CHARACTERIZATION OF A CHEMICALLY REACTIVE PROPRANOLOL METABOLITE THATBINDS TO MICROSOMAL PROTEINS OF RAT-LIVER

We have characterized a chemically reactive propranolol (PL) metabolit e which binds to proteins in rat liver microsomes. During incubation w ith rat liver microsomes (1 mg of protein) fortified with an NADPH-gen erating system, 4-hydroxypropranolol (4-OH-PL) quickly disappeared fro m the reaction medium, but none of the possible metabolite peaks was d etected under the high-performance liquid chromatographic conditions u sed. The consumption of 4-OH-PL depended on microsomes and NADPH. The reaction was not affected by inhibitors of cytochrome P450 or FAD mono oxygenase, but was markedly diminished in the presence of cytosol and ascorbic acid. The effect of cytosol was inhibited by potassium cyanid e but not by sodium benzoate or dimethyl sulfoxide, and was also not a ffected by heating at 60 degrees C for 30 min, suggesting that superox ide (SO) ion was involved in the reaction and that it was blocked by s uperoxide dismutase (SOD) present in the cytosol. Cu,Zn-SOD, purified from cytosol, effectively mimicked the suppressive effect of cytosol. Incubation of 4-OH-PL in an SO-generating system of xanthine and xanth ine oxidase generated 1,4-naphthoquinone (1,4-NQ), which was identifie d by TLC, HPLC, and GC/MS. 1,4-NQ was also formed in microsomal incuba tes containing NADPH and small amounts of microsomes (below 0.1 mg of protein). These results indicate that 4-OH-PL is converted by SO, or s ome reactive oxygen species derived from it, to 1,4-NQ which binds to proteins and is one of the reactive metabolites of PL.