Generation of alloxan radical in rat islet cells: participation of NADPH: cytochrome P-450 reductase.

Abstract

Cytotoxic actions of alloxan are thought to be caused by the hydroxyl radical (HO+) generated in a cyclic reaction involving alloxan and its reduction product, alloxan radical (HA+). The generation of HA+ and the oxidation of reduced nicotinamide adenine dinucleotide phosphate (NADPH) were observed in the reaction system of alloxan with NADPH in the presence of purified NADPH: cytochrome P-450 reductase [EC 1.6.2.4] (fp2) from rat liver microsomes. Both the generation of HA+ and the oxidation of NADPH were increased with increasing the concentrations of alloxan or fp2. These results indicate that NADPH-linked HA+ generation was catalyzed by fp2. The HA+ was generated in the reaction of alloxan with a homogenate of rat islet cells. The generation of HA+ in the reaction of alloxan with the homogenate of rat islet cells was markedly diminished by an immunogloblin fraction (Ig) of anti-fp2 serum, but not by control-Ig. The anti-fp2-Ig also inhibited the generation of HA+ in the reaction system of alloxan with NADPH in the presence of fp2. When the homogenate of islet cells from phenobarbital-treated rats was used, the generation of HA+ was significantly increased in comparison with that from the control rats. However, the homogenate of islet cells from 3-methylcholanthrene-treated rats did not have such a stimulative effect. These results suggest that the reduction of alloxan to HA+ was catalyzed by fp2 presented in islet cells.