Kinetic analysis of the transglycosidation reaction catalyzed by rabbit spleen pyridine nucleotide glycohydrolase

Properties of the transglycosidation reaction catalyzed by rabbit spleen pyridine nucleotide glycohydrolase were characterized using a modified cyanide addition method by which initial velocities of the transglycosidation (vT) and hydrolysis (vH) of pyridine nucleotides could be monitored simultaneously. (1) The vT was routinely determined with NMN and nicotinic acid used as substrates and was observed to be maximal at pH 6. Arrhenius plots of vT and vH indicated that the activation energies for transglycosidation and hydrolysis were 8.7 and 10.7 kcal/mol, respectively. (2) The enzyme showed a broad spectrum of substrate specificity with respect to both pyridine nucleotides and bases. Of the compounds tested, NMN and nicotinic acid were shown to be the best substrates when compared on the basis of Vmax/Km values. Kinetic constants for the enzyme-catalyzed transglycosidation reaction were as follows; Km(NMN) = 0.53 mM, Km(nicotinic acid), as acid form = 15 mM, apparent Vmax = 7.8 mumol/min/mg protein, in the presence of 0.2 M nicotinic acid. (3) The ratio of vT/vH was shown to be dependent on both pH and nicotinic acid concentration. However, transglycosidation versus hydrolysis partition at a fixed pH was constant regardless of the nicotinic acid concentration employed and approximated to be 1.2 x 10(4) at the maximal pH. (4) Nicotinamide, one of the most potent inhibitors for the enzyme-catalyzed hydrolysis, was shown to function as an antagonist for the transglycosidation reaction with NMN and nicotinic acid used as substrates. The inhibition mechanism with nicotinamide was purely noncompetitive with respect to nicotinic acid; on the other hand, the double reciprocal plot of the transglycosidation velocity against NMN concentration at a fixed concentration of nicotinamide was concave downwards. (5) The equilibrium constant of the reaction, NMN + 3-acetylpyridine----3-acetylpyridine mononucleotide + nicotinamide, was 0.61, whereas the conversion of NMN with nicotinic acid to nicotinic acid mononucleotide was essentially irreversible. These enzymatic properties of rabbit spleen pyridine nucleotide glycohydrolase suggested that the enzyme should not function as a glycohydrolase but as a transglycosidase and could serve in an important mechanism for an alternative biosynthetic pathway of nicotinic acid mononucleotide, one of the precursors for NAD synthesis, when nicotinic acid is supplied.