Mechanisms of transport of nontransferrin-bound iron in basolateral and canalicular rat liver plasma membrane vesicles

Abstract

Although most iron in plasma is bound to transferrin, recent evidence suggests that the nontransferrin-bound fraction contributes to hepatic iron loading and toxicity seen in iron-overload disorders. Our studies of isolated perfused rat liver previously demonstrated saturable uptake of nontransferrin-bound iron that continues despite hepatic iron overload. To further characterize the mechanism of transport of this form of iron, we measured binding of 55Fe-labeled ferrous ascorbate to rat liver plasma membrane vesicles under varying conditions. Binding of 5 mumol/L iron by both basolateral and canalicular membranes was time-dependent and linear for the first 5 sec. Initial rate of binding of ferrous ascorbate to basolateral membrane vesicles was temperature dependent and increased by calcium but, in contrast to the perfused rat liver, was not inhibited by other divalent cations. Binding velocities by basolateral membrane vesicles were saturable at increasing iron concentration (Km = 33 mumol/L, Vmax = 16 pmol/mg protein/sec). Ferrous iron binding by canalicular membrane vesicles was also temperature dependent, but initial association rates were not saturable over the concentration range studied (2 to 20 mumol/L). We conclude that nontransferrin-bound iron associates with basolateral liver plasma membrane vesicles by a saturable mechanism sensitive to temperature and calcium and consistent with a membrane carrier. Other divalent cations do not inhibit membrane association but may compete for a subsequent cytosolic binding site.