MECHANISTIC STUDY OF AMINE OXIDATION CATALYZED BY QUINONOID COFACTORS

A kinetic study was performed on the oxidative deamination of benzylam ine (BzAm) catalysed by pyrroloquinoline quinone (PQQ), a tryptophan t ryptophylquinone analogue 3-methyl-4-(3'-methylindol-2'-yl)indole-6,7- dione; TTQ(a)], and a topa quinone analogue (6-hydroxydopa quinone; TP Q(a)) using high-performance liquid chromatography under aerobic condi tions and cyclic voltammetry under anaerobic conditions. The overall k inetics are second-order in the quinone and BzAm concentrations and in dependent of the electron accepters used (dioxygen under aerobic condi tions and electrode under anaerobic conditions), The second-order rate constants vs, pH profiles are explained in terms of the acid dissocia tion of BzAm and the quinones, yielding pH-independent second-order ra te constants, which are in the order TTQ(a) > pQQ much greater than TP Q(a), Half-wave potentials of steady-state catalytic voltammograms are more positive than the redox potentials of the free quinones, indicat ing that the BzAm-reduced quinones are not quinols but the correspondi ng aminophenols, which are oxidized to iminoquinones. Iminoquinone for ms of pQQ and TTQ(a) exhibit lower catalytic activity compared with th e quinone forms, The reaction rate of TTQ(a) slows down with the time, while those of PQQ and TPQ(a) are practically independent of the reac tion time within a 15 min period. The diminished rate of TTQ(a) is rea sonably ascribed to slow hydrolysis of imino-TTQ(a) to TTQ(a). Generat ion of the iminosemiquinone radical during the catalytic reactions is evidenced by EPR spectroscopic measurements, The significance of radic al generation is discussed in the light of autoxidation of the aminoph enols. All these results are interpreted well by a transamination mech anism. In addition, an asymmetric ortho-quinone structure is proposed to be important in catalytic amine oxidation activity, and the relativ ely low catalytic activity of free TPQ(a) is ascribed to the contribut ion of a para-quinone-type electronic structure. On the basis of these results, a role for Cu-II is proposed as a transient intramolecular e lectron acceptor in copper amine oxidases to enhance the reactivity to ward dioxygen through the generation of the imino-TPQ semiquinone and as a weak ligand in TPQ-containing amine oxidases making the TPQ cofac tor bear an ortho-quinone-type electronic structure.