Inhibition of rat brain microsomal (Na⁺ +K⁺)-ATPase and K⁺-p-nitrophenylphosphatase by periodic acid

The effects of mild periodate exposure on the kinetics of (Na⁺ +K⁺)-ATPase and K⁺-p-nitrophenylphosphatase were studied using rat cerebral microsome preparations. Fifty percent inhibition of both enzyme activities was attained near 3 μM periodate concentrations. This inhibition was biphasic with time. Mg²⁺-ATPase and Mg²⁺-p-nitrophenylphosphatase activities were much less inhibited by periodate. Periodate inhibition was partially reversed by dimercaprol and dithiothreitol but not by dilution. The possible reaction products formic acid, formaldehyde, glyceraldehyde, and acetaldehyde had no inhibitory effects in similar concentrations. Periodate exposure produced no detectable changes in the activation of (Na⁺ +K⁺)-ATPase by Na⁺, K⁺, Mg²⁺, or ATP. Residues shared by both (Na⁺ +K⁺)-ATPase and K⁺-p-nitrophenylphosphatase are both critical to hydrolytic function and sensitive to mild oxidation by periodate.