Glycylglutamine: metabolism and effects on organ balances of amino acids in postabsorptive and starved subjects HERBERT LOCHS, WOLFGANG HijBL, ERICH ROTH, EMILE L. MORSE, AND

SLOBODAN GASIC, SIAMAK A. ADIBI

1st University Clinic for Gastroenterology and Hepatology, 1st Surgical University Clinic, 1st Medical University Clinic, University of Vienna, A1090 Vienna, Austria; and Clinical Nutrition Unit, Montefiore University Hospital and Department of Medicine, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15213 Lochs, ich Roth,

Herbert, Wolfgang Htibl, Slobodan Gasic, ErEmile L. Morse, and Siamak A. Adibi. Glycyl-

The reason for infusing glycine and glutamine as a dipeptide was that glutamine is not as stable in free form glutamine: metabolismand effects on organ balancesof amino as it is as a peptide (24); heat sterilization results in acids in postabsorptive and starved subjects.Am. J. Physiol. decomposition of glutamine to toxic products, such as 262 (Endocrinol. Metab. 25): El55E160, 1992.-The present ammonia. In contrast to glutamine, glycylglutamine is study was designedto investigate the metabolismof glycylgluanitamine and its effects on organ balancesof amino acids during quite stable, and previous studies in experimental intravenous infusion of this dipeptide (100 prnol. h-l kg-l) in mals (1) have established that it can be used as an postabsorptive and briefly starved (84-86 h) human subjects. efficient source of glutamine. However, as yet nothing is on the meArterial concentrations of glycylglutamine were not signifi- known about the effect of altered nutrition cantly different in postabsorptive (265 t 18 PM) and starved tabolism of this or any other dipeptide. Therefore, the (241 & 13 PM) subjects.Among the organs examined, kidney second aim of the present study was the investigation of predominated in clearance of glycylglutamine from plasma. the effect of starvation on metabolism of glycylglutaMoreover, renal clearance of glycylglutamine was reduced by mine. l

starvation (87 t 7 vs. 52 t 5 pmol/min, P < O.Ol), whereas neither splanchnic nor muscleclearance was significantly affected. Infusion of glycylglutamine raised plasma concentrations of glycine and glutamine by increasing renal releaseof these amino acids. In postabsorptive subjectsthe infusion significantly increasedsplanchnic balancesof glycine and glutamine with little or no effect on the musclebalances;the opposite was found in starved subjects.As far as other amino acids are concerned,the infusion decreasedthe musclereleaseof alanine and increased renal release of serine. We conclude that the amino acid residuesof glycylglutamine are largely metabolized by the splanchnic organs in postabsorptive subjects and by peripheral organs in starved subjects. The latter results in selective inhibition of musclereleaseof amino acids. amino acid metabolism; peptides; gluconeogenesis;ammoniagenesis HALLMARK of brief starvation is the increased amino acid release by the skeletal muscle and increased amino acid uptake by splanchnic organs (8). It is not yet clear whether there is any relation between the increased demand for substrates and the increased amino acid release by the muscle. The first aim of the present study was to investigate this problem by determining the effect of intravenous infusion of glycylglutamine on organ fluxes of amino acids in briefly starved human subjects. Glycine was used to serve as a substrate for the increased hepatic gluconeogenesis (8, 9)) and glutamine was used for the increased renal ammoniagenesis (8) associated with starvation. Two kinds of controls were used to assess the effect of glycylglutamine infusion on organ balances of amino acids in starved subjects. First, we investigated the effect of infusion of saline on organ balances of amino acids in both postabsorptive and starved subjects. A second control study was designed to compare the effect of infusion of glycylglutamine on organ balances of amino acids in postabsorptive vs. starved subjects. A METABOLIC

