Department of Genetics and Norris Cotton Cancer Center, Dartmouth Medical School, Lebanon, New Hampshire, United States of America.
PLoS One. 2011 May 11;6(5):e19710. doi: 10.1371/journal.pone.0019710.
NAD(+) is both a co-enzyme for hydride transfer enzymes and a substrate of sirtuins and other NAD(+) consuming enzymes. NAD(+) biosynthesis is required for two different regimens that extend lifespan in yeast. NAD(+) is synthesized from tryptophan and the three vitamin precursors of NAD(+): nicotinic acid, nicotinamide and nicotinamide riboside. Supplementation of yeast cells with NAD(+) precursors increases intracellular NAD(+) levels and extends replicative lifespan. Here we show that both nicotinamide riboside and nicotinic acid are not only vitamins but are also exported metabolites. We found that the deletion of the nicotinamide riboside transporter, Nrt1, leads to increased export of nicotinamide riboside. This discovery was exploited to engineer a strain to produce high levels of extracellular nicotinamide riboside, which was recovered in purified form. We further demonstrate that extracellular nicotinamide is readily converted to extracellular nicotinic acid in a manner that requires intracellular nicotinamidase activity. Like nicotinamide riboside, export of nicotinic acid is elevated by the deletion of the nicotinic acid transporter, Tna1. The data indicate that NAD(+) metabolism has a critical extracellular element in the yeast system and suggest that cells regulate intracellular NAD(+) metabolism by balancing import and export of NAD(+) precursor vitamins.
NAD(+) 既是氢化物转移酶的辅酶，也是 sirtuins 和其他消耗 NAD(+) 的酶的底物。NAD(+) 的生物合成对于延长酵母寿命的两种不同方案是必需的。NAD(+) 是由色氨酸和 NAD(+) 的三个维生素前体：烟酸、烟酰胺和烟酰胺核苷合成的。向酵母细胞补充 NAD(+) 前体可以增加细胞内 NAD(+) 水平并延长复制寿命。在这里，我们表明烟酰胺核苷和烟酸不仅是维生素，而且是可输出的代谢物。我们发现，烟酰胺核苷转运蛋白 Nrt1 的缺失会导致烟酰胺核苷的输出增加。这一发现被利用来工程化一种产生高水平细胞外烟酰胺核苷的菌株，该核苷以纯化形式回收。我们进一步证明，细胞外烟酰胺很容易在需要细胞内烟酰胺酶活性的情况下转化为细胞外烟酸。与烟酰胺核苷一样，烟酸转运蛋白 Tna1 的缺失会导致烟酸的输出增加。这些数据表明，NAD(+) 代谢在酵母系统中具有关键的细胞外元素，并表明细胞通过平衡 NAD(+) 前体维生素的输入和输出来调节细胞内 NAD(+) 代谢。
