Mechanisms for Metabolism of Ethanol to 1-Hydroxyethyl Radicals in Rat Liver Microsomes

Abstract

Experiments have been designed to reevaluate mechanisms for metabolism of ethanol to 1-hydroxyethyl radicals (HER) in rat liver microsomes. The variables tested include addition of azide, catalase, superoxide dismutase, and deferoxamine, or use of phosphate or Tris buffers. The results indicate that several mechanisms of HER formation are possible, depending on the experimental conditions used to study this process. In the presence of phosphate buffer, which has been used extensively in spite of its ability to chelate iron, HER formation is quite sensitive to changes in hydrogen peroxide availability. These results suggest that Fenton-type reactions produced the oxidizing intermediate responsible for conversion of ethanol to a free radical in phosphate buffer. However, in Tris buffer, HER formation was inhibited markedly by addition of superoxide dismutase, whereas catalase or azide had little effect. These data indicate that the apparent mechanism of radical formation may be influenced by the choice of buffer used. HER formation was almost abolished by the combination of superoxide dismutase and deferoxamine in both buffers, suggesting little enzymatic HER formation by the cytochrome P450 enzymes. When changes in HER formation were compared with rates of ethanol oxidation, it was inferred that 25 to 50% of the acetaldehyde formed during microsomal ethanol oxidation under different experimental conditions could arise via the HER intermediate.