Specificity of Amine Oxidase for Optically Active Substrates and Inhibitors

Abstract

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.

The Inhibition of Amine Oxidase and the Central Stimulating Action of the Stereoisomeric Amphetamines and 1‐phenylethylamines

“…Pratesi and Blaschko (1959), using rabbit liver as source of amine oxidase, found that the two stereoisomers of amphetamine were equally active as inhibitors. It is interesting that dexamphetamine, which in our experiments on rat liver proved to be as strong an inhibitor as (+ )-l-phenylethylamine, appears to be more active on amine oxidase from guinea-pig liver than on that from rabbit liver (Pratesi and Blaschko, 1959). Such findings emphasize the dependence of stereospecificity on animal species.…”

“…Mann and Quastel (1940) suggested that central stimulation by amphetamine might be due to inhibition of amine oxidase. However, although in rabbit and man (+)-amphetamine (dexamphetamine) is a stronger stimulant than the (-) form, Pratesi and Blaschko (1959) have shown that the two stereoisomers are equally active as inhibitors of rabbit liver enzyme.To test the idea that amphetamine owes its analeptic properties to its inhibitory action on amine oxidase, it was clearly necessary to use amine oxidase from the same species as that used for the pharmacological experiments. It was also desirable to use amine oxidase from the brain in preference to liver, because the patterns of substrate and inhibitor specificities might vary in different organs.…”

“…Mann and Quastel (1940) suggested that central stimulation by amphetamine might be due to inhibition of amine oxidase. However, although in rabbit and man (+)-amphetamine (dexamphetamine) is a stronger stimulant than the (-) form, Pratesi and Blaschko (1959) have shown that the two stereoisomers are equally active as inhibitors of rabbit liver enzyme.…”

“…There was no correlation between these two effects; thus it is unlikely that the central stimulating action of amphetamine is due to inhibition of amine oxidase.It is known that species differences exist in the pattern of substrate and inhibitor specificities Pratesi and Blaschko (1959) showed that, for a substrate as well as for an inhibitor, the presence of a centre of asymmetry in the molecule may confer different affinities (or rates of oxidation) on the. two enantiomorphs, and also that this depends on the source of the enzyme.…”

“…We studied 1-phenylethylamine because Pratesi and Blaschko (1959) found it to be a stronger inhibitor of the rabbit and cat liver enzyme than amphetamine; moreover 1-phenylethylamine had not yet been studied as a central stimulant. We have also studied the activity of dexamphetamine and (-)-amphetamine on amine oxidase from rabbit brain.…”