COFACTOR-DEPENDENT AMINE OXIDATIONS

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Mechanistic diversity in the biological oxidation of amines is a subject of much current interest. The major metabolic transformation undergone by all aliphatic amines is N-dealkylation, though this reaction is mediated by different enzymes, depending on structure and degree of N-substitution. Both cytochrome P-450 and the flavin-dependent mitochondrial monoamine oxidase (MAO) are thought to oxidize primary/secondary/tertiary amines via mechanisms which are initiated by single-electron transfer, whereas the copper amine oxidases oxidize only primary amines by way of a quinone-cofactor-mediated transamination reaction. The aim of the proposed work is to clarify, through a combination of enzymologic and model studies, a number of important questions concerning the proposed mechanisms used by these enzymes. In certain cases, understanding of the mechanism is used to guide the design and synthesis of selective inhibitors of physiologically important enzymes being studied by collaborators. Project I involves an investigation of the substrate-structure requirements of aliphatic amines for horseradish peroxidase (HRP) and lactoperoxidase, in an effort to confirm whether the mechanism involves electron-transfer oxidation. The inactivation of HRP by trans-2-phenylcyclopropylamine will also be studied. Project II addresses the mechanism of amine oxidation by the mitochondrial flavin-dependent monoamine oxidase (MAO), first through establishing a thermal (as opposed to photochemical) model reaction between amines and "high potential" flavin analogs, and subsequently elucidating the mechanism thereof. The working hypothesis is that the mechanism actually involves addition-elimination or possibly direct hydrogen atom/anion transfer rather than initial one-electron oxidation at nitrogen. New probes which might distinguish the actual mechanism will be investigated. Project III focuses on the mechanism of primary amine "transamination" mediated by the 2,4,5-trihydroxyphenylalanine (TOPA) quinone cofactor of the mammalian copper amine oxidases, including a study of the fate of known enzyme inactivators (cyclopropylamine, beta-aminopropionitrile (BAPN), 1,2-diamines, and (aminomethyl)trimethylsilane) in model quinone reaction systems. New inhibitors for lysyl oxidase and diamine oxidase will be synthesized and studied. Related to the inhibitor BAPN, is believed to be the inhibition of pyridoxal enzymes by beta-cyanoalanine, a hypothesis which will be tested in model studies.