Multiplicity of liver microsomal flavin-containing monooxygenase in the guinea pig: its purification and characterization

Two distinct forms (FMO-I and FMO-II) of flavin-containing monooxygenase were purified from the liver microsomes of guinea pig. The minimum molecular weights estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis were 54,000 for FMO-I and 56,000 for FMO-II, respectively. Tryptic digestion of these enzymes gave different electrophoretic patterns, suggesting that FMO-I and -II have distinct amino acid sequences. The amino terminal sequence of FMO-II could not be estimated probably due to its blocking while that of FMO-I was determined to be highly homologous to the rabbit liver flavin-containing monooxygenase (J. Ozols, 1989, Biochem. Biophys. Res. Commun. 163, 49-55). Absorption maxima of FMO-I and -II were recorded at 368 and 440 nm and 381 and 456 nm, respectively. Molar ratios of FAD to both of these apoenzymes were shown to be one to one. Substrate specificity of FMO-I and -II was determined using 15 compounds as the substrate. The results showed two enzymes that exhibited overlapped but different specificity toward these substrates although FMO-I had lower activity than did FMO-II with all compounds except thiobenzamide. Of particular interest, only FMO-II showed considerably high activities for primary amines, n-octylamine, and n-decylamine. Immunoglobulin G raised against FMO-II could recognize FMO-I as well as FMO-II, but the reactivity of FMO-I toward the antibody was obviously lower than that of FMO-II. Electrophoresis followed by immunostaining revealed that microsomes of lung, kidney, urinary bladder, testis, and spleen contain the same protein as FMO-II and/or FMO-I. Only lung was shown to have an additional isozyme of FAD-monooxygenase with a molecular weight apparently higher than those of FMO-I and -II. These results strongly suggest that at least two forms of flavin-containing monooxygenases distinct from the lung-type isozyme are expressed in liver of guinea pigs.