THE UPTAKE OF ZINC BY ERYTHROCYTES UNDER NEAR-PHYSIOLOGICAL CONDITIONS

The in vitro uptake of zinc by erythrocytes was measured under near-physiological conditions, using Zn-65 as a radioactive tracer. Because of the presence of serum albumin-a strong zinc ligand-a low concentration of medium free zinc was maintained. Under these conditions a high-affinity carrier for zinc transport over the cell membrane was identified. With human erythrocytes, a Michaelis constant (K(m)) of 0.2 nM with 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 the size of the internal erythrocytic Zn pool is augmented, whereas V(max) remains virtually unchanged. A model to explain this phenomenon is proposed. It is suggested that this phenomenon could underlie observations, confirmed here, that the in vitro uptake of Zn by animal erythrocytes depends on the Zn status of the animal.