Calcium-dependent zinc efflux in human red blood cells

Summary

Zinc efflux from human red blood cells is largely brought about by a saturable mechanism that depends upon extracellular Ca²⁺ ions. It has aV _max of about 35 μmol/10¹³ cells hr, aK _m for external Ca²⁺ of 1×10⁻⁴ m, and aK _m for internal Zn²⁺ of 1×10⁻⁹ m. External Zn²⁺ inhibits with aK _0.5 of 3×10⁻⁶ m. Sr²⁺ is a substitute for external Ca²⁺, but changes in monovalent anions or cations have little effect on the Zn²⁺ efflux mechanism. It is unaffected by most inhibitors of red cell transport systems, although amiloride and D-600 (methoxyverapamil, a Ca²⁺ channel blocker) are weakly inhibitory. The transport is capable of bringing about the net efflux of Zn²⁺, against an electrochemical gradient, provided Ca²⁺ is present externally. This suggests it may be a Zn²⁺:Ca²⁺ exchange, which would be able to catalyze the uphill movement of Zn²⁺ at the expense of an inward Ca²⁺ gradient, which is it self maintained by the Ca²⁺ pump.