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 recombinant 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, respectively. The concentration of FMO3 was greater than that of FMO5 in all samples, but the ratio of FMO3 to FMO5 varied from 2:1 to 10:1. Human hepatic microsomal samples also showed variable activities for the S-oxidation of methimazole. This activity was associated totally with FMO3; no participation of FMO5 was apparent. This conclusion was supported by several lines of evidence: first, the catalytic efficiency of FMO3 with methimazole was found to be 5000 times greater than that of FMO5; second, the rate of metabolism showed a direct, quantitative 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 higher K m with recombinant FMO3 than did methimazole (2 mM vs. 35 vM). However, a direct relationship between this reaction and FMO3 content in human hepatic microsomal preparations was also apparent. This result shows that even with a high K m substrate, FMO3-catalyzed metabolism can account for the majority of the product formation with some drugs. Our findings demonstrate that the contribution of FMO isoforms to human hepatic drug metabolism can be * Corresponding author.0009-2797/97/$17.00 © 1997 Elsevier Science Ireland Ltd. All rights reserved. PII S 0 0 0 9 -2 7 9 7 ( 9 7 ) 0 0 0 5 5 -0 L. H. O6erby et al. / Chemico-Biological Interactions 106 (1997) 29-45 30 assessed quantitatively on the basis of the characteristics of the enzymes expressed in Escherichia coli

Several full-length clones encoding the human and guinea pig orthologs of flavin-containing monooxygenase 5 (FMO5) have been isolated from libraries constructed with hepatic mRNA. The clones were detected by hybridization with the cDNA encoding FMO5 expressed in rabbit. The human and guinea pig cDNAs encode for proteins of 533 amino acids that contain putative pyrophosphate binding domains characteristic of mammalian FMOs. The sequences derived for the human and guinea pig FMO5 proteins are 87% identical and are 85 and 82% identical, respectively, to the sequence of rabbit FMO5. As is the case with other FMOs, FMO5 in human and guinea pig is encoded by multiple transcripts. Rabbit FMO5 expressed in Escherichia coli was purified and used to elicit antibodies in goat. These antibodies detected FMO5 in samples from livers of adult humans, rabbits, and guinea pigs and fetal livers of humans. The human and guinea pig forms of FMO5 were expressed in E. coli and characterized. Neither enzyme effectively catalyzed the metabolism of methimazole, a general FMO substrate; however, both were active with n-octylamine. The responses of the human FMO5 and guinea pig FMO5 to detergent, ions and elevated temperature are all similar to the responses described for rabbit FMO5. These results indicate that the unique properties of FMO5 from rabbit are species-independent and that this form of the flavin-containing monooxygenase is not readily classified as a drug-metabolizing enzyme.

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