0193-1849/92 $2.00 Copyright

METHODS Subjects. Seventeen healthy volunteers, as verified by history, physical examination, and laboratory tests, participated in the study. One group (postabsorptive, 8 subjects)wasstudied after an overnight fast of 12-14 h; the other group (fasted, 9 subjects) underwent the samestudy after 84-86 h of fasting. The latter subjectswere admitted to the Hospital of the University of Vienna. All of the subjects were men and were matched with respect to age (24 t 2 vs. 27 t 2 yr) and weight (75 t 3 vs. 76 t 4 kg). During the fast, subjectswere restricted to consumingnoncaloric beveragesad libitum. All subjectswere nutritionally stable as suggestedby lack of any significant changein body weight during the month before the experiment. The protocol was approved by the Ethical Committee of the University of Vienna Medical School, and each subject gave informed consent. Experimental design. All subjectsunderwent catheterization of the hepatic, renal, femoral, and cubital veins and femoral arteries. Briefly, under local anesthesia(lidocaine)polyethylene catheterswere introduced percutaneouslyinto the right femoral vein. Under fluoroscopic guidanceone catheter was positioned in the hepatic vein, another in a renal vein, and the third catheter was positioned in the femoral vein just above the inguinal ligament. The right femoral artery was catheterized for the infusion of indocyanine green (ICG), the left femoral artery for obtaining arterial blood samples,and a cubital vein for the infusion of p-aminohippurate (PAH) and the test solution. After placement of catheters, primed constant infusions of ICG and PAH were begun. The priming dose and infusion rate of ICG were 12 mg and 0.6 mg/min, respectively. The priming doseand infusion rate of PAH were 500 mg and 50 mg/min, respectively. Blood flows were not significantly changedduring the entire experiment (Table 1). A physiological saline solution (0.9%) was also infused at a constant rate of 1 ml- h-l- kg-l into the cubital vein starting at the end of the catheterization procedure. After 30 min of equilibration, three

blood samples were obtained at 15-min intervals priate catheters. As shown in Fig. 1, preliminary 0 1992 the American Physiological Society

from approexperiments El55

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Table

MUSCLE

RELEASE

1. Organ plasma flows Organ

Postabsorptive

Leg

385k57 865t68 737k50

Kidney Splanchnic

n

Starved

8 7 7

1,136+117 978t109

n

376k30

9 8 6

Values are means t SE in ml/min; n, no. of subjects used. Muscle plasma flow was measured by indocyanine green (ICG) dilution, renal plasma flow by p-aminohippurate excretion, and splanchnic plasma flow by ICG excretion.

OF

AMINO

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liquid chromatography technique (22). In addition, whole blood sampleswere obtained from the arterial catheter for ammonia analysis (4). Splanchnic plasma flow was measuredby ICG excretion (6), leg plasma flow by ICG dilution (13), and renal plasmaflow by PAH excretion (7). Calculation and statistics. Amino acid and dipeptide balances acrossorgans were calculated as a product of arterial-venous concentration differences and the respective plasma flow. A negative balance indicates net release,and a positive balance indicates net uptake. Becausethe major tissue in the leg is muscle,we designatedthe leg asmuscle. All data are expressedasmeanst SE. Statistical evaluation of the data was performed by a two-way analysis of variance with repeated measures(5). RESULTS

a 100

-

oe??,, -k

-30

I-----

-15

Saline

0

I 30

15

Time

I1

I 45

(min)

I 60

Gly-Gln

I 75

Gly-Gln

-G-

Glutamine

-m-

Glycine

I 90

I 105

I’ 120

.-I

Fig. 1. Plasma concentrations (mean t SE in 4 subjects) of glycylglutamine (Gly-Gln) and its constituent amino acid residues during infusion of saline and glycylglutamine. Glycine and glutamine concentrations were in a steady state during infusion of saline; all substrates reached a new steady state by 60 min after initiation of infusion of glycylglutamine.

showedsteady states in arterial concentrations of both glycine and glutamine during the saline infusion period. After the saline period, infusion of a 100 mM solution of glycyl+glutamine in distilled water replaced that of saline at an infusion rate of 1 ml h-l kg-l. Glycylglutamine was provided by Pfrimmer/Kabi (Erlangen, FRG). After 30 min of equilibration, triplicate blood sampleswere obtained at 15min intervals. These times were chosen after the preliminary experiment showed steady states in arterial concentrations of both glycine and glutamine after 30 min of the peptide infusion (Fig. 1). Arterial concentration of glycylglutamine reached a steady-state level after 60 min of infusion (Fig. I). Organ balancesof all amino acids were in steady state during the saline period, as well as 30 min after the beginning of the infusion of glycylglutamine (data not shown). At the end of each experiment, urine was obtained for dipeptide and amino acid analysis. We used plasma rather than whole blood for measuringconcentrations of glycylglutamine and its constituent amino acids. Consistent with our previous results (15), preliminary studies showed no uptake of glycylglutamine by human red blood cells (Morse and Adibi, unpublished observations). Moreover, it is believed that selecting plasma for analysis ensuresgreater accuracy in measuringthe organ balance of glutamine (25). Analytic methods. Blood was drawn into heparinized vials, and plasma was immediately separated by centrifugation at 4°C. Glucose concentration was determined by the glucose oxidase method (4). For amino acid and dipeptide analysis, plasmawas deproteinized by addition of 30% sulfosalicylic acid (10: 1, vol/vol); the protein-free supernatant obtained by centrifugation at 4°C was stored at -70°C. Dipeptide and amino acid concentrations were determined bv a high-nerformance

