Plasma Clearance of57Cobalt-Labelled Vitamin B12Bound in Vitro and in Vivo to Transcobalamin I and II

Radioactive-labelled B12 bound to transcobalamin I has an initial distribution volume twice the plasma volume, a fractional turnover rate of 45 to 60 per cent per day, a biologic half-life in plasma of 6 days and an extra-vascular distribution of 74 to 81 per cent. Transcobalamin II had a fractional turnover rate of 1536 and 1944 per cent per day, a biologic half-life of 12 hours and an extravascular distribution of 90 per cent.