fold enrichment in alkaline phosphatase activity (the BBM marker), a 4-fold enrichment in Na+- and K+-dependent ATPase activity (the antiluminal membrane marker), and a 3-fold reduction in succinate dehydrogenase activity (the mitochondrial marker). Our yield is roughly 2 mg of vesicles per g of healthy cortex. Generally speaking these results are very similar to those we obtain in the dog (6). Binding, metabolism, and uptake It is crucial to the interpretation of our results to be able to distinguish membrane transport effects from those associated with binding or metabolism. When the osmolarity of the "extravesicular" incubation medium is increased by the addition of sucrose we find that the amounts of both D- and L-glucose retained by the vesicles decrease as linear functions of the inverse osmotic pressure. Furthermore, when these data are extrapolated to higher osmotic pressures, the stereospecific
Proc. Natl. Acad. Sci. USA 74 (1977) P
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6
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20
FIG. 1. The timed uptake of 1 mM D- and L-glucose at 37°. (Upper) The uptake of D-glucose is shown in the presence of an initial 100 mM NaCi gradient (0), in the presence of 100 mM NaCi in equilibrium inside and outside the vesicles (A), and in the presence of a 100 mM NaCl gradient and 100 gM phlorizin (-). The uptake of L-glucose (X) is the same in each case. (Lower) The uptake of 1 mM D-glucose (0) and 1 mM L-glucose (X) in the presence of an initial 100 mM KC1 gradient. The incubation medium was buffer A containing sufficient labeled D- and L-glucose, phlorizin, and NaCl or KC1 to give the final concentrations indicated.
component of D-glucose uptake (i.e., D-glucose uptake minus L-glucose uptake) can be seen to decrease to zero as the osmotic pressure goes to infinity. These results clearly show that we are measuring stereospecific uptake into an osmotically active space rather than binding to the membranes. When the vesicular radioactivity was extracted with water and submitted to thin-layer chromatography using the solvent system 1-butanol/ethanol/water (50:32:18, vol/vol) no significant component of activity outside the glucose spot could be detected. Time-dependence of glucose uptake Sodium Dependence. The effect of sodium on D-glucose uptake is illustrated in Fig. 1 upper. The upper curve shows the uptake of D-glucose in the presence of an initial 100 mM sodium chloride gradient between the external medium and the intravesicular contents. The characteristic temporary overshoot of the intravesicular D-glucose concentration over its equilibrium value has also been observed in brush border vesicle preparations from rat (4), rabbit (5), and dog (6) kidney as well as in intestinal BBM (12). The existence of the overshoot can ultimately be traced to the sodium dependence of the D-glucose transporter in the BBM (see Fig. 1 lower). It is thought that D-glucose and sodium combine with a transport molecule at the exterior surface of the BBM and are simultaneously carried across the membrane. Thus, under the experimental conditions described above, Dglucose continues to move into the vesicular space against its concentration gradient as long as the electrochemical sodium gradient can provide the energy required. As the sodium gradient dissipates so does the D-glucose overshoot. As shown in Fig. 1 upper, preincubation of the membranes with 100 mM NaCI to remove the Na+ gradient eliminates the overshoot. The above interpretation of Fig. 1 is further supported by the results of other authors who have studied the effects of anion perme-
Biochemistry:
Turner and Silverman
Proc. Natl. Acad. Sci. USA 74 (1977)
2827
Table 1. Comparison of sodium dependence and phlorizin sensitivity for six monosaccharides and monosaccharide derivatives Initial sugar uptake* at 370, nmol/mg per 10 sec 100 mM Na+ gradient Na+ gradient + 10AM phlorizin 100 mM
Sugar
4
8
12
16
20
Incubation time, min
FIG. 2. The uptake of 1 mM D- and L-glucose at 250 in membrane vesicles preincubated with 100 mM KC1. The uptake of D-glucose is shown in the presence of a 100 mM NaCl gradient for normal membranes (A-A) and for membranes preincubated with 8 Mg of valinomycin per mg of protein (---- --). The uptake of L-glucose (X-X) is the same in each case. The vesicles were prepared in buffer A containing 100 mM KCl with or without valinomycin. The incubation medium was such that the initial extravesicular concentrations were 1 mM Tris/Hepes, 100 mM D-mannitol, 1 mM D- and L-glucose, 100 mM NaCl, and 9.1 mM KC1. In this experiment 200 Ml of incubation medium was added to 20 Ml of the final vesicle fraction.
