Imai T
J Biochem. 1979 Apr;85(4):887-99. doi: 10.1093/oxfordjournals.jbchem.a132420.
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 alpha-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 following order of relative effectiveness: GTP, guanosine 5'-tetraphosphate greater than dGTP, GDP, 2'-GMP, 3'-GMP greater than GMP, dGMP. A heat-resistant, nondialyzable factor which could replace the GTP requirement was found in the sonic extract. The Ka for GTP and the Km for NMN in the presence of GTP at 1mm were calculated to be 0.025 mM and 4.5 mM respectively. GMP, dGMP, and dCMP were found to be effective inhibitors of the enzyme when 1 mM 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.
已从棕色固氮菌的超声提取物中部分纯化出一种糖水解酶，该酶催化NMN不可逆地转化为烟酰胺和5-磷酸核糖。该酶对NMN具有高度特异性。NAD、NADP、烟酸腺嘌呤二核苷酸、烟酰胺核糖和α-NMN不会被该酶显著水解，也不会与NMN竞争。该酶还表现出对鸟苷酸衍生物的绝对依赖性，相对有效性顺序如下：GTP、5'-四磷酸鸟苷＞dGTP、GDP、2'-GMP、3'-GMP＞GMP、dGMP。在超声提取物中发现了一种耐热、不可透析的因子，它可以替代对GTP的需求。在1mM GTP存在的情况下，GTP的Ka和NMN的Km分别计算为0.025mM和4.5mM。当也存在1mM GTP时，发现GMP、dGMP和dCMP是该酶的有效抑制剂。动力学数据表明，这些单核苷酸的结合位点与活性位点或GTP结合位点不同。这种酶切割NMN的能力表明该酶在将NMN选择性转化为烟酰胺的代谢过程中发挥作用，而烟酰胺又会被细胞作为通过烟酸合成NAD的前体重新利用。
