A glycohydrolase that catalyzes the irreversible conversion of NMN to nicotinamide and ribose 5-phosphate has been partially purified from a sonic extract of Azotobacter vinelandii. The enzyme is highly specific for NMN. NAD, NADP, nicotinic acid-adenine dinucleotide, nicotinamide riboside, and α-NMN are not significantly hydrolyzed by this enzyme, nor do they compete with NMN. The enzyme also exhibits an absolute dependence on guanylic acid derivatives with the following order of relative effectiveness: GTP, guanosine 5'-tetraphos-phate>dGTP, GDP, 2'-GMP, 3'-GMP>GMP, dGMP. A heat-resistant, nondialyzable factor which could replace the GTP requirement was found in the sonic extract. The K_A for GTP and the K_m for NMN in the presence of GTP at 1mM were calculated to be 0.025mM and 4.5mM, respectively. GMP, dGMP, and dCMP were found to be effective inhibitors of the enzyme when 1mM GTP was also present. The kinetic data suggest that the binding site for these mononucleotides is distinct from the active site or the GTP binding site.
The ability of this enzyme to cleave NMN is suggestive of a metabolic role of the enzyme in selective conversion of NMN to nicotinamide, which, in turn, would be re-utilized by the cell as a precursor of NAD via nicotinic acid.