Transcobalamin II mediated delivery of albumin-bound hydroxocobalamin to human liver cells

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