Metabolism of glycylglutamine. The arterial concentrations of glycylglutamine during the infusion were not significantly different in postabsorptive (265 t 18 PM) and starved subjects (241 t 13 PM). The analysis of urine collected during the infusion showed there was very little loss of glycylglutamine or its constituent amino acids either in postabsorptive or starved subjects; in both cases Cl% of the amount infused was excreted. To determine the site of metabolism of glycylglutamine, we investigated muscle, renal, and splanchnic clearance of this dipeptide in postabsorptive and starved subjects (Fig. 2). The organ plasma flows were not significantly different in the two groups of subjects (Table l), nor did the flow rate change during the study period. The balance of peptide was positive across each organ, indicating that all organs examined narticinated in clear100

Pocrtabsorptlve Starved

l

80

60 .i 5 5 40

20

0

Kidney

Splanchnic

Fig. 2. Rates (mean t SE) of clearance of glycylglutamine by organs of postabsorptive (n = 8) and starved (n = 9) subjects. Clearance across a leg was designated as muscle. Starvation significantly (P < 0.01) reduced clearance by kidney but had no significant effect on clearance bv other organs.

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ante of glycylglutamine from plasma. Although neither splanchnic nor muscle clearance of glycylglutamine was affected by starvation, clearance by the kidney was significantly reduced. Arterial concentrations of amino acids. Starvation significantly decreased the arterial concentrations of threonine, serine, glutamate, glutamine, glycine, alanine, citrulline, and tyrosine and significantly increased arterial concentrations of valine, isoleucine, and leucine (Table 2). Infusion of glycylglutamine increased arterial concentrations of glycine and glutamine in both postabsorptive and starved subjects. There were selective alterations in arterial concentrations of amino acids that were not infused. The infusion of glycylglutamine significantly increased the plasma concentration of serine in both postabsorptive and starved subjects and significantly decreased the plasma concentration of alanine only in starved subjects. Muscle balances. Starvation significantly increased muscle release of alanine, serine, leucine, valine, methionine, isoleucine, and phenylalanine and significantly decreased muscle uptake of glutamate (Table 3). Infusion of glycylglutamine had no significant effect on muscle release of either alanine or glutamine in postabsorptive subjects but abolished muscle release of glycine and increased uptake of serine. In contrast, during starvation, the infusion of glycylglutamine significantly decreased muscle release of glutamine, glycine, alanine, and serine. The effect of infusion appeared to be selective, since it did not alter the muscle balance of other amino acids either in postabsorptive or in starved subjects. Kidney balances. The only amino acid whose renal uptake was increased by starvation was glutamine (Table 4). The renal balances of isoleucine, tyrosine, and phenylalanine, which were neutral in postabsorptive subjects, were negative in starved subjects. Starvation also inTable 2. Arterial levels of amino acids during infusion of saline or glycylglutamine in postabsorptive and starved subjects Arterial

