Lh. Overby et al., QUANTITATION AND KINETIC-PROPERTIES OF HEPATIC-MICROSOMAL AND RECOMBINANT FLAVIN-CONTAINING MONOOXYGENASE-3 AND MONOOXYGENASE-5 FROM HUMANS, Chemico-biological interactions, 106(1), 1997, pp. 29-45
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.