Clinical Science and Molecular Medicine (1977) 52, 215-21 8.
SHORT COMMUNICA TION
The forms of vitamin B,, on the transcobalamins
C A R O L I N E M. L. A. MA cD O NA L D , J. F A R Q U H A R S O N , R . G. B E SSE N T * A N D J. F. A D A M S Department of Medicine, Southern General Hospital, Glasgow, and *Department of Nuclear Medicine, Royal Infirmary, Glasgow, Scotland
(Receiued 2 August 1976; accepted 27 October 1976)
Stadtman, 1975). The functions of hydroxocobalamin and cyanocobalamin have not been established. Vitamin B12 is transported in plasma on specific carrier proteins designated transcobalamins (TC). One of these proteins, transcobalamin 11, is involved in intravascular transport of vitamin B12and in the entry of vitamin B12into the cells (Allen, 1975,1976; Mahoney & Rosenberg, 1975), and absence of this protein is characterized by a life-threatening illness (Hakami, Neiman, Canellos & Lazeron, 1971 ; Scott, Hakami, Teng & Sagerson, 1972; Hitzig, Dohman, Pluss & Vischer, 1974). In contrast the absence of the other carrier proteins in serum shows only as a Iow serum vitamin BIZconcentration and does not imperil life (Carmel & Herbert, 1969). Only a small proportion of the vitamin B12in plasma is bound to TCII (Hall, 1969; Benson, Rapazzo & Hall, 1972; Burger, Mehlman & Allen, 1975; England, Down, Wise & Linnell, 1976) but TCII mediates 33-99% of the total plasma vitamin BI2 clearance and 99% of the clearance to cells other than hepatocytes (Allen, 1976). The forms of vitamin BIZ which are normally transported on this vitally important protein have not been determined, however, and this point was therefore investigated.
SY 1. The transcobalamins from normal serum were obtained in two fractions. One contained transcobalamin I and transcobalamin I11: the other contained transcobalamin 11. The forms of vitamin B I Z in the two fractions were then examined. 2. Methylcobalamin and adenosylcobalamin were found in both fractions. Hydroxocobalamin was found in the fraction containing transcobalamin I and transcobalamin 111. Cyanocobalamin was found in both fractions in two cases, in the transcobalamin 111fraction only in one case and was absent in one case.
Key words: transcobalamins, vitamin B12. Abbreviations : TC, transcobalamin.
Introduction Four forms of vitamin B12 are normally present in human serum. Methylcobalamin is the predominant form. Adenosylcobalamin and hydroxocobalamin account jointly for an average of 33% and cyanocobalamin for an average of 2% (Linnell, Hoffbrand, Hussein, Wise & Matthews, 1974). Two of these forms have established biochemical functions: methylcobalamin in the synthesis of homocysteine from methionine and adenosylcobalamin in the isomerization of methylmalonyl coenzyme A to succinyl coenzyme A (Babior, 1975; Poston &
Materials and methods Samples of venous blood were obtained from four normal subjects by clean venepuncture with foil-wrapped syringes and transferred to foil-
Correspondence: Dr J. F. Adam, Medical Unit B, Southern General Hospital, Glasgow, Scotland.
215
Caroline M . L. A . MacDonald et al.