Levels of Amino Acids, pM

Postabsorptive Saline

Gly-Gln

Starved Saline

Gly-Gln

Taurine 77kl2 70tlO 79t9 80*7 Threonine 135k6 134ei 90*5* 93*5 93t6* 103+6$ Serine 140t9 154+9$ 29*3* 29*2 Glutamate 60t7 60t7 751+34$ 492t15* 561+15$ Glutamine 634t28 344+17$ 156&7* 210&6$ Glycine 252&E 308224 185*10* 172+7§ Alanine 307t23 3822 38t2 28+3t 30+3$ Citrulline Valine 232tll 227k12 447t19* 449t19 Cystine 78t7 78t6 62zk4 61t4 Methionine 2622 27t2 28tl 29t2 Isoleucine 62t4 63k4 145*8* 147t7 Leucine 134t7 140t8 264kll* 268t12 Tyrosine 60t2 58+3$ 48t2* 49t2 Phenylalanine 48+2 50t2 50tl 5021 Values are means * SE; n = 8 (postabsorptive) and 9 (starved) subjects. Gly-Gln, glycylglutamine (100 pmol . h-l. kg-l infusion). Difference in arterial levels during saline infusion in postabsorptive and starved subjects was significant at * P < 0.01 or t P < 0.05, respectively. Difference between arterial levels of saline and glycylglutamine infusions was significant at $ P < 0.01 or 0 P < 0.05, respectively.

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Table 3. Amino acid balances across a leg during infusion of saline or glycylglutamine in postabsorptive and starved subjects Muscle Balances of Amino Acids, pmol/min Postabsorptive Saline

Starved

Gly-Gln

Saline

Gly-Gln

4t2 652 4t2 422 Taurine -6tl -6tl -3tl -3tl Threonine -3t0.7* -2+0.4$ 3kO.4 5+0.9§ Serine 7*1* 8tl 16k2 15t2 Glutamate -17t2 -12*2$ -15t4 -15t6 Glutamine -7&l -3+0.4$ -6t2 3t2 Glycine -27+3-j-23+2$ -17t3 -2Ot5 Alanine -0.1t0.2 0.1t0.2 0.8t0.4 0.5tO.l Citrulline -4*2-f -2tl 222 l&2 Valine 1t0.5 0.4t0.2 3tl 2*1 Cystine -2t0.3* -2kO.2 Methionine -0.4t0.2 -0.4kO.2 -3t0.4* -2t0.3 Isoleucine 0.5t0.6 -0.6t0.4 -4-+1* -3&l Leucine 0.2t0.8 -0.9tl.O -2t0.4 -2kO.2 Tyrosine -1t0.5 -1t0.5 -2kO.47 -2t0.3 Phenylalanine -0.6t0.4 -0.2t0.4 Values are means t SE; n = 8 (postabsorptive) and 9 (starved) subjects. Gly-Gln, glycylglutamine (100 pmol . h-l kg-l infusion). Positive values denote net uptake and negative values denote net release. Difference in balances during saline infusions in postabsorptive and starved subjects was significant at * P < 0.01 or t P < 0.05, respectively. Balance during glycylglutamine infusion was significantly different from corresponding saline infusion at $ P < 0.01 or !$P C 0.05, respectively. l

Table 4. Amino acid balances across kidney during infusion of saline or glycylglutamine in postabsorptive and starved subjects Renal Balances

of Amino Acids, pmol/min

Postabsorptive Saline

Starved

Gly-Gln

-Saline

Gly-Gln

Taurine -1*3 3k2 7&5 11t5 Threonine -3tl -2tl -3t2 -4kl Serine -47t5 -6Ot5* -43&6 -4828 Glutamate -1823 -17+2 -32+5$ -28t5 Glutamine 41k5 -27*6* 79al$ 17+9'f 5&4 -30*4t Glycine 12+4 -28+5* Alanine -3t5 -5&3 -9t4 -9zk2 9tl Citrulline 13*1 13tl lot1 -7*4 Valine 7t3 3t3 -2t6 -12t2 -1lk2 Cystine -15t2 -12kl.f -3kl -1tl Methionine -0.6kO.4 0.1t0.3 -2*l$ -221 Isoleucine 4k2 221 1t2 -7t4 -6t2 Leucine It1 -2*2 -7kl§ -6tl Tyrosine 0.6k2.0 Phenylalanine 2tl 4tl -l+l$ O.ltl.O Values are means t SE; n = 7 (postabsorptive) and 8 (starved) subjects. Gln-Gly, glycylglutamine (100 pmol h-l kg-l infusion). Positive values denote net uptake and negative values denote net release. Balance during glycylglutamine infusion was significantly different from corresponding saline infusion at * P < 0.01 or t P < 0.05, respectively. Difference in balances during saline infusions in postabsorptive and starved subjects was significant at 6 P < 0.01 or $ P < 0.05. respectively. l

l

creased renal release of glutamate. The infusion of glycylglutamine either changed (from positive to negative) or significantly decreased renal balances of glutamine and glycine. As far as renal balances of other amino acids are concerned, the infusion significantly increased renal release of serine in postabsorptive subjects.

