Hydroperoxides selectively inhibit human erythrocyte membrane enzymes

Treatment of washed erythrocytes with tert-butyl hydroperoxide (0.5 mM, 10 min) inhibited basal Ca2+ + Mg2+-ATPase activity by 40% and calmodulin-stimulated activity by 54%. The inhibition was accompanied by the formation of methemoglobin and the aggregation of some membrane proteins into a high-molecular-weight polymer. Membranes, isolated from washed erythrocytes, showed a similar pattern of inhibition. Basal Ca2+ + Mg2+-ATPase activity was inhibited 50% at 10 min and 70% at 30 min while calmodulin-stimulated activity was inhibited 70% at 10 min and 84% at 30 min. Thiobarbituric acid-reactive products formed slowly during the first 10 min and then increased sharply between 10 and 30 min. The polymerization of membrane proteins was also observed during the tert-butyl hydroperoxide exposure. Inhibition of erythrocyte membrane enzymes was selective. The Na+ + K+-stimulated Mg2+ ATPase, like the Ca2+ + Mg2+-ATPase, was sensitive to membrane oxidation but the activities of Mg2+-ATPase and acetylcholinesterase were less inhibited by tert-butyl hydroperoxide. Acetylcholinterase was found to be very resistant to hydroperoxide treatment with less than 10% loss of activity. The effects of two other hyproperoxides on enzyme inhibition were studied also. Cumene hydroperoxide (0.5 mM) was found to be as potent as tert-butyl hydroperoxide but hydrogen peroxide at 10 mM did not produce thiobarbituric acid-reactive products or inhibit Ca2+ + Mg2+-ATPase activity until after 20 min. The selective effects of peroxides on these enzyme activities are discussed.