β-Nicotinamide adenine dinucleotide, NAD+, coenzyme I, nicotinamide adenine dinucleotide, disodium NAD+, oxidized NAD+, hydrogen transfer coenzyme, bi

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For scientific research use only. NAD+ (CAS No.: 53-84-9) exists in both oxidized (NAD+) and reduced (NADH) states. NAD+ participates in hundreds of enzymatic reactions and multiple key signaling pathways, regulating a range of fundamental life activities from energy metabolism to gene stability control. The dynamic generation, circulation, and consumption of NAD+ maintain homeostasis, ensuring the normal functioning of physiological processes and making it a core molecule supporting organisms, metabolism, and stress resistance.

NAD+ generation depends on three highly conserved metabolic pathways that work synergistically to maintain stable NAD+ levels and adapt to different metabolic states and environmental changes. The first and most important pathway is the salvage pathway, which can regenerate 80% of NAD+. This pathway uses NAM as its core substrate, with nicotinamide as a byproduct of NAD+ consumption. NAM combines with phosphoribosyl pyrophosphate (PRPP) to form nicotinamide mononucleotide (NMN). Subsequently, mononucleotide adenylate transferase combines with adenosine monophosphate (AMP) to ultimately form mature NAD+. This salvage pathway is highly efficient and energy-saving, enabling cells to recover NAD+ degradation products in real time. This is crucial for maintaining consistently high NAD+ activity in metabolically active tissues such as the liver, brain, and muscles.

The second synthetic pathway is de novo synthesis, using tryptophan as the starting material. Tryptophan is an essential amino acid that must be obtained from food. In the liver, tryptophan undergoes a series of multi-step enzymatic reactions to gradually convert into quinolinic acid, which is further converted into NMN, ultimately generating NAD+. This pathway is an important source of intracellular NAD+ replenishment, especially in cases of insufficient external NAD+ precursors, ensuring a basic supply of NAD+ throughout the body. The third pathway is the Pres-Handler pathway, using niacin (NA) as a substrate. Norepinephrine (NA) is enzymatically converted into niacin mononucleotide, which is then converted into active NAD+ through adenylate and amidation reactions. These three complementary pathways constitute a complete NAD+ production system, precisely regulating intracellular NAD+ concentration and maintaining the dynamic balance of the NAD+/NADH redox pair. This is a prerequisite for NAD+ to perform all its physiological functions.

The most important and primary function of NAD+ is as a redox coenzyme, mediating cellular energy metabolism. As a crucial electron and hydrogen carrier, NAD+ accepts hydride ions and is reduced to NADH during catabolism. NADH releases electrons, which are then oxidized back to NAD+ in oxidation reactions, thus achieving a cyclic redox process. This reversible reaction permeates the three core energy metabolism pathways of the cell: the tricarboxylic acid cycle (TCA cycle) and mitochondrial oxidative phosphorylation (OXPHOS). In the TCA cycle, NAD+ continuously accepts electrons generated from the oxidation of nutrients, producing large amounts of NADH. The generated NADH enters the mitochondrial respiratory chain, providing electrons for oxidative phosphorylation and driving the synthesis of large amounts of ATP (direct energy currency). Cells convert lipids and proteins from daily diet into usable bioenergy. Insufficient NAD+ supply hinders cellular energy production, leading to metabolic stagnation, energy deficiency, and ultimately fatigue, decreased cellular activity, and other sub-health conditions.

Besides participating in redox metabolism, NAD+ is also an important substrate for many key regulatory enzymes, mediating various non-redox signaling pathways and participating in the regulation of cell survival, aging, and repair. The most representative regulatory enzymes include sirtuins, PARP family enzymes, and CD38 hydrolases, all of which are completely dependent on NAD+ to exert their biological activity. Sirtuins are a class of NAD+-dependent deacetylases. When NAD+ levels are elevated, activated sirtuins can remove acetyl groups from histones and non-histones, thereby regulating the expression of aging-related genes, metabolism-related genes, and stress-related genes.