Abstract
1. 1.The cleavage of NAD in crayfish hepatopancreas is catalyzed chiefly by a pyrophosphatase rather than by NAD glycohydrolase (EC 3.2.2.5). This fact was confirmed by the loss in the coenzyme function of NAD for yeast alcohol dehydrogenase without a significant concomitant loss in reactivity towards cyanide and the identification of the reaction products as NMN and AMP by means of paper chromatography and Dowex 1-X2 column chromatography. 2. 2.NAD pyrophosphatase is localized chiefly in mitochondria and microsomes. The enzyme was partially purified by (NH 4) 2SO 4 fractionation. Using this preparation, the K m value for NAD was determined as 1.1·10 −3 M. The activity with reduced NAD was about 3-fold higher than with NAD. ATP and NADP are not cleaved by the crayfish enzyme. 3. 3.NAD pyrophosphatase is inactivated by heating at 60° for 1 min and no stimulation was observed by heating at 40° for 1 min. The pH optimum is 8.4. 4. 4.Enzyme activity is inhibited by mononucleotides such as AMP, GMP and UMP, and AMP inhibition is competitive with substrate. Nicotinamide did not inhibit the enzyme activity at 1·10 −3 M. 5. 5.The possible metabolic significance of AMP inhibition of NAD hydrolyzing enzymes in mammalian tissues is also discussed.
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