Late transport of vitamin B 12 by transcobalamin II.

Abstract

Abstract Current concepts of plasma transport of vitamin B 12 (B-12) hold that transcobalamin II (TC II) carries B-12 as it is absorbed from the intestine while transcobalamin I (TC I) carries it subsequently. The purpose of the present study was to determine whether TC II had a role in late B-12 transport by measuring it through its native B-12 label as detected by bioassay with Euglena gracilis . The basic procedure was the fractionation of serum by gel filtration and the subsequent assay of the native B-12 associated with the B-12-binding proteins. The identity of TC II was confirmed by bath carboxymethyl cellulose chromatography and analytic gel filtration. In 6 normal persons on typical diets nearly 30 per cent of the total native serum B-12 was bound to TC II. Neither a one week or a one month B-12-free diet significantly changed the amount of plasma B-12 bound to TC II. Three gastrectomized patients whose B-12 intake had been eliminated for 2.5 to 12 weeks also had detectable native B-12 bound to TC II. The demonstration of significant amounts of native B-12 bound to TC II, even after extended periods free of B-12 intake, suggest that TC II carries B-12 in stages of transport later than postabsorption.