US EPA

1142259 

Technical Report 

Human Liver Alcohol Dehydrogenase-Inhibition of Methanol Activity by Pyrazole, 4-Methylpyrazole, 4-Hydroxymethylpyrazole and 4-Carboxypyrazole 

Pietruszko, R 

1975 

NIOSH/00129037 

24 

1603-1607 

The effects of pyrazole (288131), 4-methylpyrazole (7554656), 4-hydroxymethylpyrazole (25222439), and 4-carboxypyrazole (37718119) on human liver alcohol-dehydrogenase (ADH) was investigated using methanol as substrate. Human livers were obtained 24 hours after death, extracted, or kept frozen. Rates of enzyme activity were measured with a recording fluorometer. The primary double reciprocal plots at varying concentrations of methanol and four different concentrations of nicotinamide-adenine-dinucleotide (NAD) of 50 to 500 micromoles (micromol) showed that the mechanism of methanol dehydrogenation was sequential. The Michaelis constant for methanol at saturating concentrations of NAD was 5.2micromol and the turnover number was 1.7 per active site per minute. Pyrazole inhibition of methanol dehydrogenation was presented at pyrazole concentrations of 1.0 and 5.0micromol. The inhibition rate constant (Ki) for pyrazole was 0.54micromol. 4-Methylpyrazole was 5 times as effective as pyrazole in inhibition of human liver ADH in the presence of methanol. 4-Hydroxymethylpyrazole was less potent as an inhibitor than either pyrazole or 4-methylpyrazole with a Ki of 6.6micromol. 4-Carboxypyrazole at concentrations of 25 and 100micromol with 500micromol NAD at methanol concentrations ranging between 3.0 and 30micromol did not inhibit human liver ADH. The authors suggest that since 4-hydroxymethylpyrazole is 70 times less effective than 4-methylpyrazole as an inhibitor of human liver ADH, the inhibitory effect of 4-methylpyrazole is not likely to be enhanced by a possible metabolic conversion to 4-carboxypyrazole. 

DCN-120306; Chemical properties; Biochemical tests; Biochemical analysis; Metabolic study; Metabolism; Enzymatic effects; Enzyme inhibitors; In vitro study; Chemical kinetics