216
wrapped glass tubes in a darkroom illuminated by two Ilford Safelights fitted with 15 W bulbs and F904 dark-brown filters. After clotting, the samples were centrifuged and the serum was removed in the darkroom and stored at -20°C in foil-wrapped glass tubes. Preparation of the transcobalamin fractions and extraction and separation of cobalamins were all carried out under darkroom conditions. Serum was fractionated on a K26/70 column (Pharmacia Ltd) containing Sephadex G-200 (Pharmacia Ltd) with a bed volume of 300 ml and a bed height of 56 cm by upward elution with degassed Tris/HCI buffer, pH 8.0, at a flow rate of 15 ml/h maintained by a persistaltic pump (12000 Varioperpex LKB Ltd). The eluent was collected continuously in 6 ml fractions in an Ultrorac fraction collector (LKB Ltd). Before separation of the test samples, the column was calibrated with 4 ml volumes of serum containing [57Co]cyanocobalamin and the eluent volumes of the two transcobalamin fractions were ascertained. The test samples were applied to the column in volumes of 4 ml and the appropriate eluent fractions were pooled to give two fractions, one containing TCII-vitamin B1 and the other containing the other (TCI and TCIII) TC-vitamin B1 complexes. The pooled fractions were stored in foil-wrapped glass containers at -20°C until dialysis in Cuphrophan tubing against distilled water for 15 h to remove buffer salts. The vitamin B12in the dialysed fractions was extracted into a concentrated aqueous extract by the method described by Farquharson & A d a m (1976) and the forms of vitamin B I 2 were separated by thin-layer chromatography in the dark by using silica gel sheets (Eastman
Chromogram Sheets no. 6061) with a solvent system of butan-2-ol/propanol/water/ammonia, 7:4:3:1,by vol. The identification of the separated forms was effected in daylight by the bioautographic method described by Linnell, MacKenzie, Wilson & Matthews (1969); the growth zones from the test sample were identified by their position relative to standards of methylcobalamin, adenosylcobalamin, hydroxocobalmin and cyanocobalamin. The amounts of each form of vitamin B I 2in the extracts were estimated by comparing the size of the growth zones obtained from the extracts with those yielded by the standards which had been applied in known amounts. Account was also taken of the effect of the diluting and concentrating procedures and the amount of each form of vitamin BIZin the extract was finally expressed as a percentage of the total in both extracts. Results The results are shown in detail in Table 1. The expression of results in the form used implies a degree of precision which is unrealistic in practice but was dictated by the disproportion in the amounts of vitamin B12in the two TC fractions. This does not affect the essential features of the results which are that methylcobalamin and adenosylcobalamin were present in both TC fractions in all cases, that hydroxocobalamin was present in all the samples but never in the TCII-vitamin Blz fractions, and that cyanocobalamin was found in three samples, being present in both T C fractions in two samples and in the TCII-vitamin BIZ fraction only in the third.
TABLE 1. Disfribution of the forms of oilamin B I Zdetected on the rranscobalamin-cifamin B12 complexes in normal subjects The amount of each form is expressed as a percentage of the total vitamin B12 in serum.
Methylcobalamin Adenosylcobalamin Hydroxocobalamin Cyanocobalamin
All forms
Subject 1
Subject 2
Subject 3
TCII Other TC fraction fractions
TCII Other TC fraction fractions
TCII Other TC fraction fractions
4
35 35
8
48
I
a
24
10 10
0 2
8
2
2
7 0 0
10
90
18
82
14
4 0
Subject 4
55 I5
TCII fraction
0
2
52 26 8 0
85
14
a6
15
4 8 0
Other TC fractions
Forms of B I Zon transcobalamitu
The predominant form of vitamin Blz was methylcobalamin, which accounted for 39-62% (average 54%), adenosylcobalamin accounting for 22-39% (average 32%), hydroxocobalamin for 8-15% (average 10%) and cyanocobalamin for up to 12% (average 4%). The ratios of methylcobalamin to adenosylcobalamin ranged from 0.5 to 1.0 for the TCIIvitamin B12fraction and from 1.0 to 3.7 for the other TC-vitamin B12 fraction. Discussion TCII and TCIII participate in intravascular transport of vitamin B I Zand facilitate entry of vitamin B12 into cells. The TCII-cyanocobalamin complex is cleared rapidly from plasma by various tissues; shortly after cell uptake, the TCII moiety is degraded and a