Human Plasma R-Type Vitamin B ,,-binding Proteins II. THE ROLE OF TRANSCOBALAMIN I, TRANSCOBALAMIN III, AND THE NORMAL GRANULOCYTE VITAMIN B,,-BINDING PROTEIN IN THE PLASMA TRANSPORT OF VITAMIN B,,*

The normal human granulocyte vitamin B,,-binding protein, transcobalamin I, and transcobalamin III, have been labeled with lZSI-labeled N-succinimidyl 3-(4-hydroxyphenyl)propionate and utilized for plasma clearance studies performed with rabbits. Both moieties of 1251-labeled granulocyte vitamin B,,-binding protein[5’Co]vitamin B,, were cleared rapidly from the plasma (>90% by 5 min) by the liver. After 30 min, the bulk of the lZ51 reappeared in the plasma in small molecular weight ( < 1000) form and was rapidly excreted in the urine. After 60 min the bulk of the [“Colvitamin B,, reappeared in the plasma bound to rabbit transcobalamin II and was subsequently taken up by a variety of tissues. Approximately 15% of the “Y-labeled granulocyte vitamin B,,-binding protein. [57Co]vitamin B,, was excreted intact into the bile during the period from 10 to 80 min after injection. The hepatic uptake of the protein*vitamin B,, complex was blocked by the prior injection of desialyzed fetuin but not by native fetuin. Similar results were obtained with Y-labeled transcobalamin 111. [5’Co]vitamin B,,. Approximately 90% of both moieties of Y-labeled transcobalamin I. [57Co]vitamin B,, had prolonged plasma survivals similar to that of 13Y-labeled bovine serum albumin. After treatment with neuraminidase, both moieties of the Y-labeled transcobalamin 1. [“Colvitamin B,, complex were cleared rapidly from the plasma by the liver in a manner that was indistinguishable from that observed in the case of untreated granulocyte vitamin B,,-binding protein and transcobalamin III. These observations indicate that desialyzed transcobalamin I and the native forms of the granulocyte vitamin B,,-binding protein and transcobalamin III are cleared from plasma by the mechanism elucidated by Ashwell and Morel1 (Ashwell, G., and Morell, A. G. (1974) Ado. Enzymol. 41, 99-128) that is capable of clearing a wide variety of asialoglycoproteins. These observations have implications concerning the function of the human R-type vitamin B,,-binding proteins, the nature of the enterohepatic circulation of vitamin B 12, the biologic significance of the mechanism described by Ashwell and Morel& and the etiology of the increased plasma concentration of human R-type protein that occurs frequently in chronic myelogenous leukemia and occasionally in hepatocellular carcinoma and other solid tumors.