NAD+

NAD+ (nicotinamide adenine dinucleotide, oxidized form) is a dinucleotide coenzyme: an adenosine and a nicotinamide riboside joined through a diphosphate bridge (PubChem CID 5892). It contains no amino acids and is not a peptide.

Redox enzymes use it to carry electrons from one reaction to another, cycling between NAD+ and its reduced form, NADH, while sirtuins, poly(ADP-ribose) polymerases (PARPs) and CD38 consume it as a cosubstrate (Verdin, 2015; Covarrubias et al., 2021).

PubChem CID 5892 gives the same formula and CAS number at 663.4 g/mol; the NADH and NADP+ rows are from PubChem.

This guide covers six areas of the NAD+ literature. Each summary below states the model used.

Redox coenzyme: NAD+/NADH and NADP+/NADPH

NAD+ and NADH shuttle electrons into mitochondrial respiration and oxidative phosphorylation (Massudi et al., 2012). NADP+ carries an extra phosphate on the adenosine ribose (PubChem CID 5885) and is made only by NAD kinases, one of which was located in the mitochondria of human HEK293A cells (Ohashi et al., 2012). Cells keep the NADP pool mostly reduced, as NADPH, as a store of reducing equivalents against oxidative damage (Pollak et al., 2007), and hold both couples in separate, compartmentalized pools (Xiao et al., 2018). Models: biochemical reviews and human cell culture.

Sirtuins and PARPs

Yeast and mouse Sir2 proteins were shown to be NAD-dependent histone deacetylases, and in yeast that activity accounted for gene silencing, suppression of rDNA recombination and longer replicative lifespan (Imai et al., 2000). In mouse fibroblasts, nicotinamide phosphoribosyltransferase (NAMPT) was rate-limiting for NAD synthesis from nicotinamide, and raising it increased cellular NAD and Sir2α activity (Revollo et al., 2004). PARPs draw on the same pool: deleting PARP-1, a major NAD+ consumer, raised NAD+ and SIRT1 activity in mouse brown fat and muscle (Bai et al., 2011).

CD38 and NAD+ breakdown

Tissues from CD38-deficient mice held 10 to 20 times more NAD than wild-type tissues, with NADase activity essentially absent from most of them, and CD38 makes about one cyclic ADP-ribose for every 100 NAD molecules it hydrolyzes (Aksoy et al., 2006). In aging mice, CD38 expression and activity rose, and CD38 was required for the age-related NAD decline and mitochondrial dysfunction, partly through SIRT3; it was also the main enzyme degrading the precursor nicotinamide mononucleotide (NMN) in vivo (Camacho-Pereira et al., 2016).

Synthesis and uptake by cells

In human cells, nicotinamide, nicotinic acid and their nucleosides entered cells readily, while extracellular NAD and NMN were first degraded outside the cell into permeable precursors that were then rebuilt into cytosolic and mitochondrial NAD (Nikiforov et al., 2011).

NAD+ and age: what the data show

In pelvic skin from 49 people aged newborn to 77, NAD+ fell with age in both sexes while PARP activity rose with age in males (Massudi et al., 2012). A magnetic resonance method in healthy volunteers found lower NAD+ and higher NADH in older brains (Zhu et al., 2015), and plasma from people aged 20 to 87 showed lower NAD+ and NADP+ but higher NADH and NADPH with age (Clement et al., 2019).

In aged mice, tissue NAD+ was about 30 percent lower while synthesis flux was maintained, pointing to faster consumption (McReynolds et al., 2021). A review spanning yeast, worms, rodents, monkeys and humans found the evidence for an overall decline limited and often confined to one tissue, especially in humans (Peluso et al., 2021).

Measurement and stability in solution

During extraction, oxidized and reduced forms interconvert enough to distort NAD+/NADH and NADP+/NADPH ratios; in cultured mammalian cells and mouse tissues, an acidic acetonitrile:methanol:water extraction gave the least interconversion, followed by prompt neutralization (Lu et al., 2018). Over 43 days at 19 and 25°C, NAD+ and NADH were most stable in Tris buffer, with faster NADH loss in sodium phosphate or HEPES (Wolfe et al., 2024). NADPH degraded faster than NADH, at rates driven by temperature, acidity and phosphate or acetate concentration (Wu et al., 1986).