The effects of cephem antibiotics and their related compounds on aldehyde dehydrogenase obtained from rat liver mitochondria were studied. A pH of 8.8 and reaction temperature 24° were the conditions for measurement of enzyme activity. The apparent Michaelis constant Km values for NAD, acetaldehyde and propionaldehyde were 3.8 × 10-5 M, 4.0 × 10-5 M and 2.5 × 10-5 M, respectively. Cefamandole, cefoperazone and cefmetazole, having a 1-methyl-5-thiotetrazol group at position 3 of the cephem ring, caused a relatively potent inhibition of aldehyde dehydrogenase. Cefmetazole and cefoperazone also showed a significant inhibition on highly purified yeast aldehyde dehydrogenase; the extent of inhibition on yeast enzyme was almost the same as that on rat mitochondrial aldehyde dehydrogenase. The decrease in enzyme activity effected by 1-methyl-1H-tetrazol-5-thiol (MTT) was greater than those of 1H-tetrazol (TZ), 1H-tetrazol-5-thiol and 1-(2-dimethylaminoethyl)-1H-tetrazol-5-thiol, but was, of course, less than that of disulfiram. Cefamandole, cefmetazole and MTT showed competitive inhibition with NAD, while TZ was uncompetitive inhibitor with respect to both NAD and acetaldehyde. Enzyme inhibition caused by disulfiram, cefmetazole and MIT increased timedependently and the addition of 2-mercaptoethanol into the medium effectively and completely restored enzyme inhibition. These results suggest that thiol group at position 5 of 1H-tetrazol ring is responsible for the type of inhibition with NAD, and methyl group at position 1 of 1H-tetrazol ring is important to exhibit a potent inhibition on aldehyde dehydrogenase.
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