ability on the overshoot phenomenon (4, 13). The uptake of L-glucose is unaffected by sodium in all cases. Phlorizin Inhibition. The uptake of D-glucose in the presence of 100MuM phlorizin and a 100 mM NaCl gradient is also shown in Fig. 1 upper. The overshoot has been completely eliminated by this competitive inhibitor of glucose transport. Again the uptake of L-glucose is not affected. Effect of Valinomycin. In the experiment illustrated in Fig. 2, the final vesicle fraction was preincubated with 100 mM KCL The timed uptake curves for D-glucose in the presence of an initial 100 mM NaCl gradient were then obtained in the absence of valinomycin and in the presence of 8Mg of valinomycin per mg of protein. Because the uptake of L-glucose is identical in the presence and absence of valinomycin it is clear that the larger D-glucose uptake in the absence of valinomycin is not a result of increased vesiculation. The same experiment when carried out for the dog (6) showed similar qualitative features and indicated that the uptake in the absence of valinomycin does indeed continue to fall slowly with time, reaching the level with valinomycin after several hours. Thus, the broad overshoot with slow return to equilibrium observed with KCI loading seems to indicate that efflux is impeded from these vesicles and that this effect is removed by valinomycin. In fact, inspection of Fig. 3 suggests that valinomycin may enhance influx as well. Valinomycin is known to provide an electrogenic shunt for K+ across the membrane. The dramatic effects of its presence on D-glucose transport in this experiment strongly imply that the transport event must also be electrogenic. Although our results can be shown to be consistent with the assumption that glucose and sodium are cotransported electrogenically as discussed earlier, further experiments employing other ionophores under a variety of initial conditions are necessary to further clarify these results. Uptake of Other Sugars. Table 1 compares the sodium dependence and phlorizin sensitivity of the initial velocity of uptake of six pyranoside derivatives. The experiments were carried out under the following conditions: (i) in the absence of sodium, (ii) in the presence of an initial 100 mM sodium chloride gradient, and (ii) in the presence of 10MgM phlorizin as well as a 100 mM sodium chloride gradient. Inspection of Table 1 reveals that D-glucose, a-methyl-D-glucoside, and
D-Glucose a-Methyl-D-glucoside D-Galactose D-Mannose 2-Deoxy-D-glucose 3-0-Methyl-D-
glucose
No Na+ 0.21 ± 0.02
3.00 I 0.09
0.79
0.13 ± 0.02 0.07 + 0.04 0.13 I 0.02 0.27 ± 0.03
2.32 4 0.12 0.90 ± 0.03 0.79 4 0.08 0.50 : 0.03
0.42 4 0.04 0.30 4 0.04 0.44 I 0.06 0.49 i 0.01
0.20 ± 0.04
0.43 + 0.01
0.36
:
:
0.02
0.01
* The corresponding uptake of L-glucose has been subtracted in each case to correct for nonstereospecific effects.
D-galactose clearly fall into one group. They show a marked sodium dependence, with their initial uptakes enhanced by over an order of magnitude in the presence of a NaCl gradient. They also exhibit considerable sensitivity to inhibition by 10 gM phlorizin, with approximately 75% reduction in initial uptake. On the other hand, 2-deoxy-D-glucose and 3-0-methyl-Dglucose show relatively little sodium dependence or phlorizin sensitivity. D-Mannose seems to lie between these two groups. The specificity characteristics of sugar transport in human BBM were further studied by determining the inhibitory potency of various substrates on the initial uptake of 1 mM Dglucose. The results are shown in Table 2 and are expressed as Table 2. Inhibitory effects of various substances on the initial (10 sec) uptake of 1 mM D-glucose* at 370 Inhibitor
a-Methyl-D-glucoside D-Galactose 5-Thio-D-glucose 2-Deoxy-D-glucose
3-O-Methyl-D-glucose D-Mannose D-Fructose
L-Glucose Phlorizin
Phloretin
Anti-BBM antibody
Concentration, mM 5 25 50 25 50 25 50 25 50 25 50 25 50 25 50 0.001 0.01 0.01 0.10 0.06 mg/mg of protein
Relative uptaket 0.45 I 0.01 0.59 A 0.01 0.51 i 0.02 0.61 4 0.05 0.57 I 0.01 0.94 4- 0.02 0.81 + 0.03 0.83 + 0.05 0.74 0.02 1.01 + 0.05 1.03.+ 0.08 0.92 + 0.05 0.95 I 0.03 0.90 ± 0.04 1.00 0.67 ± 0.02 0.26 i 0.04 0.84 + 0.04 0.74+0.04 1.02 + 0.03
* The corresponding uptake of L-glucose has been subtracted in each case to correct for nonstereospecific effects. t Uptake is given relative to the uptake in the presence of 50 mM Lglucose.
