Human Liver Mitochondrial Monoamine Oxidase

Kinetic data have been obtained concerning model interactions of partially purified monoamine oxidase preparations prepared from human liver mitochondria with the use of Triton X-100 or sonic treatment. The effect of pH upon apparent Michaelis and inhibitor constants of benzylamines and n-alkylamines indicates that the nonprotonated species of these substrates are bound by the enzyme. Ammonia competitively inhibits the enzyme. Primary aliphatic alcohols, which are electronic analogues of nonprotonated monoamines, are also competitive inhibitors. Alcohol inhibitor constants do not vary with pH. The inhibitory effects of mixtures of ammonia and benzyl alcohol suggest that hydroxy and amino groups of interactants are bound at the same enzyme site. The affinities of both the n-alkylamine substrates and the l-hydroxy-n-alkane inhibitors for the enzyme depend upon the number of methylene residues in these interactants. The effects of the chain lengths of the n-alkylamines and the alcohol analogues upon enzyme affinities are remarkably similar. In the case of both types of l-substituted n-alkanes with less than 6 methylene residues, the contribution of each methylene residue to the change in standard free energy that determines the alkane-enzyme complex at 25° is constant and amounts to - 1.3 kcal per mole. We interpret these data as evidence that the enzyme active center contains both electrophilic and hydrophobic binding sites. Substrate inhibitions observed with high concentrations of benzylamine and n-butylamine are consistent with this hypothesis. The kinetic data obtained with Triton X-100 preparations agree with data obtained with sonically treated preparations. We were unable to detect discrepancies in the kinetic data of either preparation, which might have suggested the presence of multiple monoamine oxidase activities. On the other hand, the mitochondrial activity differed in several respects from that of the monoamine oxidase of human plasma.