Abstract
Recent investigations suggest that Ca 2+-ATPase from fish gills is very sensitive to Zn 2+ (Hogstrand et al., 1996. Am. J. Physiol. 270, R1141–R1147). The effect of free Zn 2+ ion on the human erythrocyte plasma membrane Ca 2+-ATPase was investigated to explore the possible extension of this finding to humans. Membrane vesicles were prepared and the Ca 2+-ATPase activity was measured as Ca 2+-stimulated ATP hydrolysis and as ATP-dependent Ca 2+ transport. The Zn 2+ ion inhibited the erythrocyte Ca 2+-ATPase by reducing V max and increasing the K 0.5. While in the Ca 2+ transport assay only the V max was affected at lower Zn 2+ concentrations (50–100 pM), reduction of V max was always accompanied by an affinity decrease in the ATP hydrolysis assay. The Ca 2+-ATPase was found to be inhibited by Zn 2+ at extremely low concentrations. The IC 10 and IC 50 for Zn 2+, at a Ca 2+ concentration of 1.0 μM, were estimated at 4 and 80 pM, respectively. Although the Ca 2+-ATPase might be more sensitive in vitro than in vivo conditions, the results suggest that physiological concentrations of Zn 2+ may reduce the activity of the erythrocyte Ca 2+-ATPase. Furthermore, disturbance of Ca homeostasis may be a mechanism causing Zn toxicity during exposure.
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