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Splanchnic balances. Starvation significantly increased splanchnic uptake of glycine, serine, threonine, methionine, isoleucine, and leucine and significantly increased splanchnic release of glutamate (Table 5). The most dramatic effect of the infusion of glycylglutamine was an increase in splanchnic uptake of glutamine and glycine in the postabsorptive subjects. This effect was blunted in the starved subjects. The infusion had no significant effect on glutamine uptake, and the increase in glycine uptake was smaller in the starved than in the postabsorptive subjects. The infusion significantly increased splanchnic uptake of serine and threonine in postabsorptive subjects and significantly increased splanchnic release of citrulline in both groups of subjects. Metabolic effects of the infusion. Arterial concentration of neither ammonia nor glucose was affected by the infusion of glycylglutamine either in postabsorptive or starved subjects. However, starvation significantly (P < 0.01) decreased plasma concentrations of ammonia (28 + 3 vs. 20 t 3 PM) and glucose (4.54 t 0.22 vs. 3.85 t 0.27 mM). DISCUSSION

Our study provides new insight into the metabolism of amino acids during starvation and infusion of glycylglutamine, a dipeptide that is being considered for inclusion in amino acid solutions for parenteral nutrition (2). Furthermore, it is the first investigation of metabolism of a dipeptide in altered nutrition. Effect of staruation. Despite considerable interest in the effect of brief starvation on amino acid metabolism, there has not been any previous study of its effect on kidney balances of amino acids in humans. The present Table 5. Amino acid balances across splanchnic organs during infusion of saline or glycylglutamine in postabsorptive and starved subjects Splanchnic

Balances

of Amino Acids, pmol/min

Postabsorptive Saline

Gly-Gln

Starved Saline

Gly-Gln

Taurine 5t3 7*2 8t4 9t4 Threonine 14t2 19t2* 30+2$ 30+2 Serine 22*4 31t5* 36*4§ 39t3 Glutamate -62&B -63-+8 -19+6$ -18t6 Glutamine 71t9 141*16* 58k14 67t13 Glycine 16t5 33+5t 35+3§ 46*3* Alanine 104*18 lllt16 132t12 119t9 Citrulline -8t1 -12+1t -521 -8+lt Valine 11t5 6t3 23t7 15t4 Cystine 4*2 3*1 4tl 2t0.5 Methionine 4tl 4tl 8+1$ 8*1 Isoleucine 321 2tl 9*2$ 7+1 Leucine 423 4t2 15+4$ 12&2 Tyrosine 10+2 9t2 13tl 13tl Phenylalanine 4*1 5tl 8tl 8tl Values are means t SE; n = 7 (postabsorptive) and 6 (starved) subjects. Gly-Gln, glycylglutamine (100 pmol . h-l kg-’ infusion). Positive values denote net uptake and negative values denote net release. Balance during glycylglutamine infusion was significantly different from corresponding saline infusion at * P < 0.01 or t P < 0.05, respectively. Difference in balances during saline infusions in postabsorptive and starved subjects was significant at $ P c 0.01 or $ P < 0.05, respectively. l

