Some properties of soluble monoamine oxidase preparations from guinea-pig liver

Monoamine oxidase (MAO) (Monoamine: O-oxidoreductase (doaminating) E.C.1.4.3.4) was partially purified from soluble extracts of guinea-pig liver, which contain about 10 per cent of the activity present in whole honiogenatos. Higher yields of enzyme were recovered in soluble extracts derived from sonicated homogcnates.The substrate inhibitor specificity of the soluble enzyme was very similar to that for crude particulate fractions or mitochondrial fractions when measured by manoraetric assay with molecular oxygen as final electron acceptor. The behaviour of the enzyme in other oxido-redactase systems, namely in the reduction of ietrazolium salts and NAD,were studied extensively. Marked differences in substrate specificity were found for the soluble enzyme depending on the nature of the final electron acceptor and on the type of preparation used as source of enzyme. The soluble enzyme showed a marked dependence on added NAD for teira-Eolium reduction, the extent of this effect varying according to the substrate tested. The MAD dependence was correlated with the ability of various amine substrates to stimulate the enzymic reduction of NAD in the absence of tetra-solium salts. NAD had no effect on the enzyme when assayed manometrically. However, soluble enzyme derived from sonicated homogenate showed marked alterations in its substrate specificity in the NAD-reduetase assay. In addition, the effect of added NAD in the tetrazolimu-reductase assay was negligible compared to that seen in unsonicated preparations. Various data led to the conclusion that the NAB-dependent step(s) in the tetrazoliura-reductase system of soluble enzyme preporations was probably inactivated by sonication, although the total amine-tetrazolium reductase activity was in fact increased by sonication.These. findings led to the formulation of possible pathways involved in the enzymic reduction of NAD and tetrazolium by monoamines. These are discussed in the light of current concepts about the chemical constitution of the enzyme.Various attempts were made to detect isoenzymes of monoamine-oxidase by gel electrophoresis. In the best preparations, only one mobile (anodic) form of the enzyme was demonstrated after polyarylamide gel electrophoresis of soluble extracts from guinea-pig liver.