Possible roles of free radicals in alcoholic tissue damage

Hepatic microsomes metabolize ethanol to a free radical metabolite which forms adducts with the spin trapping agents PBN (phenyl-N-t-butylnitrone) and DMPO (5,5-dimethyl-l-pyrroline N-oxide). This ethanol radical has been identified as the l-hydroxyethyl radical through the use of 13C-labelled ethanol. A role of the cytochrome P-450 enzymes in the generation of the l-hydroxyethyl radical was suggested by requirements for oxygen and NADPH, as well as inhibition in the presence of SKF 525-A and imidazole. In contrast, the ESR signal intensity of the l-hydroxyethyl radical was diminished when either catalase, or the iron chelating agent deferoxamine, was added to the microsomal incubations, and was increased by the addition of ADP-Fe. These observations suggest that the ethanol radicals may arise secondary to iron-catalyzed formation of hydroxyl radicals from hydrogen peroxide. This possibility was supported by enhanced rates of l-hydroxyethyl radical formation when microsomal catalase activity was inhibited by the addition of sodium azide, or by pretreatment of rats with aminotriazole. However, the reaction was relatively insensitive to scavengers of the hydroxyl radical. Thus, the mechanism of l-hydroxyethyl radical formation could involve two cytochrome P-450-dependent pathways: generation of hydrogen peroxide required for a Fenton reaction, as well as direct catalytic formation of the ethanol radical.