QUANTITATION AND KINETIC-PROPERTIES OF HEPATIC-MICROSOMAL AND RECOMBINANT FLAVIN-CONTAINING MONOOXYGENASE-3 AND MONOOXYGENASE-5 FROM HUMANS

Variable amounts of flavin-containing monooxygenase isoforms 3 and 5 ( FMO3 and FMO5) are present in microsomal preparations from adult, male , human liver. Quantitation with monospecific antibodies and recombina nt isoforms as standards showed levels of FMO3 and of FMO5 that ranged from 12.5 to 117 and 3.5 to 34 pmol/mg microsomal protein, respective ly. The concentration of FMO3 was greater than that of FMO5 in all sam ples, but the ratio of FMO3 to FMO5 varied from 2:1 to 10:1. Human hep atic microsomal samples also showed variable activities for the S-oxid ation of methimazole. This activity was associated totally with FMO3; no participation of FMO5 was apparent. This conclusion was supported b y several lines of evidence: first, the catalytic efficiency of FMO3 w ith methimazole was found to be similar to 5000 times greater than tha t of FMO5; second, the rate of metabolism showed a direct, quantitativ e relationship with FMO3 content; third, the plot of the relationship between metabolism and FMO3 content extrapolated close to the origin. A second reaction, the N-oxidation of ranitidine, exhibited a much hig her K-m with recombinant FMO3 than did methimazole (2 mM vs. 35 mu M). However, a direct relationship between this reaction and FMO3 content in human hepatic microsomal preparations was also apparent. This resu lt shows that even with a high K-m substrate, FMO3-catalyzed metabolis m can account for the majority of the product formation with some drug s. Our findings demonstrate that the contribution of FMO isoforms to h uman hepatic drug metabolism can be assessed quantitatively on the bas is of the characteristics of the enzymes expressed in Escherichia coli . (C) 1997 Elsevier Science Ireland Ltd.