2828
Biochemistry: Turner and Silverman
Proc. Natl. Acad. Sci. USA 74 (1977)
E a)
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**
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1o
~5
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1 4 8 1/[glucose], mM '
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FIG. 3. A Scatchard plot of the initial stereospecific uptake of D-glucose as a function of glucose concentration. Uptake is shown in the presence of a 100 mM NaCl gradient (E), in the presence of a 100 mM NaCl gradient and 1 ,M phlorizin (A), and in the presence of a 100 mM KCI gradient (@). The incubation medium was buffer A containing sufficient D- and L-glucose, phlorizin, and NaCl or KCI to give the final concentrations indicated. The inset shows the difference between D-glucose uptake in the presence of NaCl and that in the presence of KCl (0).
a fraction of the D-glucose uptake in the presence of 50 mM L-glucose. Inspection of Table 2 (and 1) suggests that a-methyl-D-glucoside, D-galactose, and 5-thio-D-glucose share the D-glucose transporter at the BBM in human kidney (in order of decreasing affinity). In contrast, 2-deoxy-D-glucose, 3-O-methyl-D-glucose, D-mannose, and D-fructose seem to have little, if any, affinity for the transporter. Phlorizin gives more than an order of magnitude greater inhibition of D-glucose uptake than its aglycone, phloretin. Finally, an anti-BBM antibody raised against intact purified BBM from human kidney (14) has no detectable inhibitory capability with respect to D-glucose uptake.
Quantitative characterization of the D-glucose transporter The initial uptake of D-glucose as a function of concentration is illustrated in Fig. 3 in the form of a Scatchard plot. This experiment was carried out at 250 rather than 370 in order to ensure a better estimate of the initial velocity of uptake. The three curves represent uptake in the presence of an initial 100 mM NaCl gradient, in the presence of a sodium gradient and 1 AiM phlorizin, and in the presence of an initial 100 mM KCI gradient. The sodium-independent uptake seems to represent a single transporter site with a Km - 36 mM and a Vml 74 nmol/min per mg of protein. The characterization of the sodium-dependent uptake can be found by subtracting the sodium-independent uptake at each concentration from that found in the presence of sodium. These results are shown in the inset of Fig. 3. This plot is definitely curvilinear, indicating either cooperative effects or the existence of two (or more) sodium-dependent transporters in our preparation. A curvilinear Scatchard plot is also seen at 370 with the human and at 250 in our experiments with the dog. Assuming that the inset of Fig. 3 represents two independent sites, we estimate that the "high-affinity transporter" has a Km 0.3 mM and a Vmax 2.5 nmol/min per -
-
-
FIG. 4. The stereospecific sodium-dependent uptake of D-glucose as a function of concentration in the absence (o) and presence (&) of 1 AM phlorizin is plotted according to Lineweaver and Burk. The points represent the same experiment shown in Fig. 3.
mg of protein and that the "low-affinity transporter" has a Km 6 mM and a Vmax 8 nmol/min per mg of protein. Fig. 4 is a Lineweaver-Burk plot of the same data shown in the inset of Fig. 3. The inhibitory effect of 1 AtM phlorizin has also been included. The competitive nature of phlorizin inhibition of the "high-affinity transporter" is obvious. The KI for phlorizin calculated from the graph is 0.8 AtM. Fig. 5 documents the Na+ dependency of D-glucose uptake into BBM vesicles at 37°. The apparent Km for Na+ is estimated tobe l 114mM. -
DISCUSSION We have investigated D-glucose transport in a plasma membrane vesicle preparation from human kidney cortex. Our results are qualitatively similar to our own findings in the dog (6) and to those of others in the rat (4) and rabbit (5). We see two components of stereospecific D-glucose uptake: a sodiumindependent component with a relatively low affinity (Km36 mM) for D-glucose and a sodium-dependent, phlorizinsensitive component which, as our results indicate, may represent several transport systems. If we extrapolate from other mammalian renal tissue, the enzymatic marker profile of our final vesicle fraction reveals a mixture of brush border (alkaline phosphatase) and antiluminal (Na+- and K+-dependent ATPase) membranes, with a .'a) 0.8 0
a,
O 0.6C
-r E 0.4 CL
x
T
° 0.2-
0.02
0.04
0.06
1/[Na],
0.08
0.10
mMX
FIG. 5. The stereospecific sodium-dependent initial (10 sec) uptake of 1 mM D-glucose as a function of initial extravesicular sodium concentration at 37° plotted according to Lineweaver and Burk. The extravesicular NaCl concentration was varied by replacing it iso-osomotically with KC1.