significant fraction of the vitamin Blz in as yet unidentified form leaves the cell and enters the plasma in the free state. The TCIII-cyanocobalamin complex is also cleared rapidly but exclusively by the liver; part of the complex is excreted in bile, and the TCIII moiety of the remainder is degraded and the vitamin BI in unknown form is excreted into plasma bound to TCII. The fate of the TCI-vitamin B12 complex which is cleared relatively slowly is not known (Cooksley, England, Louis, Down & Tavill, 1974; Burger, Schneider, Mehlman & Allen, 1975; Nexo & Gimsing, 1975; Allen, 1975, 1976; Schneider, Burger, Mehlman & Allen, 1976). Leaving aside the unknown quantities of the function of TCI and the affinities of the TC for the various forms of vitamin B12,the absence of hydroxocobalamin from the TCII-vitamin B complex would therefore suggest that this form, unlike the others which can enter all cells, can enter only hepatocytes and is not excreted from hepatocytes to plasma. Such concepts, however, leave open to question the origins of non-dietary hydroxocobalamin in plasma and do not accord with observations on the uptake of hydroxocobalamin by cells other than hepatocytes both in uivo and in uitro (Linnell, 1975). A more interesting and acceptable concept is that TCI effects the cell entry and exit of hydroxocobalamin and is solely responsible for this function in cells other than hepatocytes. Acknowledgment
J.F.A. acknowledges with thanks grants from
217
the Secretary of State for Scotland on the advice of the Advisory Committee on Medical Research. References ALLEN,R.H. (1975) In: Progress in Hematology, vol. IX, pp. 57-84. Ed. Brown, E.B. Grune and Stratton, New York. ALLEN,R.H. (1976) The plasma transport of vitamin BIZ.British Journal of Haematology, 33, 161-171. BABIOR, B.M. (1975) In: Cobalamin. Biochemistry and Pathophysiology, pp. 141-212. Ed. Babior, B.M. John Wiley and Sons, New York. BENSON,R.E., RAPAZZO,M.E. & HALL,C.A. (1972) Late transport of vitamin BIZ by transcobalamin 11. Journal of Laboratory and Clinical Medicine, 80,448495. BURGER, R.L., MEHLMAN, C.S. & ALLEN,R.H. (1975) Human plasma R-type vitamin BIZbinding proteins. I. Isolation and characterisation of transcobalamin I, transcobalamin I11 and the normal granulocyte vitamin Blz-binding protein. Journal of Biological Chemistry, 250, 7700-7706. BURGER,R.L., SCHNEIDER, R.J., MEHLMAN, C.S. & ALLEN,R.H. (1975) Human plasma R-type vitamin Blz binding proteins. 11. The role of transcobalamin I, transcobalamin I11 and the normal granulocyte vitamin B12-binding protein in the plasma transport of vitamin B12. Journal of Biological Chemistry. 250, 7707-771 3. CARMEL, R. & HERBERT, V. (1969) Deficiency of vitamin B12-bindingalpha globulin in two brothers. Blood, 33, 1-12. COOKSLEY, W.G.E., ENGLAND, J.M., LOUIS,L., DOWN, M.C. & TAVILL, A.S. (1974) Hepatic vitamin B12 release and transcobalamin I1 synthesis in the rat. Clinical Science and Molecular Medicine, 47, 531-545. J.C. ENGLAND, J.M., DOWN,M.C., WISE,I.J. & LINNELL, (1976) The transport of endogenous vitamin B1z in normal human serum. Clinical Science and Molecular Medicine, 51, 47-52. FARQUHARSON, J. & ADAMS, J.F. (1976) The forms of vitamin BIZ in foods. British Journal of Nutrition, 36, 127-136. N., NEIMAN, P.E., CANELLOS. G.P. & LAZARON, HAKAMI, J. (1971) Neonatal megaloblastic anaemia due to inherited transcobalamin I1 deficiency in two siblings New England Journal of Medicine, 285, 1163-1170. HALL,C.A. (1969) Transport of vitamin B1z in man. British Journal of Haematology, 16,429433. HITZIG,W.H., DOHMAN, V., PLUS, H.J. & VISCHER, D. (1974) Hereditary transcobalamin I1 deficiency: clinical findings in a new family. Journal ofpediutrics, 85, m-628. LINNELL,J.C. (1975) In: Cobalamin. Biochemistry and Pathophvsioloav. DD. - - 287-333. Ed. Babior, B.M. John Wilei and Sons, New York. LINNELL,J.C., MACKENZE, H.M., WILSON, J. & MATTHEWS.D.M. (1969) Patterns of plasma cobalamins in control subjects and in cases of vitamin B12 deficiency.Journal of Clinical Pathology, 22,545-550. LINNELL,J.C., HOFFBRAND, A.V.. HIJSEIN. H.A.A., WISE,I.J. & M A ~ E W D.M. S , (1974) Tissue distribption of coenzyme and other forms of vitamin BIZ,m control subjects and patients with pernicious anaemia. Clinical Science and Molecular Medicine, 26,163-172. MAHONEY, M.J. & ROSENBERG, L.E. (1975) In: Cobalamin: Biochemistry and Pathophysiology. pp. 369402. Ed. Babior. B.M. John Wiley and Sons, New York.
