Mechanism-Based Inhibitors of Prostaglandin ω-Hydroxylase: (R)- and (S)-12-Hydroxy-16-heptadecynoic Acid and 2,2-Dimethyl-12-hydroxy-16-heptadecynoic Acid

12-Hydroxy-16-heptadecynoic acid has been shown to selectively inactivate cytochrome P450 4A4, a pulmonary cytochrome P450 enzyme that catalyzes the ω-hydroxylation of prostaglandins [Muerhoff, A. S.; Williams, D. E.; Reich, N. O.; CaJacob, C. A.; Ortiz de Montellano, P. R.; Masters, B. S. S. J. Biol. Chem. 1989, 264, 749–756]. Potent, specific inhibitors of this enzyme are required to explore its physiological role. In a continuing effort to develop such agents, the two enantiomers of 12-hydroxy-16-heptadecynoic acid have been stereospecifically synthesized, their absolute stereochemistry confirmed, and the dependence of enzyme inactivation on absolute stereochemistry determined using cytochrome P450 4A4 purified from the lungs of pregnant rabbits. The 12S enantiomer is roughly twice as active (K_I = 1.8 μM, t_1/2 = 0.7 min) as the 12R enantiomer (K_I = 3.6 μM, t_1/2 = 0.8 min), but the chirality of the hydroxyl group is not a major determinant of the specificity for the prostaglandin ω-hydroxylase. The flexibility of the acyclic skeleton of the inhibitor may account for the relatively low enantiomeric discrimination. 2,2-Dimethyl-12-hydroxy-16-heptadecynoic acid, an analogue that cannot undergo β-oxidation, has also been synthesized as a potential in vivo inhibitor of the enzyme and has been shown to inactivate the purified enzyme with K_I = 4.9 μM and t_1/2 = 1.0 min. These acetylenic agents, particularly the dimethyl analog, are promising in vivo inhibitors of cytochrome P450 4A4.