Abstract
An otherwise noncytostatic flux of H 2O 2 from glucose and glucose oxidase became cytostatic to cultured Chinese Hamster Ovary (CHO) cells when horseradish or thyroid peroxidase was added to the culture medium. Electron spin resonance (ESR) measurements showed that one or more factors present in the culture medium promote the one-electron oxidation of a reduced nitroxide or glutathione in an H 2O 2/peroxidase-dependent process. Moreover, a reduced nitroxide conferred significant protection against the cytostatic effect of H 2O 2/peroxidase. Cytostatic effects were not only seen in the presence of the active H 2O 2/peroxidase system, but also in media which had been preexposed to H 2O 2/peroxidase but no longer contained an active H 2O 2 generating system. It is suggested that peroxidases oxidize one or more factors in tissue culture media to free radicals, which react with nearby components of cells or form toxic products, causing growth inhibition. If similar free radical precursors are present in tissue fluids, some of the toxicity of H 2O 2 in vivo may be due to peroxidase-mediated endogenous free radical generation.
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