抄録
The flavin-containing monooxygenases (FMOs) are classified into five isozymes (FMO1-FMO5) which are mainly expressed in the liver, the kidney and the lung. However, the function of each isozyme has not been confirmed yet. Thus, to analyze the enzyme functions we isolated cDNA encoding each isozyme and established heterologous expression systems. The cDNA encoding mouse FMO1, rat FMO1, FMO2, FMO3 or FMO5 was cloned. The rat FMO2 cDNA encoded a polypeptide chain which was 100 amino acids shorter than that encoded by macaque, rabbit or guinea-pig FMO2. The mouse and rat FMO1 expressed in yeast showed the catalytic activities toward the major substrates of FMO, while the activity of the other isozymes was not detectable. Human FMO cDNAs were also isolated. The human FMO2 cDNA had an in-frame stop codon which aligns with a Gln codon in the macaque, rabbit and guinea-pig FMO2. Thus, human FMO2 cDNA encoded a truncated polypeptide. The heterologously expressed human FMO2 was catalyticaly inactive. In contrast, when the stop codon changed to the GIn codon in human FMO2 cDNA, the modified enzyme catalyzed the oxidation of substrates. The expressed FMO1, FMO3 and FMO5 metabolized methimazole. Imipramine and n-octylamine were oxidized only by the expressed FMO1 and FMO5, respectively. Furthermore, the N-oxidation of trimethylamine (TMA) by human FMO1, FMO3 and FMO5 expressed in E. coli. was examined. TMA was metabolized only by the human FMO1 and FMO3. The Km values for FMO1 and FMO3 were 4.2 mM and 75.6 μM, respectively. The Vmax/Km value of FMO3 was about 136 times higher than that of FMO1. These results suggest that FMO3 plays a major role in the metabolism of TMA in humans.
These observations suggest that FMO1 and FMO3 have broad spectrum for substrates among FMO isozymes, though FMO show substrate specificity in general.