218
Caroline M , L. A . MacDonaId et al.
NExO, E. & GIMSING, P. (1975) Turnover in humans of iodine- and cobalamin-labelled transcobalamin I and of iodine-labelled albumin. Scandinavian Journal of Clinical Laboratory Investigation, 35, 39 1-398. POSTON,J.M. & STADTMAN, T.C. (1975) In: Cobalaniin. Biochemistry and Pathophysiology, pp. I 1 1-140. Ed. Babior, B.M. John Wiiey and Sons, New York. SCHNEIDER, R.J., BURGER,R.L., MEHLMAN, C.S. &
ALLEN,R.H. (1976) The role and fate of rabbit and human transcobalamin I1 in the plasma transport of vitamin B I 2 in the rabbit. Journal of Clinical Investigation, 51, 27-38 Scon, C.R., HAKAMI, N., TENG,C.C. & SAGERSON, R.N. (1972) Hereditary transcobalamin I1 deficiency: the role of transcobalamin I1 in vitamin B I ,-mediated reactions. Journal of Pediatrics, 81, 1106-1 101.
SHORT COMMUNICA TION
The forms of vitamin B,, on the transcobalamins
C A R O L I N E M. L. A. MA cD O NA L D , J. F A R Q U H A R S O N , R . G. B E SSE N T * A N D J. F. A D A M S Department of Medicine, Southern General Hospital, Glasgow, and *Department of Nuclear Medicine, Royal Infirmary, Glasgow, Scotland
(Receiued 2 August 1976; accepted 27 October 1976)
Stadtman, 1975). The functions of hydroxocobalamin and cyanocobalamin have not been established. Vitamin B12 is transported in plasma on specific carrier proteins designated transcobalamins (TC). One of these proteins, transcobalamin 11, is involved in intravascular transport of vitamin B12and in the entry of vitamin B12into the cells (Allen, 1975,1976; Mahoney & Rosenberg, 1975), and absence of this protein is characterized by a life-threatening illness (Hakami, Neiman, Canellos & Lazeron, 1971 ; Scott, Hakami, Teng & Sagerson, 1972; Hitzig, Dohman, Pluss & Vischer, 1974). In contrast the absence of the other carrier proteins in serum shows only as a Iow serum vitamin BIZconcentration and does not imperil life (Carmel & Herbert, 1969). Only a small proportion of the vitamin B12in plasma is bound to TCII (Hall, 1969; Benson, Rapazzo & Hall, 1972; Burger, Mehlman & Allen, 1975; England, Down, Wise & Linnell, 1976) but TCII mediates 33-99% of the total plasma vitamin BI2 clearance and 99% of the clearance to cells other than hepatocytes (Allen, 1976). The forms of vitamin BIZ which are normally transported on this vitally important protein have not been determined, however, and this point was therefore investigated.
