Elizabeth S. Roberts

Mechanism-based inhibition of mouse P4502b-10 by selected arylalkynes

Suicide inhibitors of cytochrome P450 families are excellent tools to predict which isoforms mediate the metabolism/activation of a variety of chemical agents. We compared the inhibitory effects of several arylalkynes on mouse cytochromes P450 with published data for the rat model. The inhibition of P4502b specific dealkylation of benzyloxyresorufin by 2-ethynylnaphthalene (2-EN), 5-phenyl-1-pentyne (PPY), 4-phenyl-1-butyne (PBY), and 9-ethynylphenanthrene (9-EPh) was measured in hepatic microsomes from male mice treated with 1,4-bis[2-(3,5-dichloropyridyloxy)]-benzene (TCPOBOP) to induce cytochrome P4502b. Pulmonary microsomes were prepared from untreated mice. 9-EPh, 2-EN, and PPY caused a time-, concentration-, and NADPH-dependent loss in P4502b activity in both tissues. PBY, however, demonstrated this type of inhibition only in liver microsomes. The IC50 was calculated for both liver and lung microsomes and compared with published Ki (concentration required for half-maximal inhibition) or KI (concentration required for half-maximal inactivation) values for the rat. PPY, PBY, and 9-EPh were equally effective inhibitors of mouse P4502b and rat P4502B1. 2-EN was a 5- to 10-fold less potent inhibitor of mouse P4502b, as compared with the rat, even though it was shown to bind to the active site of the mouse isoform as demonstrated by its metabolism to 2-naphthylacetic acid. These data suggest that the active site of the mouse P4502b enzyme is functionally similar to the rat P4502B isoform, with the exception of the disparity in its susceptibility to inactivation by 2-EN as measured by the Ki values.

RADICAL INTERMEDIATES IN THE CATALYTIC CYCLES OF CYTOCHROME P-450

Schemes are presented summarizing current knowledge of the mechanism of action of cytochrome P-450 when it functions either as a monooxygenase with molecular oxygen as the oxygen donor or as a peroxygenase with peroxy compounds as the oxygen donor. In the process, a large variety of physiologically occurring and foreign compounds undergo hydroxylation and oxy and peroxy radicals are generated. In addition, cytochrome P-450 catalyzes reductive reactions, including a recently discovered reaction in which organic hydroperoxides are cleaved to yield hydrocarbons and aldehydes or ketones. The reaction is believed to involve homolysis of the oxygen-oxygen bond and generation of an alkoxy radical, with beta-scission of the latter followed by reduction of the secondary radical to the hydrocarbon. Evidence has been obtained that lipid hydroperoxides are physiological substrates for this reductive cleavage reaction catalyzed by cytochrome P-450.