NAD的生物合成，代谢和检测

NAD+的生物学特性和生物合成

NAD+，NADP，NADH和NADPH一起在细胞作为氢化物的受体和供体的能量代谢中起着关键作用。与许多磷酸化产物一样，NAD+是通过从从头合成的许多较小单位而产生的。NAD+的分解代谢曾经被认为是一个非特异性过程。现在已经认识到，NAD+分解代谢与控制细胞内外的细胞基因表达的信号密切相关，CA²⁺激活和凋亡。

研究表明，色氨酸是脊椎动物和几乎所有真核生物中NAD+的先驱。除非有足够量的NAD+前体烟酰胺（NAM）和烟酸，否则有缺陷饮食的人有可能患有Pellagra的风险。

色氨酸通过一系列酶（如griboamyl二加氧酶或色氨酸二氧酶）的作用将色氨酸转化为烟酸单核苷酸（NAMN）（NAMN）等等NaMN,像烟酰胺单核苷酸(NMN能否),是converted to nicotinic acid adenine coenzyme l (NaAD) after adenylation by nicotinamide mononucleotide adenyl (group) transferase (Nmnats). NaAD relies on the action of NAD+ glutamine synthetase (Nadsyn1) to convert to NAD+. NAD+ digestive enzymes can break the force between Nam and ADP-ribose groups within the NAD+ structure to produce Nam. Nam, Na, and nicotinamide riboside (NR) are present in our daily diet. Nam can be generated in the presence of pre-B-cell clonogenic enhancer factor (PBEF). Na can be generated via bacterial degradation pathways in the presence of phosphoribosyltransferase (Naprt). NR generates NaMN in the presence of nikamide inosine kinases (Nrk1 and Nrk2). Na and Nam are eventually converted to nicotinic acid and N-methylnicotinamide in the body and excreted. Because the substances controlling the 2 remedial nicotinic acid synthesis pathways are encoded by different genes, the expression of the 2 substances varies in all vertebrate tissues.

图1.酵母和哺乳动物的NAD生物合成途径（Revollo）等。，2004）。

NAD+代谢机制

NAD+ digestive enzymes can be inhibited by Nam. ART, PARP, CD38 and Sirtuins each have a Nam site that binds to their substrates and enzyme intermediates. Each of these enzymes can be inhibited by Nam through inter-substrate base exchange. The presence of Nam in vivo is critical for NAD+ metabolism because of the different inhibition products. Nam in fungi and many bacteria can be used to synthesize Na, but the lack of genes encoding nicotinamide enzymes in vertebrate chromosomes prevents humans from using this pathway for Na synthesis (unless symbiotic bacteria in the host GI tract can help).

A gene for Nam N-methyltransferase, which is different from fungal but can convert Nam to N-methylnicotinamide in vitro, is present in humans. Moreover, a homologue of an influenza virus gene expressed in humans has Nam phosphoribosyltransferase activity, which was confirmed in a peptide called PBFE.

PBFE in the cell can promote the synthesis of NAD+, which has a protective effect on the cell. The extracellular PBEF also has two activities: 1. This polypeptide can cooperate with stem cell factor and interleukin to promote the formation of B cell precursor colony PS; 2. It has Visfatin activity. Visfatin is a newly discovered adipocytokine secreted by visceral adipocytes that binds to and activates the non-insulin binding site of the insulin receptor, activating the insulin receptor signaling pathway, thereby mimicking insulin action and lowering body glucose. This factor can also promote the differentiation, synthesis and accumulation of adipose tissue. Visfatin cDNA sequence is identical to PBEF. Visfatin also functions intracellularly as a nikosphoribosyltransferase (Nampt), which regulates NAD+ production and thus Sirt1 activity, and is involved in cell proliferation, differentiation and regulation of cell death. To some extent, PBEF has a role in alleviating Nam-mediated inhibition of extracellular NAD* digesting enzymes and participates in the process of NAD+ biosynthesis cycle partially outside the cell.

脊椎动物中的细胞外NAD+起源于整个细胞膜上的间隙连接蛋白43的细胞解析和易位。NAD+通过CD38起作用以生成NAM和CADP-ribose。当焦磷酸（PRPP）足够时，PBEF能够将细胞外NAM转换为NMN。CD73是一种与NADN同源的细胞外酶，NMN流感病毒的NMN核苷酸酶将NMN转换为NR。

图2. NAD+代谢及其生理功能（XIE等., 2020).

与NAD代谢相关的小分子主要包括NAD+，NADH，NADP+，NADPH，NMN，NAM，NAM，烟酰胺核糖（NAR）和ATP的氧化形式。创意蛋白质组学具有基于UPLC/MS的NAD代谢相关的小分子检测平台，可实现quantitative and qualitative analysis of NAD metabolism- 相关的小分子。

参考

1. Revollo, J. R., Grimm, A. A., & Imai, S. I. (2004). The NAD biosynthesis pathway mediated by nicotinamide phosphoribosyltransferase regulates Sir2 activity in mammalian cells.生物化学杂志,279（49），50754-50763。
2. Xie，N.，Zhang，L.，Gao，W。等。（2020）。NAD+代谢：病理生理机制和治疗潜力。信号转导和靶向治疗,5（1），1-37。