SY 1. The transcobalamins from normal serum were obtained in two fractions. One contained transcobalamin I and transcobalamin I11: the other contained transcobalamin 11. The forms of vitamin B I Z in the two fractions were then examined. 2. Methylcobalamin and adenosylcobalamin were found in both fractions. Hydroxocobalamin was found in the fraction containing transcobalamin I and transcobalamin 111. Cyanocobalamin was found in both fractions in two cases, in the transcobalamin 111fraction only in one case and was absent in one case.
Key words: transcobalamins, vitamin B12. Abbreviations : TC, transcobalamin.
Introduction Four forms of vitamin B12 are normally present in human serum. Methylcobalamin is the predominant form. Adenosylcobalamin and hydroxocobalamin account jointly for an average of 33% and cyanocobalamin for an average of 2% (Linnell, Hoffbrand, Hussein, Wise & Matthews, 1974). Two of these forms have established biochemical functions: methylcobalamin in the synthesis of homocysteine from methionine and adenosylcobalamin in the isomerization of methylmalonyl coenzyme A to succinyl coenzyme A (Babior, 1975; Poston &
Materials and methods Samples of venous blood were obtained from four normal subjects by clean venepuncture with foil-wrapped syringes and transferred to foil-
Correspondence: Dr J. F. Adam, Medical Unit B, Southern General Hospital, Glasgow, Scotland.
215
Caroline M . L. A . MacDonald et al.
216
wrapped glass tubes in a darkroom illuminated by two Ilford Safelights fitted with 15 W bulbs and F904 dark-brown filters. After clotting, the samples were centrifuged and the serum was removed in the darkroom and stored at -20°C in foil-wrapped glass tubes. Preparation of the transcobalamin fractions and extraction and separation of cobalamins were all carried out under darkroom conditions. Serum was fractionated on a K26/70 column (Pharmacia Ltd) containing Sephadex G-200 (Pharmacia Ltd) with a bed volume of 300 ml and a bed height of 56 cm by upward elution with degassed Tris/HCI buffer, pH 8.0, at a flow rate of 15 ml/h maintained by a persistaltic pump (12000 Varioperpex LKB Ltd). The eluent was collected continuously in 6 ml fractions in an Ultrorac fraction collector (LKB Ltd). Before separation of the test samples, the column was calibrated with 4 ml volumes of serum containing [57Co]cyanocobalamin and the eluent volumes of the two transcobalamin fractions were ascertained. The test samples were applied to the column in volumes of 4 ml and the appropriate eluent fractions were pooled to give two fractions, one containing TCII-vitamin B1 and the other containing the other (TCI and TCIII) TC-vitamin B1 complexes. The pooled fractions were stored in foil-wrapped glass containers at -20°C until dialysis in Cuphrophan tubing against distilled water for 15 h to remove buffer salts. The vitamin B12in the dialysed fractions was extracted into a concentrated aqueous extract by the method described by Farquharson & A d a m (1976) and the forms of vitamin B I 2 were separated by thin-layer chromatography in the dark by using silica gel sheets (Eastman
Chromogram Sheets no. 6061) with a solvent system of butan-2-ol/propanol/water/ammonia, 7:4:3:1,by vol. The identification of the separated forms was effected in daylight by the bioautographic method described by Linnell, MacKenzie, Wilson & Matthews (1969); the growth zones from the test sample were identified by their position relative to standards of methylcobalamin, adenosylcobalamin, hydroxocobalmin and cyanocobalamin. The amounts of each form of vitamin B I 2in the extracts were estimated by comparing the size of the growth zones obtained from the extracts with those yielded by the standards which had been applied in known amounts. Account was also taken of the effect of the diluting and concentrating procedures and the amount of each form of vitamin BIZin the extract was finally expressed as a percentage of the total in both extracts. Results The results are shown in detail in Table 1. The expression of results in the form used implies a degree of precision which is unrealistic in practice but was dictated by the disproportion in the amounts of vitamin B12in the two TC fractions. This does not affect the essential features of the results which are that methylcobalamin and adenosylcobalamin were present in both TC fractions in all cases, that hydroxocobalamin was present in all the samples but never in the TCII-vitamin Blz fractions, and that cyanocobalamin was found in three samples, being present in both T C fractions in two samples and in the TCII-vitamin BIZ fraction only in the third.
