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1. The disappearance of pyridine nucleotides during incubation with mosquito homogenates proceeds through the hydrolysis of the pyrophosphate linkage of these compounds as demonstrated by the formation of NMN and AMP from NAD(+). This reaction was also demonstrated by the loss in the coenzyme functioning property of NAD(+) (yeast alcohol dehydrogenase reaction) without a concomitant loss in reactivity towards cyanide. Transglycosidase activity was not observed in the mosquito homogenates, and low concentrations of nicotinamide did not inhibit the NAD(+) splitting activity of these homogenates. These observations are all in accord with the presence in these homogenates of a NAD(+) pyrophosphatase rather than a NADase. 2. The NAD(+) pyrophosphatase is destroyed by boiling, is not heat-activated, and has a pH optimum at pH8.75. In addition to NAD(+), other dinucleotides such as NADP(+), the 3-acetylpyridine and thionicotinamide analogues of NAD(+) and the thionicotinamide analogue of NADP(+), function as substrates in the hydrolysis catalysed by the pyrophosphatase. 3. A decrease in the specific activity of NAD(+) pyrophosphatase was observed during larval development, and a barely detectable activity was found in the pupa and adult. 4. Enzyme activity per organism increased in the larva but decreased to a very low value in the pupa and adult. These results indicate that the decrease in specific activity was due to a decrease in enzyme concentration rather than an increase in amounts of protein.

Nicotinamide-adenine dinucleotide pyrophosphatase in the growing and aging mosquito