title: Flavin-containing monooxygenase activity in human liver microsomes.
abstract: Human liver microsomal flavin-containing monooxygenase activity has been studied using dimethylaniline N-oxidation and thiobenzamide S-oxidation. Except for one subject, the capacity of human liver microsomes to mediate these reactions were markedly increased at pH 8.4 compared to pH 7.4. The mean dimethylaniline N-oxidase activities at pH 7.4 and 8.4 in the four subjects tested were 2.49 +/- 1.13 and 6.59 +/- 4.04 nmol mg-1 min-1, respectively (mean +/- SD, N = 4). The mean thiobenzamide S-oxidase activities at pH 7.4 and 8.4 were 1.39 +/- 0.51 and 2.74 +/- 1.28 nmol mg-1 min-1, respectively. At pH 7.4, an antibody to the human liver NADPH-cytochrome P-450 reductase inhibited dimethylaniline N-oxidation between 4 and 38%. The same antibody had no effect on this reaction at pH 8.4. Except for one subject, a battery of cytochrome P-450 inhibitors also had little effect on this reaction. Further, preincubating human microsomes at 45 degrees C in the absence of NADPH for 4 min destroyed approximately 90% of the dimethylaniline N-oxidase activity. These data collectively suggested that the flavin-containing mono-oxygenase is the major enzyme mediating this reaction in human liver microsomes. In contrast to dimethylaniline N-oxidation, thiobenzamide S-oxidation was significantly inhibited by the anti-reductase at both pH 7.4 and 8.4, respectively. These data indicate that cytochromes P-450 contribute significantly to this reaction in human liver microsomes.
mesh_terms: D000293:Adolescent; D000328:Adult; D004789:Enzyme Activation / Q000187:drug effects; D005260:Female; D006801:Humans; D007120:Immunochemistry; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology*; D008875:Middle Aged; D009251:NADPH-Ferrihemoprotein Reductase / Q000276:immunology / Q000378:metabolism; D010084:Oxidation-Reduction; D010105:Oxygenases / Q000276:immunology / Q000378:metabolism*; D013854:Thioamides / Q000378:metabolism
chemical_list: D013854:Thioamides; C015539:thiobenzamide; D010105:Oxygenases; C020066:dimethylaniline monooxygenase (N-oxide forming); D009251:NADPH-Ferrihemoprotein Reductase