Inhibition of Nicotinamidase by Nicotinamide Adenine Dinucleotide

Feedback inhibition by NAD was shown with the nicotinamidases of a number of microorganisms. The purified enzyme from Fleischmann's yeast was found to have a molecular weight of 110,000 and consist of presumably identical subunits with a molecular weight of 26,000. A Lineweaver-Burk plot of the NAD inhibition is concave upward at low concentrations of NAD; at 6 mm NAD, the linear plot is that of a competitive inhibitor with an apparent Ki of 7 x 10-4 m. Other competitive inhibitors include NMN (Ki = 6.5 x 10-5 m) and 3-acetylpyridine (Ki = 4.6 x 10-4 m). ADP, ADP-ribose, and PP-ribose-P were all noncompetitive inhibitors with Ki values of approximately 1 x 10-4 m. Inactivation of enzyme by urea can be prevented or partially reversed by the presence of NAD, NMN, or 3-acetylpyridine. The inactivation by urea was time-dependent and could be reversed by subsequent addition of substrate. p-Chloromercuribenzoate inhibition is competitive, with an apparent Ki of 2 x 10-7 m, and was reversed by dithiothreitol. The properties of the nicotinamidases of yeast and rabbit liver microsomes differ markedly.