TABLE 1. Disfribution of the forms of oilamin B I Zdetected on the rranscobalamin-cifamin B12 complexes in normal subjects The amount of each form is expressed as a percentage of the total vitamin B12 in serum.
Methylcobalamin Adenosylcobalamin Hydroxocobalamin Cyanocobalamin
All forms
Subject 1
Subject 2
Subject 3
TCII Other TC fraction fractions
TCII Other TC fraction fractions
TCII Other TC fraction fractions
4
35 35
8
48
I
a
24
10 10
0 2
8
2
2
7 0 0
10
90
18
82
14
4 0
Subject 4
55 I5
TCII fraction
0
2
52 26 8 0
85
14
a6
15
4 8 0
Other TC fractions
Forms of B I Zon transcobalamitu
The predominant form of vitamin Blz was methylcobalamin, which accounted for 39-62% (average 54%), adenosylcobalamin accounting for 22-39% (average 32%), hydroxocobalamin for 8-15% (average 10%) and cyanocobalamin for up to 12% (average 4%). The ratios of methylcobalamin to adenosylcobalamin ranged from 0.5 to 1.0 for the TCIIvitamin B12fraction and from 1.0 to 3.7 for the other TC-vitamin B12 fraction. Discussion TCII and TCIII participate in intravascular transport of vitamin B I Zand facilitate entry of vitamin B12 into cells. The TCII-cyanocobalamin complex is cleared rapidly from plasma by various tissues; shortly after cell uptake, the TCII moiety is degraded and a significant fraction of the vitamin Blz in as yet unidentified form leaves the cell and enters the plasma in the free state. The TCIII-cyanocobalamin complex is also cleared rapidly but exclusively by the liver; part of the complex is excreted in bile, and the TCIII moiety of the remainder is degraded and the vitamin BI in unknown form is excreted into plasma bound to TCII. The fate of the TCI-vitamin B12 complex which is cleared relatively slowly is not known (Cooksley, England, Louis, Down & Tavill, 1974; Burger, Schneider, Mehlman & Allen, 1975; Nexo & Gimsing, 1975; Allen, 1975, 1976; Schneider, Burger, Mehlman & Allen, 1976). Leaving aside the unknown quantities of the function of TCI and the affinities of the TC for the various forms of vitamin B12,the absence of hydroxocobalamin from the TCII-vitamin B complex would therefore suggest that this form, unlike the others which can enter all cells, can enter only hepatocytes and is not excreted from hepatocytes to plasma. Such concepts, however, leave open to question the origins of non-dietary hydroxocobalamin in plasma and do not accord with observations on the uptake of hydroxocobalamin by cells other than hepatocytes both in uivo and in uitro (Linnell, 1975). A more interesting and acceptable concept is that TCI effects the cell entry and exit of hydroxocobalamin and is solely responsible for this function in cells other than hepatocytes. Acknowledgment
J.F.A. acknowledges with thanks grants from
217
the Secretary of State for Scotland on the advice of the Advisory Committee on Medical Research. References ALLEN,R.H. (1975) In: Progress in Hematology, vol. IX, pp. 57-84. Ed. Brown, E.B. Grune and Stratton, New York. ALLEN,R.H. (1976) The plasma transport of vitamin BIZ.British Journal of Haematology, 33, 161-171. BABIOR, B.M. (1975) In: Cobalamin. Biochemistry and Pathophysiology, pp. 141-212. Ed. Babior, B.M. John Wiley and Sons, New York. BENSON,R.E., RAPAZZO,M.E. & HALL,C.A. (1972) Late transport of vitamin BIZ by transcobalamin 11. Journal of Laboratory and Clinical Medicine, 80,448495. BURGER, R.L., MEHLMAN, C.S. & ALLEN,R.H. (1975) Human plasma R-type vitamin BIZbinding proteins. I. Isolation and characterisation of transcobalamin I, transcobalamin I11 and the normal granulocyte vitamin Blz-binding protein. Journal of Biological Chemistry, 250, 7700-7706. BURGER,R.L., SCHNEIDER, R.J., MEHLMAN, C.S. & ALLEN,R.H. (1975) Human plasma R-type vitamin Blz binding proteins. 