MOUSE TRANSCOBALAMIN - 110 BIOSYNTHESIS AND UPTAKE BY L-929 CELLS

Mouse [neoplastic] fibroblast (L-929) cells, in culture, synthesized and secreted into the growth medium a vitamin B12-binding substance which was identical to mouse transcobalamin II (TC II) as judged by gel filtration on Sephadex G-200, ion-exchange chromatography on DEAE-cellulose and CM-cellulose and the ability to facilitate cellular B12 uptake by L-929 cells. The secretion of mouse fibroblast binder was blocked by cycloheximide and puromycin; and in both cases the cells'' ability to secrete this binder was partially restored when the inhibitor was removed. Within 30 h after the cells were exposed to [57Co]B12 bound to mouse serum TC II (MW .apprx. 38,000) the [57Co]B12 was bound to a large molecular weight intracellular binder (MW .apprx. 120,000) which was not released into the culture medium. During this same incubation period, the cells released free [57Co]B12 and [57Co]B12 bound to a protein which had the same elution volume as mouse serum TC II on Sephadex G-200.