Factors affecting the intracellular generation of free radicals from quinones.

Isolated hepatocytes do not liberate appreciable amounts of superoxide into the external medium. Simple quinones stimulate the release of superoxide up to 15 nmol/min/10(6) hepatocytes. Superoxide release stimulated by a variety of simple quinones and more complex antitumor quinones was maximal at a quinone one-electron reduction potential of -70 mV. This was qualitatively similar to the pattern of superoxide formation seen with NADH-cytochrome b5 reductase and NADH: ubiquinone oxidoreductase. Superoxide production by NADPH-cytochrome P-450 reductase was maximal at a quinone single-electron reduction potential at -200 mV. Phenobarbital pretreatment had no effect on superoxide formation by hepatocytes suggesting that NADPH-cytochrome P-450 reductase activity is not rate limiting for quinone stimulated superoxide formation. Sulfonated stilbenes, specific inhibitors of anion exchange, had no effect on the release of superoxide by hepatocytes suggesting that superoxide is not transported through anion channels in the plasma membrane. Pretreatment of hepatocytes with 10(-5) M diethyldithiocarbamate produced over a two fold increase in the release of superoxide.