GENERATION OF A FREE-RADICAL FROM CALPHOSTIN-C BY MICROSOMAL CYTOCHROME-P450 REDUCTASE IN RAT AND HUMAN LIVER

Calphostin C (cal C) is a potent and specific protein kinase C inhibit or that is thought re, be photoactivated to generate a reactive peryle nesemiquinone free radical intermediate. In this electron spin resonan ce spectrometry study, we show, for the first time, an enzymatic light -independent mechanism of cal C free radical formation. Such a free ra dical was generated in liver microsomes from rat and humans, and its f ormation was dependent on the presence of NADPH, functional microsomes , and protein and cal C concentrations. Inhibitor and purified enzyme studies showed that cytochrome P450 reductase is responsible for the c al C free radical observed, which is probably a perylenesemiquinone.