A number of optically active amines have been tested as substrates or inhibitors of amine oxidase of rabbit and guinea-pig liver. The two stereoisomers of fl-hydroxyphenethylamine were oxidized at the same rate by rabbit liver, but the guinea-pig liver extracts oxidized the D form more rapidly than the L form. The two stereoisomers of amphetamine were equally active as inhibitors of the rabbit liver oxidase, but with guinea-pig liver extracts dexamphetamine, the (+) form, was more potent as an inhibitor. In both species, 2-hydroxy-l-phenylethylamine was a weaker inhibitor than I-phenylethylamine; in the rabbit liver (+) forms of these two amines were more potent as inhibitors.The enzyme amine oxidase, which takes part in the biological inactivation of 5-hydroxytryptamine and many sympathomimetic amines, acts on compounds in which an amino group is attached to a terminal carbon atom. Compounds like amphetamine (a-methylphenethylamine) (I) and its derivatives, substances in which the amino group is not attached to a terminal carbon atom, are not oxidized; they act as competitive inhibitors of the enzyme (see Blaschko, 1952). Mann and Quastel (1940) suggested that thQ central stimulating action of amphetamine might be due to its activity as an inhibitor of amine oxidase. This suggestion found some support in the observation that substances like amphetamine, in which the carbon atom adjacent to the benzene ring was not substituted, were stronger central stimulants, and at the same time stronger inhibitors of amine oxidase, than compounds like ephedrine, which carry a hydroxyl group in this position (Blaschko, 1940).In the present paper the substrate and inhibitor specificities of a number of amines for amine oxidase have been studied. Attention was given to the question whether the steric configuration of amines with an asymmetric carbon atom had an influence upon their affinity for the enzyme. The substrates used were phenethylamine (II), tyramine (p-hydroxyphenethylamine) (III) and f-hydroxyphenethylamine (IV); of the last compound the D and L forms were available; the steric configuration of these two forms was determined *Present address: Istituto di Chimica Farmaceutica.dell'UniversitA di Pavia, Italy. some time ago (Pratesi and Grassi, 1953). A few experiments with both 8m-and 8p-dihydroxy- N-methylphenethylamine (V and VI respectively) are also reported. Phenylephrine, the (-) form of compound V, and the (+) form were available; we also used oxedrine (Sympatol), the (±) form of compound VI, as well as the (+) form; the latter compound is now known to have the L configuration (Pratesi, La Manna, Campiglio and Ghislandi, 1958). The inhibitors tested were: amphetamine (1), 1-phenylethylamine (VII) and 2-hydroxy-l-phenylethylamine (VIII). Of all three compounds the two enantiomorphs were available.
