THE HEPATOCELLULAR METABOLISM OF 4-HYDROXYNONENAL BY ALCOHOL-DEHYDROGENASE, ALDEHYDE DEHYDROGENASE, AND GLUTATHIONE-S-TRANSFERASE

It has previously been reported that isolated rat hepatocytes rapidly and completely metabolize high concentrations of 4-hydroxy-2,3-(E)-nonenal (4-HNE). However, until this report, the degree to which oxidative-reductive and nonoxidative metabolic pathways function in the depletion of 4-HNE by isolated rat hepatocytes has been speculative. The objective of the present study was to quantitate the extent to which cellular aldehyde dehydrogenases (ALDH; EC 1.2.1.3.), alcohol dehydrogenase (ADH; EC 1.1.1.1.), and glutathione S-transferases (GST; EC 2.5.1.18) function simultaneously during hepatocellular metabolism of 4-HNE. Hepatocytes were incubated with varying concentrations of 4-HNE (50, 100, 250 mu M) and reversed-phase HPLC was used to quantitate 4-HNE and the oxidative and reductive metabolites, 4-hydroxy-2-nonenoic: acid and 1,4-dihydroxy-2-nonene, respectively. Conjugative metabolism of 4-HNE was determined from the depletion of cellular reduced glutathione (GSH) and concomitant formation of a GSH-4-HNE adduct detected as 2,4-dinitrofluorobenzene derivatives measured by reversed-phase HPLC. Hepatocellular elimination of 4-HNE was estimated at rates of 1.666, 0.902, and 0.219 nmol min(-1) 10(6) hepatocytes(-1) for 50, 100, and 250 mu M aldehyde, respectively. At aldehyde concentrations of 50, 100, and 250 mu m the maximal concentrations of oxidative (acid) metabolites formed were 5.9, 12.7, and 28.9 nmoles 10(6) hepatocytes(-1), whereas the concentrations of the reductive (diol) metabolite were 0.4, 12.6, and 42.3 nmoles 10(6) hepatocytes(-1), respectively. The presence of 4-methylpyrazole or cyanamide abolished formation of the reductive metabolite 1,4-dihydroxy-2-nonene or the oxidative metabolite 4-hydroxy-2-nonenoic acid in hepatocyte suspensions. At all 4-HNE concentrations evaluated, hepatocellular glutathione was not completely depleted by the aldehyde and the depletion of cellular reduced GSM corresponded to the production of the GSH-4-HNE conjugate. Metabolism by the alcohol/aldehyde dehydrogenase pathways accounted for approximately 10% of the 4-HNE elimination, while bioconversion by GST represents 50-60% of the total 4-HNE removal by hepatocytes. The enzymatic pathways responsible for the remaining 40% of 4-HNE metabolism remain to be identified. Taken together these results describe the quantitative and dynamic importance of oxidative, reductive, and nonoxidative routes in the metabolism and detoxification of 4-HNE. (C) 1995 Academic Press, Inc.