Repeated administration of propranolol (PL) to rats causes the inhibition of cytochrome P450-2D (P450-2D) enzyme. We recently found that 4-hydroxypropranolol (4-OH-PL) was biotransformed to 1,4-naphthoquinone (1,4-NQ) by superoxide (SO) anions in medium containing rat liver microsomes and NADPH and proposed that the binding of the quinone to P450-2D apoproteins might be one of mechanisms for the enzyme inhibition [Narimatsu et al. (1995) Chem. Res. Toxicol. 8, 721-728]. In this study, we have searched for possible sources of SO for the conversion of 4-OH-PL to 1,4-NQ in rat liver microsomes and determined the radioactivity covalently bound to microsomal proteins after incubation of radioactive PL and 4-OH-PL with rat liver microsomes. Elimination of 4-OH-PL from a mixture containing microsomes and NADPH was suppressed by carbon monoxide. Antibodies raised to P450-2B1 and -3A2 partially, and antibody against NADPH-cytochrome P450 reductase (fp2) markedly suppressed the reaction. 1,4-NQ was formed concomitantly with 4-OH-PL elimination by a reconstituted preparation of fp2. Binding studies using naphthalene ring (NR)- and side chain (SC)-radiolabeled PL and 4-OH-PL showed that radioactivity covalently bound to microsomal proteins was much higher from 4-OH-PL than from PL for the NR-labeled compounds, but higher from PL than from 4-OH-PL for the SC-labeled compounds. These results suggest that the 4-OH-PL formed from PL by P450-2D enzyme is converted to 1,4-NQ with loss of the side chain, and the 1,4-NQ accounts for most of the radioactivity covalently bound to microsomal proteins, including the P450-2D enzymes. The SO for conversion of 4-OH-PL to 1,4-NQ is supplied mainly by fp2 with some contribution by P450 enzymes.