NAD+

Redox chemistry

NAD+ carries reducing equivalents from glycolysis, the tricarboxylic acid cycle and fatty-acid oxidation to complex I of the electron transport chain. The NAD+/NADH ratio therefore constrains flux through several dehydrogenase-dependent steps, and researchers measure it directly or infer it from lactate/pyruvate and β-hydroxybutyrate/acetoacetate ratios. The related phosphorylated pair NADP+/NADPH serves anabolic and antioxidant chemistry and is measured separately.

Consuming enzymes

• Sirtuins remove acetyl and other acyl groups from lysine residues, releasing nicotinamide and 2'-O-acyl-ADP-ribose
• PARPs build poly(ADP-ribose) chains during DNA damage response, a major NAD+ sink under genotoxic stress
• CD38 and CD157 generate cyclic ADP-ribose and NAADP as calcium-signaling messengers

Salvage and precursors

Because those reactions cleave the glycosidic bond, cells rebuild NAD+ mainly through the salvage pathway, in which NAMPT converts nicotinamide to nicotinamide mononucleotide and NMNAT completes the dinucleotide. Precursor compounds including nicotinamide riboside, NMN and nicotinic acid are widely studied as ways to load this pathway, and comparative work between direct NAD+ and precursors is an active area.

Aging and stress models

Tissue NAD+ levels have been reported to decline with age in several species, with proposed contributors including elevated CD38 expression and increased PARP activity. Rodent studies have examined mitochondrial function, metabolic parameters and DNA repair capacity when the pool is manipulated. These are laboratory observations in model systems, not demonstrated outcomes in people.

One further practical point runs through this literature: the intact coenzyme is a charged, membrane-impermeant molecule, so studies that add NAD+ to intact cells are measuring something different from studies that raise the pool through precursors or through enzyme inhibition. Distinguishing those experimental routes is essential when comparing published results across laboratories.

Bring the vial to room temperature before opening. NAD+ dissolves quickly in water, but solutions are acidic and the molecule hydrolyzes under alkaline conditions, so buffer to the pH your assay requires and avoid prolonged exposure above neutral. Prepare working solutions fresh where possible, keep them cold and protected from light, and aliquot rather than repeatedly thawing a single stock. For enzyme kinetics, verify concentration spectrophotometrically at 260 nm using the published extinction coefficient rather than relying on weighed mass alone. Store unopened vials at −20 °C.