抄録
In order to clarify the regulation mechanism of NMN glycohydrolase, a number of purine and pyrimidine nucleotides were tested as activators for the enzyme. Among naturally occurring nucleotides, pppGpp was shown to be the most potent activator (KA=0.0087mM). The effectiveness of these nucleosides estimated from Vmax/KA was in the order: pppGpp>ppGpp>pppppG>ppppG>pGpp>GTP_??_2'-GMP_??_GppppG>dGTP>ITP>GDP=2'-O-methylGTP. XTP, UTP, dTTP gave no effect. In contrast, ATP, CTP, and 7-methylGTP were inhibitory. A comparison of the data indicated that activation of the enzyme is specific for the base structure, guanine, and that effectiveness of nucleotides as activators results from the negatively charged phosphate moiety. Consistent with these results, inorganic polyphosphates (tri-, tetra-, and pentaphosphate) and 5-phosphoribose 1-pyrophosphate were shown to function as inhibitors of the enzyme. The inhibition mechanism was shown to be competitive with GTP. However, apparent K1 values for these inhibitors were dependent on the activator concentration. Furthermore, a shift of cooperativity type and progressive increase of positive cooperativity concomitant with increase of PRPP concentration were observed. These results indicate that this enzyme may be allosteric and an important regulatory component of pyridine nucleotide cycle metabolism.
