Co-metabolism of ethanol, ethanol-derived acetaldehyde, and 4-hydroxynonenal in isolated rat hepatocytes.
Our laboratory has previously reported on the ability of 4-hydroxynonenal (4-HNE), a primary product of lipid peroxidation, to inhibit acetaldehyde metabolism in isolated mouse liver mitochondria. The purpose of the present study is to determine whether the co-metabolism of ethanol and 4-HNE compromises the elimination of either substrate in isolated rat hepatocytes. Hepatocytes were isolated and incubated with ethanol and 4-HNE. Ethanol elimination and acetaldehyde accumulation were monitored by gas chromatography, whereas 4-HNE elimination and metabolite accumulation were measured by UV detection and reversed-phase HPLC at 202 nm. In the absence of 4-HNE, hepatocytes metabolized ethanol at an initial rate of 9.4 nmol/min/million cells. Ethanol elimination was moderately inhibited by the presence of 4-HNE. Accumulation of ethanol-derived acetaldehyde was not apparent in incubations with only ethanol. In contrast, in incubations containing both substrates, ethanol-derived acetaldehyde accumulation exceeded that observed in hepatocytes exposed only to ethanol and was proportional to the 4-HNE concentration in the incubations. In all instances, the rate of 4-HNE elimination was not compromised by the presence of ethanol. Accordingly, ethanol metabolism did not alter the oxidative or conjugative metabolism of 4-HNE. However, the reductive metabolism of 4-HNE was affected by the presence of ethanol, wherein accumulation of 1,4-dihydroxy-2-nonene increased > 2-fold of that observed in incubations with only 4-HNE. To determine further if 4-HNE and ethanol are metabolized through the same metabolic pathways, cells were preincubated with either 4-methylpyrazole or cyanamide to inhibit alcohol dehydrogenase (E.C. 1.1.1.1.) and aldehyde dehydrogenase (E.C. 1.2.1.2.), respectively. Expectantly, 4-methylpyrazole blocked the formation of 1,4-dihydroxy-2-nonene, but had no effect on the rate of 4-HNE elimination. In contrast, cyanamide substantially inhibited the formation of 4-hydroxy-2-nonenoic acid, decreased the rate of 1,4-dihydroxy-2-nonene formation, but did not decrease the elimination rate of 4-HNE. Overall, these results support our previous observation that 4-HNE inhibits acetaldehyde metabolism and establish that ethanol and 4-HNE are metabolized through the same alcohol dehydrogenase- and aldehyde dehydrogenase-mediated pathways. These data continue to suggest that, as a consequence of enhanced lipid peroxidation resulting from chronic ethanol consumption, increased 4-HNE levels could compromise cellular elimination of ethanol-derived acetaldehyde and thus function in the potentiation of alcoholic liver fibrosis.
Hartley
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DP
School of Pharmacy and Hepatobiliary Research Center, University of Colorado Health Sciences Center, Denver 80262, USA.
Petersen
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DR
D000079:Acetaldehyde / Q000378:metabolism*; D000426:Alcohol Dehydrogenase / Q000502:physiology; D000444:Aldehyde Dehydrogenase / Q000502:physiology; D000447:Aldehydes / Q000493:pharmacokinetics*; D000818:Animals; D002478:Cells, Cultured; D002849:Chromatography, Gas; D002851:Chromatography, High Pressure Liquid; D000431:Ethanol / Q000493:pharmacokinetics*; D015227:Lipid Peroxidation / Q000502:physiology; D008099:Liver / Q000201:enzymology*; D008104:Liver Cirrhosis, Alcoholic / Q000503:physiopathology; D008297:Male; D008657:Metabolic Clearance Rate / Q000502:physiology; D051379:Mice; D051381:Rats; D017207:Rats, Sprague-Dawley
D000447:Aldehydes; D000431:Ethanol; D000426:Alcohol Dehydrogenase; D000444:Aldehyde Dehydrogenase; D000079:Acetaldehyde; C027576:4-hydroxy-2-nonenal