Substrate‐ and stereoselective inhibition of human brain monoamine oxidase by 4‐dimethylamino‐α, 2‐dimethylphenethylamine (FLA 336)

Monoamine oxidase (MAO, monoamine 0,: oxidoreductase, EC 1.4.3.4) is thought to exist as two catalytically active forms, termed MAO-A and MAO-B, where the -A form of the enzyme is inhibited by low concentrations of the acetylenic inhibitor clorgyline, and the -B form inhibited by low concentrations of deprenyl (Johnston 1968; Knoll & Magyar 1972). According to this definition, in the human brain, 5hydroxytryptamine (5-HI") is metabolized by MAO-A alone, whereas benzylamine is metabolized by MAO-B alone (Tipton et al 1973; White & Glassman 1977). In both human and rat brain, the MAO-A is thought to be localized predominantly, but not exclusively, in the neuronal tissue, whereas the -B form of the enzyme is predominantly non-neuronal in nature (Student & Edwards 1977; Oreland et al 1980; Fowler et al 1980b). Monoamine oxidase inhibitors have long been used for the treatment of certain types of depression which are not responsive to treatment by the tricyclic inhibitors of noradrenaline uptake (for discussion, see e.g. Pare 1979). One of the drawbacks of treatment with classical M A 0 inhibitors is the so-called 'cheese effect'-a pressor response to the tyramine found in various foodstuffs (Blackwell & Marley 1966). The MAO-B selective inhibitor deprenyl has been reported not to have this effect (Knoll 1976; Elsworth et al 1978), but this compound does not appear to be useful in the treatment of depression (Mendis et al 1981). The phenethylamine analogue FLA 336 (4-dimethylamino-a, 2dimethylphenethylamine), an MAO-A selective inhibitor in the rat, also appears to be without an appreciable 'cheese' reaction (Ask, Hogberg, Schmidt, Kiessling, Ross, text in preparation). In the present study, the effect of the optical isomers of this compound upon the activity of human brain MAO-A and -B has been studied. In addition, the principal metabolite of FLA 336(+) has been investigated. Brains were taken at autopsy from 7 individuals (age range 23-79 years), without histories of neurologic, psychiatric, or liver disease. The brain regions studied (hypothalamus, medulla oblongata, thalamus, nucleus accumbens, cortex gyrus cinguli and frontal cortex) were dissected from the brains as described by Adolfsson ct al (1979), put into air-tight packages, and frozen at -70 "C. The frozen brain regions were pulverized in a mortar filled with liquid nitrogen. The tissues were not allowed to thaw during this procedure. The frozen powders were placed in tubes, weighed, and kept frozen. Before use, the powders were resuspended in 'sucrose buffer' (0.25 M sucrose, I0 mM potassium phosphate, pH 7.8) to a protein concentration of 1.5 mg ml-l. This homogenizing procedure, which allows the rapid storage of autopsy material, has been shown not to affect the substrate-specificity or kinetic properties of either MAO-A or -B (Fowler et al 1980b). Monoamine oxidase activity was assayed radiochemically by the method of Callingham & Laverty (1973), as modified by Fowler et al (1979). with [14C]5-HT and [14C]benzylamine as substrates for MAO-A and -B, respectively. All assays were carried out under an atmosphere of air, and, unless otherwise shown, at a substrate concentration of 50 p ~ . Specific activities of MAO, corrected for the efficiencies of extraction of the deaminated metabolites into the organic layer of the assay medium (see Fowler et al 1980a). are expressed as nmol (of substrate metabolized) (mg protein)-' min-l. Protein concentrations were determined by the method of Markwell et a1 (1978) because this method is not affected by high concentrations of sucrose. Human serum albumin was used as the protein standard. The radioactive substrates for MAO, 5-hydroxytryptamine-[side chain-2-14C]binoxalate and benzylamine [methylene-14C]hydrochloride were obtained from New England Nuclear (Boston, Mass., U.S.A.) and ICN Pharmaceuticals Inc. (Irvine, CA, U.S.A.) respectively. (+)and (-)-FLA 336 and (+)+-monomethylamino-a.2-dimethylphenethylamine (FLA 788 (+)), the principal metabolite of FLA 336 (+), were