THE UPTAKE OF ZINC BY ERYTHROCYTES UNDER NEAR-PHYSIOLOGICAL CONDITIONS

The in vitro uptake of zinc by erythrocytes was measured under near-ph ysiological conditions, using Zn-65 as a radioactive tracer. Because o f the presence of serum albumin-a strong zinc ligand-a low concentrati on of medium free zinc was maintained. Under these conditions a high-a ffinity carrier for zinc transport over the cell membrane was identifi ed. With human erythrocytes, a Michaelis constant (K(m)) of 0.2 nM wit h respect to free medium zinc was measured and a V(max) of 4.5 nmoles Zn transported per h/g dry wt. The K(m) for medium Zn increases when t he size of the internal erythrocytic Zn pool is augmented, whereas V(m ax) remains virtually unchanged. A model to explain this phenomenon is proposed. It is suggested that this phenomenon could underlie observa tions, confirmed here, that the in vitro uptake of Zn by animal erythr ocytes depends on the Zn status of the animal.