Thiol oxidation coupled to DT-diaphorase-catalysed reduction of diaziquone. Reductive and oxidative pathways of diaziquone semiquinone modulated by glutathione and superoxide dismutase.

Abstract

DT-diaphorase [NAD(P)H:quinone oxidoreductase; EC 1.6.99.2] catalysed the two-electron reduction of the anti-tumour quinone 2,5-bis-(1-aziridinyl)-3,6-bis(ethoxycarbonylamino)-1,4-benzoquino ne (AZQ) to the hydroquinone form (AZQH2). Although DT-diaphorase catalysis of AZQ was not significantly affected by pH, the hydroquinone product was effectively stabilized by protonation at pH values below 7, whereas, above that pH, hyroquinone autoxidation, evaluated in terms of H2O2 production, increased exponentially. The autoxidation of AZQH2 entailed the formation of diverse radicals, such as O2-.,HO., and the semiquinone form of AZQ (AZQ-.), which contributed to different extents to the e.p.r. spectrum. Superoxide dismutase enhanced the autoxidation of AZQH2 and suppressed the e.p.r. signal ascribed to AZQ-., in agreement with a displacement of the equilibrium of the semiquinone autoxidation reaction (AZQ-.+O2 in equilibrium with AZQ+O2-.) upon enzymic withdrawal of O2-.. GSH increased the steady-state concentration of AZQH2 formed during DT-diaphorase catalysis and inhibited temporarily its autoxidation. This effect was accompanied by oxidation of the thiol to the disulphide within a process involving glutathionyl radical (GS.) formation, the relative contribution of which to the e.p.r. spectrum was enhanced by increasing GSH concentrations. GS. formation in this experimental model can be rationalized as originating from the reaction of GSH with AZQ-., rather than with O2-. or HO., for thiol oxidation was not affected significantly by superoxide dismutase, and GS. formation was insensitive to catalase. In addition, GSH suppressed the e.p.r. signal attributed to AZQ-.. No glutathionyl-quinone conjugate was detected during the DT-diaphorase-catalysed reduction of AZQ; although the chemical requirements for alkylation were partly fulfilled (quinone ring aromatization and acid-assisted aziridinyl ring opening), the negligible dissociation of GSH (GS(-)+H+ in equilibrium with GSH) at low pH prevented any nucleophilic addition to occur. Therefore the redox transitions of AZQ during DT-diaphorase catalysis seemed to be centred on the semiquinone species, the fate of which was inversely affected by catalytic amounts of superoxide dismutase and large amounts of GSH: the former enhanced AZQ-. autoxidation and the latter favoured AZQ-. reduction. Accordingly, superoxide dismutase and GSH suppressed the semiquinone e.p.r. signal. These results are discussed in terms of three interdependent redox transitions (comprising one-electron transfer reactions involving the quinone, oxygen and the thiol) and the thermodynamic and kinetic properties of the reactions involved.