NON-TRANSFERRIN IRON UPTAKE BY TROPHOBLAST CELLS IN CULTURE - SIGNIFICANCE OF A NADH-DEPENDENT FERRIREDUCTASE

This study shows that trophoblast cells in culture are able to take up Fe-59 from both Fe(III)nitrilotriacetate (NTA) and Fe-ascorbate. Fe i n the presence of ascorbate is assumed to be Fe(III) in equilibrium wi th Fe(II). Kinetic parameters for non-transferrin iron uptake are dete rmined from initial rate experiments, yielding V-max = 366 pmol/mg pro tein/5 min and K-m = 0.96 mu M for Fe(III)NTA and V-max = 4043 pmol/mg protein/5 min and K-m = 1.3 mu M for Fe-ascorbate. Since trophoblast cells in culture reduce extracellular Fe(III)CN, and uptake of Fe-59 f rom Fe-ascorbate is higher than that from Fe(III)NTA, it is suggested that reduction of Fe(III) precedes uptake. Uptake of Fe-59 from both F e-ascorbate and Fe(III)NTA is inhibited by Fe(II)chelator ferrozine an d membrane-impermeable Fe(III)CN, further supporting this hypothesis. Studies with microvillous membrane vesicles (MMV) and basal membrane v esicles (BMV) reveal the presence of a NADH-dependent ferrireductase. Reduction of Fe(III)CN follows Michaelis-Menten kinetics, both with re spect to [NADH] and [Fe]. NADPH is ineffective as electrondonor. The r ate of Fe(III)CN reduction by BMV is 2.5 times higher compared to MMV, while K-m values for Fe(III)CN and NADH are not significantly differe nt. These results reveal that a transmembrane NADH-dependent ferriredu ctase plays a role in uptake of non-transferrin iron. The possibility that this enzyme system is involved in iron transfer across the basal membrane is discussed. (C) 1998 W. B. Saunders Company Ltd.