Summary. Vitamin B12 absorbed from the gut is transported on all the vitamin B12 binding proteins (transcobalamins) and not primarily on Transcobalamin II. Absorbed vitamin B12 can still be detected on the vitamin B12 binding proteins after 10 days suggesting that the very rapid plasma clearance of the TCII‐vitamin B12 complex previously reported was probably due to damage to TCII prior to re‐injection.
Vitamin B12 binder III is part of Transcobalamin I complex showing immunological identity with other binders in tissue fluids (R binders). Heparin given in vivo or in vitro prevented separation of Binder III from TCI and this effect was neutralized by protamine.