Iodosylbenzene derivatives as oxygen donors in cytochrome P-450 catalyzed steroid hydroxylations

Abstract

The mechanism of cytochrome P-450 catalyzed steroid hydroxylations in rat liver microsomes was investigated employing derivatives of iodosylbenzene as O donors. The model steroid substrate androstenedione which was hydroxylated in positions 7.alpha., 6.beta. and 16.alpha. was used in reactions supported by NADPH, iodosylbenzene and iodosylbenzene derivatives. Evidence for cytochrome P-450 involvement in iodosylbenzene-sustained androstenedione hydroxylation included inhibition by substrates and modifiers of cytochrome P-450. The order of O2 donors efficiency was (diacetoxyiodo)-2-nitrobenzene > (diacetoxyiodo)-2-chlorobenzene > 2-nitroiodosylbenzene > (dinitratoiodo)-2-nitrobenzene > (diacetoxyiodo)benzene > (diacetoxyiodo)-2-methylbenzene 4-(diacetoxyiodo)toluene > iodosylbenzene. The capacity of the oxidation agents to serve as O2 donors in cytochrome P-450 dependent steroid hydroxylation is probably dependent on several factors e.g. the tendency of iodosyl compounds to associate, which decreases coordination with the heme Fe, the presence of bulky substituents in the 2 position (decreases association) and the presence of electron-withdrawing substituents (tends to decrease coordination with the heme Fe). The rates of 7.alpha., 6.beta. and 16.alpha. hydroxylation of androstenedione catalyzed by (diacetoxyiodo)-2-nitrobenzene were 108-, 130-, and 167-fold higher, respectively, than the rates of the NADPH-supported reactions. The rate-limiting step in NADPH-sustained cytochrome P-450 catalyzed reactions is probably the rate of reduction of cytochrome P-450.