A rapid spectrophotometric assay of mono-amine oxidase based on the rate of disappearance of kynuramine.

Studies in this laboratory (l-3) have dealt with the metabolism of amines derived from tryptophan, such as tryptamine and serotonin. Tryptophan may give rise to additional amines through decarboxylation of kynurenine and 3-hydroxykynurenine. Makino and Arai (4) were the first to suggest this pathway and later showed (5) that m&se liver homogenates convert kynuramine, the decarboxylation product of kynurenine, to 4-hydroxyquinoline which was identified by its paper chromatographic and spectral properties. The mechanism of this conversion has not been studied in further detail. Kynuramine (1) has now been found to be rapidly oxidized by monoamine oxidase (MAO) but not by diamine oxidase (Scheme 1). The former enzyme conceivably could oxidatively deaminate kynuramine to an aldehyde II which would either condense to 4-hydroxyquinoline (4(1H)-quinolone, III) or undergo further oxidation to an acid IV or the lactam, 2,4dihydroxyquinoline (formulated as 4-hydroxy-2(lH)-quinolone, V). Intramolecular (nonenzymatic) condensation of the aminoaldehyde proved to be faster than the further oxidation of the aldehyde to the acid or lactam. The overall reaction could be conveniently followed in situ in a self-recording spectrophotometer and has been developed into a rapid method for the assay of monoamine oxidase. The details of this procedure are presented in the present report.