Microsomal hydroxylation of specifically deuterated monosubstituted benzenes. Evidence for direct aromatic hydroxylation.

The aromatic hydroxylation of six pairs of selectively deuterated monosubstituted benzenes was investigated with rat liver microsomes of various induction states. The substrates studied included 3,5-D2C6H3X (1a-6a) and 2,4,6-D3C6H2X (1b-6b), where X = Br, CN, NO2, OCH3, CH3, or Ph, respectively. The deuterium content of the ortho, meta, and para hydroxylated metabolites, as well as side chain oxidation products from 4 and 5, was determined by capillary gas chromatography-mass spectroscopy. These data were analyzed according to a hypothetical model in which a molecule of substrate can undergo either direct aromatic hydroxylation (defined as obligatory and complete loss of deuterium from the site of hydroxylation) or indirect aromatic hydroxylation (defined as the obligatory and complete shift of deuterium to an adjacent position, followed by its partial loss as governed by a kinetic deuterium isotope effect). From this and other analyses of the data the following conclusions were reached. (1) The relative extent of meta hydroxylation increased and the total yield of metabolites decreased as the substituents X became more electron withdrawing. (2) The induction state of the microsomes altered the regioselectivity of hydroxylation (2, 3, 4, or side chain) noticeably and predictably but had little or no effect on the retention or loss of deuterium during each hydroxylation. (3) With each substrate and at each ring position hydroxylation was found to occur by a combination of direct and indirect mechanisms. (4) The relative importance of direct vs. indirect mechanisms did not vary in a simple manner with either the position of hydroxylation or the nature of the substituent X.(ABSTRACT TRUNCATED AT 250 WORDS)