Ascorbate is the primary reductant of the phenoxyl radical of etoposide in the presence of thiols both in cell homogenates and in model systems.

摘要: Phenoxyl radicals are intermediates in the oxidation of phenolic compounds to quinoid derivatives (quinones, quinone methides), which known act as ultimate mutagenic, carcinogenic, and cytotoxic agents by directly interacting with macromolecular targets or generating toxic reactive oxygen species. One-electron reduction phenoxyl may reverse oxidative activation quinoids, thus preventing their effects. In present work, we studied interactions ascorbate, thiols (glutathione, dihydrolipoic acid, metallothioneins), combinations thereof radical generated tyrosinase-catalyzed VP-16 [etoposide, 4'-demethylepipodophyllotoxin-9-(4,6-O-ethylidene-beta-D-glucop yra noside)], a hindered phenol widely used an antitumor drug. We found liquid chromatography-ionspray mass spectrometry electron spin resonance (ESR) that tyrosinase caused its o-quinone aromatized derivative via intermediate formation radical. Both ascorbate (GSH, metallothioneins) were able reduce prevent oxidation. The characteristic ESR signal was quenched reductants. semidehydroascorbyl detected presence ascorbate; did not produce signals spectra. combinations, plus GSH metallothionein acted independently additively reducing Ascorbate more reactive: VP-16-dependent commenced only after complete ascorbate. preceded quenching metallothionein. also toward than but concentration maintained at expense regeneration from dehydroascorbate acid. spectra, continuously then abruptly substituted signal. When incubated retina hepatocyte homogenates, two-phase lag period observed for appearance signal: ascorbate-dependent part (semidehydroascorbyl observable, sensitive oxidase) thiol-dependent (no mersalyl acid). About 50% could be accounted endogenous (as revealed treatment peroxidase+cumene hydroperoxide).(ABSTRACT TRUNCATED AT 400 WORDS)