The liver microsomal FAD-containing monooxygenase. Spectral characterization and kinetic studies.

A FAD-containing monooxygenase isolated from pig liver microsomes migrates as a single band upon electrophoresis in polyacrylamide gels in the presence of dodecyl sulfate. The minimum molecular weight based on mass of amino acids per mole of flavin is 64,000. However, the catalytically active enzyme exists as aggregating units of the monomer. Neither oxygen nor organic substrates perturbed the spectrum of the oxidized flavoprotein and their binding to this form of the enzyme could not be detected. Anaerobically NADPH bleaches the flavoprotein, and in the presence of both NADPH and oxygen a remarkably stable intermediate form of the enzyme, with an absorption band at 375 nm, is observed. The spectrum of the intermediate resembles that of a peroxyflavin. The monooxygenase catalyzes NADPH- and oxygen-dependent oxygenations of nucleophilic nitrogen- or sulfur-containing compounds. Kinetic studies carried out with a model organic nitrogen substrate (trimethylamine) and a sulfur substrate (methimazole) gave similar patterns. The kinetic data are consistent with an ordered Ter-Bi mechanism with an irreversible step between the second and third substrate where NADPH is added first, followed by oxygen, and the oxidizable organic substrate is added last. If NADPH is the first substrate added, then NADP+ must be the last product released since NADP+ is competitive with NADPH.