Monoamine oxidase in bovine thyroid tissue.

Two functional forms of monoamine oxidase (MAO) defined as MAO A and MAO B have been described in several tissues. In this study the characteristics of MAO present in a particulate subcellular fraction of bovine thyroid tissue were investigated. The selective inhibitors of MAO, clorgyline (A form) and deprenyl (B form) were used. The monoamines 5-hydroxytryptamine (5-HT) (preferred of the A form), tyramine (both forms) and beta-phenylethylamine (PEA) (B Form) were used as substrates. MAO activity towards 5-HT was markedly inhibited by clorgyline. Tyramine oxidation was very sensitive to clorgyline and the curve obtained was in accordance with the presence of a high proportion of the A form. MAO activity towards PEA was also markedly inhibited by clorgyline. Deprenyl was not able to induce modifications in the MAO activity except when it was used at very high concentrations. According to these results the bovine thyroid tissue contains predominantly the A form of MAO. In this tissue PEA was deaminated by the A form of the enzyme.