Inhibition of cardiac sarcoplasmic reticulum Ca2+-ATPase activity by menadione

2-Methyl-1,4-naphthoquinone (menadione) inhibits Ca2+-ATPase activity of cardiac sarcoplasmic reticulum membrane vesicles in a time- and concentration-dependent way; after 60 min of preincubation an apparent Ki value of 33.5 microM was calculated. Inhibition is not reversible in that it persists even after the drug is removed and Ca2+-ATPase activity is assayed in a menadione-free medium. GSH (2 mM), but not DTT, is able to prevent and reverse the inhibition of Ca2+-ATPase by menadione. The relative importance of menadione metabolism in the inhibition of Ca2+-ATPase was studied in cell-free systems composed of vesicles and subcellular fractions containing metabolizing enzymes. Under these experimental conditions, 105,000g supernatants isolated from heart or liver that biotransform menadione through DT-diaphorase reduce the inhibition of Ca2+-ATPase activity determined by menadione. Also liver microsomes that biotransform menadione through NADPH-cytochrome P450 reductase decrease the inhibition by menadione. By contrast, cardiac microsomes that do not biotransform the drug do not influence the effect of menadione. These results indicate that, under the experimental conditions used for this study, menadione does not require metabolism to inhibit cardiac sarcoplasmic reticulum Ca2+-ATPase activity.