Role of extrahepatic alcohol dehydrogenase in rat ethanol metabolism.
Rat alcohol dehydrogenase exhibits three isoenzymes with very different capacities of ethanol oxidation and with characteristic distribution in tissues. ADH-1 (class II isoenzyme, Km = 5 M) is especially concentrated in the most external organs: auditive, bucal, and nasal mucoses, cornea, esophagus, stomach, rectum, penis, and vagina. ADH-2 (class III isoenzyme) is present in all organs but has a poor activity with ethanol. ADH-3 (class I isoenzyme, Km = 1.4 mM) is the main liver isoenzyme, also present in lung, intestine, kidney, and sexual organs. At 33 mM ethanol and pH 7.5, total hepatic activity (3.5 +/- 0.6 units) represents 90% of the whole activity in the male rat, while the remaining 10% is distributed in many organs. The skin is the extrahepatic organ with the highest total activity (88 +/- 15 mU) followed by testis and small intestine. ADH-3 accounts for 96% of total activity (90% hepatic and 6% extrahepatic) and ADH-1 contributes with 4% (extrahepatic). However, in conditions that may be found in the digestive tract mucose after ethanol ingestion (pH 7.5, 1 M ethanol), stomach and small intestine activities represent 10% of the liver activity at 33 mM ethanol. Therefore, oral administration of ethanol will result in a higher contribution of the extrahepatic activity than will intravenous or intraperitoneal administration, because of the great ADH-1 content of the digestive tract. On the other hand, pyrazole inhibition constants at pH 7.5 for ADH-1 (33 mM) and ADH-3 (4.2 microM) are much higher than those at pH 10.0 (0.56 mM and 0.4 microM) and indicate that at the usual concentration of inhibitor only ADH-3 activity will be effectively suppressed. ADH-1 will be, therefore, responsible in part for the residual ethanol oxidation activity in pyrazole-treated rats.
Department de Bioquímica i Biologia Molecular, Facultat de Ciències, Universitat Autònoma de Barcelona, Bellaterra, Spain.
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Parés
D000426:Alcohol Dehydrogenase / Q000378:metabolism*; D000818:Animals; D000431:Ethanol / Q000378:metabolism*; D005260:Female; D000077604:Fomepizole; D007527:Isoenzymes / Q000378:metabolism*; D007700:Kinetics; D008099:Liver / Q000201:enzymology; D008297:Male; D009928:Organ Specificity; D011720:Pyrazoles / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains
D007527:Isoenzymes; D011720:Pyrazoles; D000431:Ethanol; C031280:pyrazole; D000077604:Fomepizole; D000426:Alcohol Dehydrogenase