Plasma Turnover of 57Cobalt‐Vitamin B12 Bound to Transcobalamin I and II

The plasma turnover of radioactive vitamin B12 bound in vitro to each of the two plasma binders (transcobalamins) was studied after intravenous injection. The observed T 1/2 values for the radioactive B12 were 9–10 days when bound to transcobalamin I and about 1.5 hours when bound to transcobalamin II. In both cases, the plasma curves yielded similar final slopes, corresponding to T 1/2 values of 9.3 and 9.8 days. In the study with transcobalamin II, this is shown to be due to rapid reappearance in the plasma of the radiocobalt label bound to transcobalamin I.