Inhibition of human alcohol and aldehyde dehydrogenases by cimetidine and assessment of its effects on ethanol metabolism.

Previous studies have reported that cimetidine, an H2-receptor antagonist used to treat gastric and duodenal ulcers, can inhibit alcohol dehydrogenases (ADHs) and ethanol metabolism. Human alcohol dehydrogenases and aldehyde dehydrogenases (ALDHs), the principal enzymes responsible for metabolism of ethanol, are complex enzyme families that exhibit functional polymorphisms among ethnic groups and distinct tissue distributions. We investigated the inhibition by cimetidine of alcohol oxidation by recombinant human ADH1A, ADH1B1, ADH1B2, ADH1B3, ADH1C1, ADH1C2, ADH2, and ADH4, and aldehyde oxidation by ALDH1A1 and ALDH2 at pH 7.5 and a cytosolic NAD(+) concentration. Cimetidine acted as competitive or noncompetitive inhibitors for the ADH and ALDH isozymes/allozymes with near mM inhibition constants. The metabolic interactions between cimetidine and ethanol/acetaldehyde were assessed by computer simulation using the inhibition equations and the determined kinetic constants. At therapeutic drug levels (0.015 mM) and physiologically relevant concentrations of ethanol (10 mM) and acetaldehyde (10 µM) in target tissues, cimetidine could weakly inhibit (<5%) the activities of ADH1B2 and ADH1B3 in liver, ADH2 in liver and small intestine, ADH4 in stomach, and ALDH1A1 in the three tissues, but not significantly affect ADH1A, ADH1B1, ADH1C1/2, or ALDH2. At higher drug levels, which may accumulate in cells (0.2 mM), the activities of the weakly-inhibited enzymes may be decreased more significantly. The quantitative effects of cimetidine on metabolism of ethanol and other physiological substrates of ADHs need further investigation.

Oxidation of methanol, ethylene glycol, and isopropanol with human alcohol dehydrogenases and the inhibition by ethanol and 4-methylpyrazole.

Human alcohol dehydrogenases (ADHs) include multiple isozymes with broad substrate specificity and ethnic distinct allozymes. ADH catalyzes the rate-limiting step in metabolism of various primary and secondary aliphatic alcohols. The oxidation of common toxic alcohols, that is, methanol, ethylene glycol, and isopropanol by the human ADHs remains poorly understood. Kinetic studies were performed in 0.1M sodium phosphate buffer, at pH 7.5 and 25°C, containing 0.5 mM NAD(+) and varied concentrations of substrate. K(M) values for ethanol with recombinant human class I ADH1A, ADH1B1, ADH1B2, ADH1B3, ADH1C1, and ADH1C2, and class II ADH2 and class IV ADH4 were determined to be in the range of 0.12-57 mM, for methanol to be 2.0-3500 mM, for ethylene glycol to be 4.3-2600mM, and for isopropanol to be 0.73-3400 mM. ADH1B3 appeared to be inactive toward ethylene glycol, and ADH2 and ADH4, inactive with methanol. The variations for V(max) for the toxic alcohols were much less than that of the K(M) across the ADH family. 4-Methylpyrazole (4MP) was a competitive inhibitor with respect to ethanol for ADH1A, ADH1B1, ADH1B2, ADH1C1 and ADH1C2, and a noncompetitive inhibitor for ADH1B3, ADH2 and ADH4, with the slope inhibition constants (K(is)) for the whole family being 0.062-960 µM and the intercept inhibition constants (K(ii)), 33-3000 µM. Computer simulation studies using inhibition equations in the presence of alternate substrate ethanol and of dead-end inhibitor 4MP with the determined corresponding kinetic parameters for ADH family, indicate that the oxidation of the toxic alcohols up to 50mM are largely inhibited by 20 mM ethanol or by 50 µM 4MP with some exceptions. The above findings provide an enzymological basis for clinical treatment of methanol and ethylene glycol poisoning by 4MP or ethanol with pharmacogenetic perspectives.