Biochemistry:
Turner and Silverman
minor contamination by mitochondria, lysosomes, and probably microsomes. Restricting ourselves to the available data on renal sugar transport in humans, we would be hard put to identify precisely which type of membrane was responsible for the above observed sugar transport mechanisms. However, comparison of the characteristics of the sodium-dependent component of D-glucose uptake in human tissue, as defined by our experiments, with results from more highly purified brush border membranes from rat, rabbit, and dog indicates striking similarities with respect to (i) Km, (ii) Na+-dependent electrogenic transport, (iii) competitive inhibition by phlorizin, and (iv) specificity of transport and inhibition. Moreover, the transport of D-glucose at the antiluminal membrane appears to be sodium-independent as we have demonstrated in the dog (6). Thus, we feel that there is compelling evidence that the sodium-dependent D-glucose uptake we observe is principally due to BBM. Obviously we cannot comment on the sidedness of the human BBM vesicles; however, preliminary studies in our laboratory suggest that our dog BBM vesicles are almost all right-side-out. It is of interest that in the human as well as the dog (6) the initial uptake data can be interpreted to indicate the existence of two (or more) Na+-dependent D-glucose transporters in the BBM. Alternatively, these observations could be the result of cooperative effects between subunits of the same transporter. Obviously more work is needed to clarify this matter. Our results provide an important rationale for extrapolating findings, such as those on model Fanconi states, across species barriers. Moreover, it is hoped that the present documentation of specificity and kinetic characteristics of the BBM glucose transporter in normal humans will contribute to a secure basis for future studies on pathological sugar transport mechanisms in human beings.
Proc. Natl. Acad. Sci. USA 74 (1977)
2829
The authors thank R. Rutherford and S. McGugan for excellent
technical and secretarial assistance. This work was supported by the Medical Research Council of Canada (MT4590). M.S. is a Medical Research Council Scholar. The costs of publication of this article were defrayed in part by the
payment of page charges from funds made available to support the research which is the subject of the article. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact.
1. Silverman, M. (1976) Biochim. Biophys. Acta 457,303-351. 2. Heidrich, H.-G., Kinne, R., Kinne-Saffran, E. & Hannig, K. (1972) J. Cell Biol. 54, 232-245. 3. Forstner, G. G., Sabesin, S. M. & Isselbacher, K. J. (1968) Biochem. J. 106,381-390. 4. Kinne, R., Murer, H., Kinne-Saffran, E., Thees, M. & Sachs, G. (1975) J. Membr. Biol. 21, 375-395. 5. Beck, J. C. & Sacktor, B. (1975) J. Biol. Chem. 250, 86748680. 6. Turner, R. J. & Silverman, M. (1977) J. Supramol. Struct. Suppl. 1,141. 7. Schmitz, J., Preiser, H., Maestraaci, D., Ghosh, B. K., Cerda, J. J. & Crane, R. K. (1973) Biochim. Biophys. Acta 323,98-112. 8. Bessey, 0. A., Lowry, 0. H. & Brock, M. J. (1946) J. Biol. Chem. 164, 321-329. 9. Post, R. A. & Sen, A. K. (1967) in Methods in Enzymology, eds. Estabrook, R. W. & Pullman, M. E. (Academic Press, New York), Vol. 10, pp. 762-769. 10. Pennington, R. J. (1961) Biochem. J. 80,649-654. 11. Lowry, 0. H., Rosebrough, N. J., Farr, A. L. & Randall, R. J.
(1951) J. Biol. Chem. 193, 265-275.
12. Hopfer, U., Sigrist-Nelson, K. & Murer, H. (1975) Ann. N.Y. Acad. Sci. 264,414-427. 13. Murer, H. & Hopfer, U. (1974) Proc. Natl. Acad. Sci. USA 71, 484-488. 14. Chant, S. & Silverman, M. (1977) Kidney Int., in press.
