1. The yeast Candida boidinii was grown on glucose as carbon source with a range of amines and amino acids as nitrogen sources. Cells grown on amines contained elevated activities of catalase. If the amines contained N-methyl groups, formaldehyde dehydrogenase, formate dehydrogenase and S-formylglutathione hydrolase were also elevated in activity compared with cells grown on (NH4)2SO4. 2. Cells grown on all the amines tested, but not those grown on urea or amino acids, contained an oxidase attacking primary amines, which is referred to as methylamine oxidase. In addition, cells grown on some amines contained a second amine oxidase, which is referred to as benzylamine oxidase. 3. Both amine oxidases were purified to near homogeneity. 4. Benzylamine oxidase was considerably more stable at 45 and 50°C than was methylamine oxidase. 5. Both enzymes had a pH optimum in the region of 7.0, and had a considerable number of substrates in common. There were, however, significant differences in the substrate specificity of the two enzymes. The ratio V/Kapp.m increased with increasing n-alkyl carbon chain length for benzylamine oxidase, but decreased for methylamine oxidase. 6. Both enzymes showed similar sensitivity to carbonyl-group reagents, copper-chelating agents and other typical ‘diamine oxidase inhibitors’. 7. The stoicheiometry for the reaction catalysed by each enzyme was established. 8. The kinetics of methylamine oxidase were examined by varying the methylamine and oxygen concentrations in turn. A non-Ping Pong kinetic pattern with intersecting double-reciprocal plots was obtained, giving Km values of 10mum for O2 and 198mum for methylamine. The significance of this unusual kinetic behaviour is discussed. Similar experiments were not possible with the benzylamine oxidase, because it seemed to have an even lower Km for O2. 9. Both enzymes had similar subunit Mr values of about 80000, but the benzylamine oxidase behaved as if it were usually a dimer, Mr 136000, which under certain conditions aggregated to a tetramer, Mr 288000. Methylamine oxidase was mainly in the form of an octamer, Mr 510000, which gave rise quite readily to dimers of Mr 150000, and on gel filtration behaved as if the Mr was 286000.

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