Some parameters affecting the activity of monoamine oxidase in purified bovine brain mitochondria

Gabay, S.; Achee, F.M.; Mentes, G.

Journal of Neurochemistry 27(2): 415-424


ISSN/ISBN: 0022-3042
PMID: 9476
DOI: 10.1111/j.1471-4159.1976.tb12263.x
Accession: 068517859

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Some parameters affecting the activity of monoamine oxidase (MAO) [EC] in purified beef brain mitochondria were investigated, and diversities in enzyme properties were found as a function of substrate. The deamination of the biogenic amines serotonin, dopamine, tyramine, tryptamine, phenylethylamine, and 2 non-physiological amines kynuramine and m-iodobenzylamine, was studied. Anions in high concentrations inhibited enzyme activity with kynuramine being most affected. Among the biogenic amines, the activity with the indolalkylamines showed greater sensitivity to monovalent anions such as Cl- than to polyvalent ions such as PO43- whereas the opposite was true with the phenylalkylamines. Pyrophosphate ion had little or no effect on MAO activity, regardless of substrate. The inhibition of kynuramine and serotonin deamination was non-competitive but mixed competitive inhibition was found with tyramine and phenylethylamine. The activity of MAO was markedly affected by pH, and it was previously reported that the substrates showed different pH optima in their oxidation. The effect of pH on activity was attributed in part to changes in the ionization of the substrate and the hypothesis that the true substrate is the non-protonated amine. This was reflected in kinetic studies showing high substrate inhibition with increased pH. It was calculated that phenylethylamine would have the highest percentage of un-ionized amine at pH 8.2 and 9.1. At these pHs, there was more pronounced inhibition with high substrate concentrations of phenylethylamine than with the other substrates. There was little inhibition with high substrate concentrations of tyramine which was the most ionizable of the substrates tested. When Km values obtained at pH 7.4, 8.2 and 9.1 were corrected for ionization of the substrate, the corrected Km was lowest at pH 7.4 for all substrates. Less than 50% of MAO activity was lost when beef brain mitochondria were heated at C for 20 min. There was only a slight variation with substrate in the thermal inactivation experiments. The mitochondrial membrane environment surrounding the enzyme imposes certain restrictions on the enzymatic activity with respect to the different substrates which, in turn, are also affected by such parameters as pH and ions. The relationship of these factors to the question of enzyme multiplicity was discussed.