11. The role of transcobalamin I, transcobalamin I11 and the normal granulocyte vitamin B12-binding protein in the plasma transport of vitamin B12. Journal of Biological Chemistry. 250, 7707-771 3. CARMEL, R. & HERBERT, V. (1969) Deficiency of vitamin B12-bindingalpha globulin in two brothers. Blood, 33, 1-12. COOKSLEY, W.G.E., ENGLAND, J.M., LOUIS,L., DOWN, M.C. & TAVILL, A.S. (1974) Hepatic vitamin B12 release and transcobalamin I1 synthesis in the rat. Clinical Science and Molecular Medicine, 47, 531-545. J.C. ENGLAND, J.M., DOWN,M.C., WISE,I.J. & LINNELL, (1976) The transport of endogenous vitamin B1z in normal human serum. Clinical Science and Molecular Medicine, 51, 47-52. FARQUHARSON, J. & ADAMS, J.F. (1976) The forms of vitamin BIZ in foods. British Journal of Nutrition, 36, 127-136. N., NEIMAN, P.E., CANELLOS. G.P. & LAZARON, HAKAMI, J. (1971) Neonatal megaloblastic anaemia due to inherited transcobalamin I1 deficiency in two siblings New England Journal of Medicine, 285, 1163-1170. HALL,C.A. (1969) Transport of vitamin B1z in man. British Journal of Haematology, 16,429433. HITZIG,W.H., DOHMAN, V., PLUS, H.J. & VISCHER, D. (1974) Hereditary transcobalamin I1 deficiency: clinical findings in a new family. Journal ofpediutrics, 85, m-628. LINNELL,J.C. (1975) In: Cobalamin. Biochemistry and Pathophvsioloav. DD. - - 287-333. Ed. Babior, B.M. John Wilei and Sons, New York. LINNELL,J.C., MACKENZE, H.M., WILSON, J. & MATTHEWS.D.M. (1969) Patterns of plasma cobalamins in control subjects and in cases of vitamin B12 deficiency.Journal of Clinical Pathology, 22,545-550. LINNELL,J.C., HOFFBRAND, A.V.. HIJSEIN. H.A.A., WISE,I.J. & M A ~ E W D.M. S , (1974) Tissue distribption of coenzyme and other forms of vitamin BIZ,m control subjects and patients with pernicious anaemia. Clinical Science and Molecular Medicine, 26,163-172. MAHONEY, M.J. & ROSENBERG, L.E. (1975) In: Cobalamin: Biochemistry and Pathophysiology. pp. 369402. Ed. Babior. B.M. John Wiley and Sons, New York.
218
Caroline M , L. A . MacDonaId et al.
NExO, E. & GIMSING, P. (1975) Turnover in humans of iodine- and cobalamin-labelled transcobalamin I and of iodine-labelled albumin. Scandinavian Journal of Clinical Laboratory Investigation, 35, 39 1-398. POSTON,J.M. & STADTMAN, T.C. (1975) In: Cobalaniin. Biochemistry and Pathophysiology, pp. I 1 1-140. Ed. Babior, B.M. John Wiiey and Sons, New York. SCHNEIDER, R.J., BURGER,R.L., MEHLMAN, C.S. &
ALLEN,R.H. (1976) The role and fate of rabbit and human transcobalamin I1 in the plasma transport of vitamin B I 2 in the rabbit. Journal of Clinical Investigation, 51, 27-38 Scon, C.R., HAKAMI, N., TENG,C.C. & SAGERSON, R.N. (1972) Hereditary transcobalamin I1 deficiency: the role of transcobalamin I1 in vitamin B I ,-mediated reactions. Journal of Pediatrics, 81, 1106-1 101.
