Imai T, Anderson B M
Arch Biochem Biophys. 1987 Apr;254(1):241-52. doi: 10.1016/0003-9861(87)90100-7.
Nicotinamide mononucleotide (NMN) is not only an intermediate for the biosynthesis but also a degradation product of pyridine cofactors in animal tissues. Among the animal tissues tested, the highest NMN catabolizing activity was detected in beef liver (5.6 mumol/min/g tissue). This activity was 16 times higher than the NAD hydrolysis catalyzed by the liver NAD glycohydrolase. As a result of enzymatic analysis of the NMN splitting process, two types of enzyme responsible for this catabolism were partially purified and identified as a membrane-bound 5'-nucleotidase and a cytoplasmic nicotinamide riboside (NR) phosphorylase. No specific NMN glycohydrolase could be found in contrast to results observed in bacterial systems. The 5'-nucleotidase and NR phosphorylase constitute an obligatory process of the pyridine nucleotide cycle. The dephosphorylation and phosphorolysis catalyzed suggest that these enzymes could serve as an important mechanism for salvaging the ribose and nicotinamide moieties of NMN and pyridine nucleotides in the cell and a process that could be regulated at the mononucleotide level by this "NMN cycle" rather than by a NAD glycohydrolase cycle. In addition to the enzymatic properties of these enzymes, a regulatory mechanism by nucleotides such as ATP was also demonstrated.
烟酰胺单核苷酸（NMN）不仅是生物合成的中间体，也是动物组织中吡啶辅因子的降解产物。在所测试的动物组织中，牛肉肝脏中检测到最高的NMN分解代谢活性（5.6微摩尔/分钟/克组织）。该活性比肝脏NAD糖水解酶催化的NAD水解高16倍。通过对NMN分解过程的酶促分析，部分纯化了负责这种分解代谢的两种酶，并鉴定为膜结合的5'-核苷酸酶和细胞质烟酰胺核糖（NR）磷酸化酶。与细菌系统中观察到的结果相反，未发现特异性的NMN糖水解酶。5'-核苷酸酶和NR磷酸化酶构成了吡啶核苷酸循环的一个必要过程。所催化的去磷酸化和磷酸解表明，这些酶可能是细胞中挽救NMN和吡啶核苷酸的核糖和烟酰胺部分的重要机制，并且这一过程可能通过这种“NMN循环”而非NAD糖水解酶循环在单核苷酸水平上受到调节。除了这些酶的酶学性质外，还证明了核苷酸如ATP的调节机制。
