Enzymatic and non-enzymatic reduction of N-acetyl-p-benzoquinone imine and some properties of the N-acetyl-p-benzosemiquinone imine radical.

N-Acetyl-p-benzoquinone imine (NAPQI) is the postulated hepatotoxic intermediate in acetaminophen overdosage. NAPQI was rapidly metabolized by NADPH-cytochrome P-450 reductase, with an apparent Km of 1.8 to 4.0 microM and an apparent Vmax of 29.4 mumoles per min per mg, and exhibited substrate inhibition of metabolism at NAPQI concentrations above 10 microM. NADPH was oxidized by NAPQI at a slower rate in the absence of enzyme. NAPQI did not appear to undergo redox cycling at an appreciable rate to form superoxide, and it did not stimulate oxygen utilization or superoxide release by rat isolated hepatocytes. Electron spin resonance studies failed to show formation of a free radical by chemical or enzymatic reduction of NAPQI under anaerobic conditions in aqueous media.