Studies of Bungarus fasciatus venom NAD glycohydrolase. Inhibition, modification, and transglycosidation reactions.

The properties of the NAD-binding domain of Bungarus fasciatus venom NAD glycohydrolase were investigated by a number of biochemical techniques. Inhibitor studies indicated that an intact dinucleotide was required for effective enzyme recognition. Adenosine derivatives, AMP, ADP, ATP, adenosine diphosphoribose (ADP-rib), and 2'-phosphoadenosine diphosphoribose (phospho-ADP-rib), were linear competitive inhibitors of the NADase-catalyzed reaction. Nicotinamide analogs were noncompetitive inhibitors with Ki values that varied with the nature and position of substituent groups. Selective enzyme inactivation by 2,4-pentanedione indicated the importance of a lysinyl residue for NADase activity. The importance of a carboxyl group was also indicated by the sensitivity of the NADase toward N-ethyl-5-phenyl-isoxazolium-3'-sulfonate (Woodward's Reagent K). Reagents which modified sulfhydryl, histidyl, and arginyl residues were not effective in inactivating B. fasciatus venom NADase. The purified snake venom NADase catalyzed a transglycosidation (pyridine base exchange) reaction. The functioning of a variety of substituted pyridine bases in this reaction was demonstrated by the formation of the corresponding NAD analogs. Kinetic studies of the transglycosidation reaction were consistent with the partitioning of an enzyme-ADP-rib intermediate between water and the substituted pyridine base. Concentrations of pyridine bases required for enzyme inhibition differed significantly from those needed for the pyridine base exchange reaction, suggesting two sites or two forms of the NADase differing in affinity for pyridine bases. A number of NAD analogs were also shown to be effective ribosyl donors in the transglycosidation reaction.