TRANSCOBALAMIN-II-MEDIATED DELIVERY OF ALBUMIN-BOUND HYDROXOCOBALAMINTO HUMAN LIVER-CELLS

We show that hydroxocobalamin bound to human serum albumin can dissoci ate and bind to transcobalamin II present in serum. Human liver cells in culture exposed to hydroxocobalamin bound to albumin incorporated l ess of the vitamin than when similar amounts of unbound hydroxocobalam in or cyanocobalamin were present. In the presence of transcobalamin I I, a 4.5-fold increase in cellular uptake occurred, but this amount wa s less than when hydroxocobalamin or cyanocobalamin were added to tran scobalamin II. These results indicate that albumin, by binding hydroxo cobalamin, can alter the dynamics of binding to transcobalamin II and the subsequent cellular incorporation of this form of the vitamin.