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AMINO

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data show that brief starvation causes kidney release of branched-chain and aromatic amino acids and glutamate (Table 4). Since, except for glutamate, there is no synthesis of these amino acids in the kidney, the release suggests increased proteolysis in the kidney which, like muscle, serves to export amino acids for metabolism elsewhere. Glutamine, glycine, and alanine are all considered substrates for renal ammoniagenesis (18, 19, 23). Our data show that, among these amino acids, glutamine is the principal substrate for enhanced renal ammoniagenesis. Brief starvation uniquely increased renal uptake of glutamine and renal release of glutamate (Table 4). On the other hand, brief starvation had no significant effect on renal balance of either glycine or alanine (Table 4). Previous studies of the effect of brief starvation on splanchnic balances of amino acids in humans have produced conflicting results. Felig et al. (11) reported a significant increase in splanchnic balance of alanine. This was not supported by the result of the recent studies of Eriksson et al. (10) who found that brief starvation has no significant effect on splanchnic balance of alanine in humans. Our data (Table 5) support the finding of Eriksson et al. (10). However, our data provide ample evidence for the view that brief starvation stimulates hepatic gluconeogenesis (9). The splanchnic uptake of a number of amino acids was significantly increased (Table 5). It is pertinent to note that the increased splanchnic uptake of amino acids was not unique to gluconeogenic amino acids but also included nongluconeogenic amino acids, such as leucine (Table 5). Finally, the study of Pozefsky et al. (20) showed that brief starvation increases muscle release of gluconeogenic amino acids. The present results show that, in addition to these amino acids, muscle increases the release of amino acids that are not gluconeogenic, such as leucine (Table 3). Effect of glycylglutamine infusion. The most noteworthy effect of glycylglutamine infusion on metabolism of amino acids, aside from the constituent amino acids, was a modest but significant decrease in the muscle release of alanine in starved subjects (Table 3). The impact of this curtailment of muscle release was also evident from a significant decrease in arterial concentration of alanine during the infusion of glycylglutamine in starved subjects (Table 2). This conclusion is further supported by lack of a significant effect of the infusion on either renal or splanchnic balance of alanine (Tables 4 and 5). The effect of glycylglutamine infusion on the skeletal muscle release of alanine was selective, since it occurred in starved but not in postabsorptive subjects. It therefore appears that muscle release of alanine is susceptible to inhibition by exogenous substrates only when there is increased utilization of this amino acid. The increased utilization of alanine during starvation was indicated by decreased plasma concentration (Table 2), despite its increased skeletal muscle release (Table 3). The mechanism of decrease in muscle release of alanine does not appear to include a reduction in muscle proteolysis, since the release of other amino acids such as tyrosine and branched-chain amino acids were not affected by the

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infusion

MUSCLE

RELEASE

(Table 3).

Metabolism of glycylglutamine and its constituent amino acids. The data show that the efficient utilization

of glycylglutamine was not impaired in starved human subjects, since the loss in the urine during the infusion was trivial. Furthermore, the arterial concentration of glycylglutamine did not differ in postabsorptive and starved human subjects. The kidney appeared to be the predominant organ in clearance of glycylglutamine from plasma. Although starvation significantly reduced the clearance by kidney, it did not eliminate its predominance over other organs. The present results allow a conclusion about the mechanism of reduced renal clearance of glycylglutamine in starved subjects. Previous studies in isolated perfused liver and muscle preparations (14, 16, 21) have shown that the mechanism of dipeptide clearance by these tissues is largely hydrolysis by enzymes in the plasma membrane, in the case of liver, and in the sarcolemmal membrane, in the case of skeletal muscle. These organs do not appear to have a transport system for peptides. On the other hand, in vivo (3) and in vitro (12) studies in experimental animals have shown the presence of an active system for uptake of dipeptides in kidney. The fact that the splanchnic and muscle clearances of glycylglutamine were not significantly different in postabsorptive and starved human subjects suggests that membrane hydrolysis was not impaired by starvation. Therefore the reduction in clearance of glycylglutamine by kidney could indicate a reduction in the activity of the peptide transport system in this tissue. The infusion of glycylglutamine significantly increased arterial concentrations of constituent amino acids (Table 2), but the increases were quite modest. For example, in starved subjects, the increase was 14% for glutamine and 25% for glycine. Among the organs studied, kidney appeared to be responsible for these increases. Kidney was the only organ that changed from net uptake to net release of free glycine and free glutamine during the infusion (Table 4). The only exception was the renal glutamine balance in starved subjects, which was greatly decreased during the infusion but still was positive. Based on these data, two mechanisms could be suggested to account for the role of kidney in raising the arterial concentrations of glycine and glutamine as follows: 1) hydrolysis of glycylglutamine followed by the release of constituent amino acids and 2) inhibition of glycine and glutamine uptake by glycylglutamine. Infusion of glycylglutamine not only increased plasma concentrations of constituent amino acids but also increased the plasma concentration of serine in both postabsorptive and starved subjects (Table 2). The mechanism of this increase in plasma concentration appeared to be the increased renal release of serine (Table 4), most likely the result of increased synthesis of serine from glycine (17) due to increased uptake either in free or peptide form. It is pertinent to note that the increase in renal release of serine was statistically significant only in the postabsorptive subjects (Table 4). The fact that splanchnic organs and muscle were not involved in raising plasma serine concentration was suggested by the following observations. There was neither splanchnic nor

