Inhibitory characteristics of the mycotoxin penicillic acid on (Na +-K +)-activated adenosine triphosphatase

Abstract

Penicillic acid, a cardioactive mycotoxin produced by various Penicillium molds, is a potent and selective inhibitor of membrane (Na +-K +)-adenosine triphosphatase (ATPase). A broad range of inhibition of activity by the toxin was demonstrated with a half-maximal concentration equal to 1.8 × 10 −8M. Inhibition was time and pH dependent and complete after 20–30 min preincubation within a narrow range of physiological pH. Kinetic evaluation of cationic substrate activation of (Na +-K +)-ATPase indicated competitive inhibition with regard to Na + concentration and noncompetitive inhibition with regard to K + concentration. Also K + -dependent p-nitrophenyl phosphatase activity was not significantly altered by penicillic acid, and uncompetitive inhibition with regard to ATP activation of the enzyme was demonstrated. Preliminary binding studies indicated that inhibition of ATPase activity could be partially restored by repeated washing and by incubation with dithiothreitol and cysteine. Penicillic acid (high concentrations) impaired [ 3H]ouabain binding to membrane preparations but this effect was noncompetitive, indicating different sites of action for the two inhibitors. A significant linear correlation between reactive enzyme sulfhydryl content [SH] and ATPase activity in the presence of varying concentrations of toxin also was noted. It is postulated that penicillic acid inhibition of (Na +-K +) -ATPase occurs via critically accessible membrane thiol receptors regulating Na +-dependent phosphorylation of the transport enzyme.