Comparison of aniline hydroxylation by hemoglobin and microsomal cytochrome P450 using stable isotopes.

Hemoglobin (Hb) and cytochrome P450 carry out aromatic ring hydroxylation of aniline. In the presence of reductants and Hb, para- and ortho-aminophenol were formed. Under [18O]O2, 100% of product was labeled; no incorporation occurred with [18O]H2O. Deuterium (1.9%) was detectable in p-aminophenol formed from p-[2H]aniline by Hb, as compared with 6% retention observed with cytochrome P450. These observations are consistent with a mechanism for Hb-dependent reaction involving formation of an iron-oxo complex competent to hydroxylate substrate. Hb-mediated reactions may represent a source of extrahepatic metabolism since Hb is a major carrier for small organic molecules. The similarities of P450- and Hb-mediated aniline hydroxylation using stable isotopes preclude their use as in vivo probes.