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muscle release of serine during the infusion of glycylglutamine in postabsorptive subjects (Tables 3 and 5). Furthermore, splanchnic and muscle uptake of serine were actually increased by the infusion in postabsorptive subjects. Even in starved subjects, in which there was muscle release of serine, the muscle release was significantly decreased by the infusion of glycylglutamine (Table 3). Another striking finding of the present study was that there were exceptions in increases in muscle and splanchnit balances of amino acid residues of glycylglutamine during the infusion of this dipeptide. The exceptions were the muscle glutamine balance in postabsorptive subjects and the splanchnic glutamine balance in starved subjects. These data suggest that factors other than plasma levels are involved in tissue metabolism of glutamine. In conclusion, the present study offers the following information that may have relevance in the use of glycylglutamine in parenteral nutrition: 1) despite a reduction in renal clearance, starvation does not alter metabolism of glycylglutamine; 2) nutritional state has a striking effect on metabolism of constituent amino acids, they are utilized largely in splanchnic organs in the postabsorptive state and in peripheral organs in starvation; and 3) the increased utilization of glycine and particularly glutamine in the peripheral organs is accompanied by selective inhibition of muscle release of amino acids that are substrates for gluconeogenesis and ammoniagenesis without inhibition of starvation-induced muscle proteolysis. We thank Fritz Tichy for excellent technical assistance and Nancy Scholar for assistance in preparing the manuscript. This work was supported by National Institute of Diabetes and Digestive and Kidney Diseases Grant DK-15861. Address for reprint requests: H. Lochs, 1st Univ. Clinic for Gastroenterology and Hepatology, Univ. of Vienna, Wahringer Gurtel 18-20, A1090 Vienna, Austria. Received 31 July 1990; accepted in final form 24 September 1991. REFERENCES 1. Abumrad, S. A. Adibi.

N. N., E. L. Morse,

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and

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glutamine in peptide form: clinical applications of old and new observations. Metub. CZin. Exp. 38: 89-

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B. A. Krzysik, and A. L. Drash. Metabolism of intravenously administered dipeptides in rats: effects on amino acid pools, glucose concentration and insulin and glucagon secretion. Clin. Sci. MOL. Med. 52: 193-204, 1977.

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New York: Academic, 1974. 5. BMDP. Statistical Software. Berkeley: Univ. of California Press, 1985. 6. Bradley, S. E., F. J. Ingelfinger, G. P. Bradley, and J. J. Curry. The estimation of hepatic blood flow in man. J. CZin. Inuest. 24: 890-897, 1945. 7. Brun, C. A rapid method for the determination of para-aminohippuric acid in kidney function tests. J. Lab. CZin. Med. 37: 955-958, 1951. G. F., Jr. Starvation in man. N. EngZ. J. Med. 282: 6688. Cahill, 675,197O. 9. Chiasson, J. L., R. L. Atkinson, A. D. Cherrington, U. Keller, B. C. Sinclair-Smith, W. W. Lacy, and J. E. Liljenquist. Effects of fasting on gluconeogenesis from alanine in non-

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diabetic man. Diabetes 28: 56-60, 1979. Eriksson, L. S., M. Olsson, and 0. Bjorkman. metabolism of amino acids in healthy subjects: effect of fasting. Metub. Clin. hp. 37: 1159-1162, 1988. 11. Felig, P., 0. E. Owen, J. Wahren, and G. F. Cahill, acid metabolism during prolonged starvation. J. Clin. 10.

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Ganapathy, V., J. F. Mendicino, and F. H. Leibach. Transport of glycyl+proline into intestinal and renal brush border vesicles from rabbit. J. Biol. Chem. 256: 118-121, 1981. 13. Jorfeldt, L., and J. Wahren. Leg blood flow during exercise. 12.

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Glycylglutamine: metabolism and effects on organ balances of amino acids in postabsorptive and starved subjects.

The present study was designed to investigate the metabolism of glycylglutamine and its effects on organ balances of amino acids during intravenous in...
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