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Transcobalamin I and other Human R‐Binders: Purification, Structural, Spectral and Physiological Studies
Author(s) -
Nexø Ebba
Publication year - 1978
Publication title -
scandinavian journal of haematology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.904
H-Index - 84
eISSN - 1600-0609
pISSN - 0036-553X
DOI - 10.1111/j.1600-0609.1978.tb02451.x
Subject(s) - cobalamin , chemistry , catabolism , ligand (biochemistry) , intrinsic factor , dissociation (chemistry) , amino acid , cyanocobalamin , biochemistry , stereochemistry , chromatography , enzyme , organic chemistry , vitamin b12 , receptor
Transcobalamin I (TCI), a human R‐binder of unknown function, has been purified in the native state by ‘labile ligand affinity chromatography’. By this method protein denaturing agents are avoided, because desorption takes place by dissociation of the insolubilized ligand (cobalamin) simply by a shift in temp. The amino terminal sequence of TCI is identical to those of other R‐binders, and the amino acid composition varies only slightly. A common phylogenetic origin for the R‐binders and intrinsic factor is suggested by the similarity in amino acid composition. Spectral analyses of aquo‐, hydroxo‐, azido‐ and cyanocobalamin, free or attached to TC I suggest that the sixth ligand is donated to the cobalamin from the protein. The observed changes in the spectra are in keeping with the hypothesis that TC I and other R‐binders are enzymes. Turnover studies with iodine‐labelled TC I saturated with cyanocobalamin revealed that it is metabolized with a fractional catabolic rate of 0.15 d ‐1 , and that the cobalamin is liberated only on degradation of the protein. From these data the amount of cobalamin transported by TC I can be calculated to be approximately 0.1 nmol per day, thus TC I does not play any significant role in the daily transport of cobalamin.