Synthesis and Characterization of Models for the 2,4,5-Trihydroxyphenylalanine (Topa)-Derived Cofactor of Mammalian Copper Amine Oxidases, and Initial Amine Reactivity Studies

The mammalian copper amine oxidases effect the oxidative deamination of primary amines through utilization of an ''active carbonyl'' cofactor, shown recently to be the quinone form (TPQ) of a protein-based 2,4,5-trihydroxyphenylalanine (TOPA) residue. We synthesized three models for the cofactor in both reduced (benzenetriol) and oxidized (hydroxyquinone) forms, which differ in the nature of the alkyl substituent mimicking the connection to the protein backbone: hydantoinylmethyl, phthalimidoethyl, and pivalamidoethyl. The quinone forms were capable of deaminating benzylamine in aqueous CH3CN both stoichiometrically and catalytically (in the presence of O-2), but incapable of deaminating non-benzylic amines. In order to clarify the various reactions potentially occurring during aerobic autorecycling deamination, we studied the pH-dependent benzenetriol --> hydroxyquinone autoxidation as well as the possible reaction of amines with the benzenetriol forms. The latter undergo stoichiometric substitution with amines to give (alkylamino)resorcinols, not via cyclohexadienone tautomerization previously proposed, but via a redox cycling mechanism involving condensation of the amines with traces of hydroxyquinone present in the benzenetriol preparations. This observed substitution regiochemistry, as well as structural characterization of the hydroxyquinone arylhydrazine derivatives, confirms that amines react exclusively at the electrophilic C5 carbonyl position of TPQ models. Both the hydroxyquinone and benzenetriol forms were found to react with ethylenediamine in the presence of O-2 to give 6-hydroxy-7-(2-pivalamidoethyl)quinoxaline, consistent with the postulated generation of such moiety when lysyl oxidase is inactivated by ethylenediamine.