title: Variant-specific differences in human unsaturated transcobalamin II.
abstract: Electrophoresis and subsequent autoradiography of 57Co-cobalamin (57 Co-Cbl)-labeled serum show intensity differences between the genetic variants of human transcobalamin II (TC2), suggesting differences in the unsaturated (apo-) TC2 concentration. In order to distinguish between variant-specific differences in the Cbl binding affinity and those in the total-TC2 concentration, techniques were developed to determine total, apo-, and holo-TC2. Prolonged incubation at 37 degrees C with a 20-fold excess of 57Co-Cbl resulted in an almost complete exchange of endogenously bound Cbl, which allowed determination of the total TC2. The holo-TC2 concentration of both gene products in TC2 heterozygotes could be estimated by comparison of the labeling levels of apo- and total TC2, using densitometric quantification of the autoradiographs. By means of ion-exchange chromatography, TC2 could be separated from other Cbl-binding proteins, permitting a simple quantitative assay of apo- and total TC2, the results of which correlate fairly well with those measured by an immunoadsorption assay. The results obtained in the present investigation indicate that the variant-specific variation in the apo-TC2 concentration is caused by differences in the total-TC2 concentration rather than in the Cbl binding affinity.
mesh_terms: D001059:Apoproteins / Q000378:metabolism; D004586:Electrophoresis; D006801:Humans; D009439:Neuraminidase / Q000378:metabolism; D011110:Polymorphism, Genetic; D011485:Protein Binding; D014155:Transcobalamins / Q000235:genetics / Q000378:metabolism*; D014805:Vitamin B 12 / Q000378:metabolism*
chemical_list: D001059:Apoproteins; D014155:Transcobalamins; D009439:Neuraminidase; D014805:Vitamin B 12