Calf-Spleen Nicotinamide-Adenine Dinucleotide Glycohydrolase. Properties of the Active Site

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Abstract
The interaction between the nicotinamide adenine dinucleotide binding domain of calf spleen NAD glycohydrolase and its ligands has been studied. The use of competitive inhibitors, structurally related to different portions of the NAD molecule (i.e. adenosine and nicotinamide moieties), revealed the considerable importance of the binding between the pyrophosphate linkage and probably an arginyl residue of the active site. This interaction allows the positioning of the substrate in a conformation which permits catalysis to occur. The binding between the 2′‐hydroxyl of the adenosine moiety and a residue of the active site, which exists in NAD‐linked dehydrogenases, is probably missing in the calf spleen NAD glycohydrolase, based on the inhibition by salicylates, 2′‐deoxyadenosine 5′‐monophosphate and the hydrolysis of the 2′‐deoxyadenosine analogue of NAD. The NAD glycohydrolase could be completely inactivated by 2,3‐butanedione, an arginyl‐modifying reagent. The reaction followed pseudo‐first‐order kinetics and the modification was found to be reversible. Woodward's reagent K, a reagent for carboxyl residues, partially inactivated the enzyme, which resulted in a change of the NAD glycohydrolase kinetic parameters Km and V. The inactivation rate was complicated by a parallel decomposition of the reagent.