Substrate specificity of the rabbit lung flavin-containing monooxygenase for amines: oxidation products of primary alkylamines

Substrate activity of a flavin-containing monooxygenase isolated from rabbit lung microsomes has been examined with a number of primary, secondary, and tertiary amines. Of the secondary and tertiary amines tested, trifluoperazine, prochlorperazine, N, N-dimethyloctylamine, desmethylperazine, and N-methyloctylamine half-saturate the enzyme at concentrations less than 100 microM. Although the lung enzyme does not exhibit detectable substrate activity with primary arylamines, it catalyzes N-oxygenation of alkylamines to oximes. Studies on the mechanism for the oxidation of n-dodecylamine suggest that the amine is first oxidized to the hydroxylamine which is then further oxidized to the oxime. This interpretation is based on product identification, kinetic studies, and changes in the ratio of hydroxylamine to oxime formed as a function of initial substrate concentration. Kinetic constants calculated for the oxidation of n-dodecylamine and n-dodecylhydroxylamine indicate that the latter saturates the enzyme at a 100-fold lower concentration than that required for the parent amine, and the hydroxylamine is the dominant product only at saturating concentrations of the amine. The ratio of substrate-dependent NADPH and O2 consumption and product formation (hydroxylamine + 2 X oxime) is approximately 1.0:0.9:0.7. Although the reason for the less than stoichiometric yield of products is not known, uncoupling of the enzyme by primary amines does not appear to be a major factor since substrate-dependent increase in H2O2 formation is never more than 3% of substrate-dependent O2 consumption.