烟酰胺腺嘌呤二核苷酸Nicotinamide adenine dinucleotide

氧化还原发生在烟酰胺的吡啶环上：NAD⁺ + 2e⁻ + H⁺ → NADH，环 C4 位接受一个氢负离子，芳香性吡啶鎓变为 1,4-二氢吡啶。NADH 在 340 nm 有特征吸收而 NAD⁺ 没有，这是许多酶活性测定的依据。标准还原电势 E°′ = −0.32 V。NAD⁺ 主要参与分解代谢（糖酵解、三羧酸循环），其磷酸化形式 NADP⁺/NADPH 主要用于合成代谢和抗氧化。

The redox chemistry happens on the nicotinamide pyridine ring: NAD⁺ + 2e⁻ + H⁺ → NADH, with hydride added at C4, turning the aromatic pyridinium into a 1,4-dihydropyridine. NADH absorbs at 340 nm and NAD⁺ does not, the basis of many enzyme assays. Its standard reduction potential E°′ is −0.32 V. NAD⁺ mainly serves catabolism (glycolysis, the citric acid cycle), while its phosphorylated form NADP⁺/NADPH serves biosynthesis and antioxidant defence.

1906 年哈登和扬发现酵母提取液中有一种可透析、耐热的“辅酶”是发酵所必需的。1929 年哈登和冯·奥伊勒-切尔平因糖发酵研究共获诺贝尔化学奖。1936 年瓦尔堡确定烟酰胺部分负责传递氢，并阐明其作用机理。1937 年埃尔维耶姆发现烟酸能治愈犬黑舌病，从而揭示了癞皮病是烟酸缺乏症。

In 1906 Harden and Young found that yeast extract contains a dialysable, heat-stable 'coferment' needed for fermentation. Harden and von Euler-Chelpin shared the 1929 Nobel Prize in Chemistry for work on sugar fermentation. In 1936 Warburg showed that the nicotinamide part carries hydrogen and explained how it works. In 1937 Elvehjem found that nicotinic acid cures black tongue in dogs, revealing pellagra as niacin deficiency.

糖酵解和三羧酸循环生成的 NADH 把电子交给线粒体呼吸链，每个 NADH 约产生 2.5 个 ATP。NAD⁺ 还是去乙酰化酶（sirtuin）和 PARP 酶的底物，参与 DNA 修复和衰老调控，因此 NAD⁺ 前体（烟酰胺核糖、NMN）成为研究热点。体内 NAD⁺ 可由色氨酸或烟酸（维生素 B₃）合成。

NADH from glycolysis and the citric acid cycle hands electrons to the mitochondrial respiratory chain, yielding about 2.5 ATP each. NAD⁺ is also consumed by sirtuin deacylases and PARP enzymes, which take part in DNA repair and ageing, so NAD⁺ precursors such as nicotinamide riboside and NMN are active research topics. The body makes NAD⁺ from tryptophan or from niacin (vitamin B₃).