REVERSIBLE INHIBITION OF NADPH-CYTOCHROME P450 REDUCTASE BY ALPHA-LIPOIC ACID

NADPH-cytochrome-P450 reductase both purified from rat hepatic microso mes and involved in microsomal fraction was inactivated by treatment w ith alpha-lipoic acid. Since alpha-lipoic acid contains disulfide bond , it reacts with SH-groups of the reductase via the reaction of thiol- disulfide exchange resulting in the loss of the enzyme reducing activi ty. NADP(+) completely protected reductase from the inactivation. The modification of reductase was reversible: the modified enzyme was part ially reactivated with dithiothreitol and dihydrolipoic acid in the ca se when cytochrome c was used as a substrate of reductase. In the case when inorganic substrate, K3Fe(CN)(6), was used for assay the activit y of modified reductase no reactivation was observed. It was found tha t the order of the reaction of inactivation of membrane-bound microsom al reductase is equal to 1.2 +/- 0.2, which is in an agreement with ps eudo-first order kinetics, and the second-order-rate constant of 26 M( -1)min(-1). The results have shown that well known therapeutic agent a lpha-lipoic acid is an efficient inhibitor of both purified and micros omal reductase. (C) 1995 Academic Press, Inc.