PLASMA TRANSPORT OF VITAMIN-B12

Human plasma contains at least 3 vitamin B12 binding proteins that were designated as transcobalamin (TC)I, TCII and TCIII. TCII consists of a single polypeptide chain with a MW of 38,000, contains a single vitamin B12-binding site and the association constant for TCII and vitamin B12 is .apprx. 1011 M-1. Cellular uptake appeared to consist of an initial step in which the TCII-vitamin B12 entered the cell. The complex could also enter the cell in intact form by pinocytosis and TCII was subsequently degraded by lysosomal enzymes. TCI and TCIII eluted together from Sephadex G150 with an apparent MW of 150,000 and could be distinguished by chromatography on DEAE-cellulose and isoelectric focusing and were glycoproteins which differ in their carbohydrate moieties. The association constant for these R-type proteins and vitamin B12 is .apprx. 1011 M-1. Both TCI and III was taken up by hepatocytes by a common mechanism. The terminal galactose residues of these glycoproteins were responsible for the binding of the complex to the complex to the plasma membrane receptor and the complex was eventually catabolized by lysosomes.