NAD+

Nicotinamide adenine dinucleotide. Not a peptide: two nucleotides, one carrying nicotinamide and the other adenine, joined tail to tail through a diphosphate bridge.

It is drawn in the PepXtide structural library as a heavy-atom model, and it is the only nucleotide cofactor in that library.

Three separate routes build it in human cells, which is unusual for a cofactor and is the reason the literature on it is so large. The de novo route runs from tryptophan through the kynurenine pathway; the Preiss-Handler route starts from nicotinic acid; and the salvage route, whose rate-limiting enzyme is nicotinamide phosphoribosyltransferase, recycles the nicotinamide released by the enzymes that consume the cofactor.

It has two quite different jobs, and conflating them is a common error. In redox chemistry it is a hydride carrier, with a standard potential near -320 millivolts, accepting hydride at the nicotinamide ring for glyceraldehyde-3-phosphate dehydrogenase, isocitrate dehydrogenase, malate dehydrogenase and respiratory complex I, and being regenerated without net consumption. In signalling chemistry it is a substrate that is destroyed: the sirtuin deacylases, the poly(ADP-ribose) polymerases and the CD38 and BST1 glycohydrolases all cleave the nicotinamide-ribose bond, releasing nicotinamide and transferring or hydrolysing the ADP-ribose portion. The second class is why cellular concentration is a regulated variable rather than a constant.

Its solution behaviour is directional and matters for assay design. The oxidised form degrades by base catalysis at high pH, while the reduced form degrades by acid catalysis at low pH, so the two have opposite sensitivities and a buffer good for one is poor for the other; work on this reports a combined optimum near pH 8.5 when both species must be held. Degradation has been localised to the phosphate-phosphate linkage, the nicotinamide-ribose bond and the dihydropyridine ring.

Buffer choice has a measurable effect independent of pH. Comparative work over six weeks at room temperature found both forms comparatively stable in Tris, with markedly faster loss in sodium phosphate or HEPES, and loss rates roughly doubling between 19 and 25 degrees Celsius. Supplier data give solubility around 10 mg/mL in phosphate-buffered saline and advise against holding aqueous solutions beyond a day. In the laboratory the compound is standard in spectrophotometric dehydrogenase assays read at 340 nanometres, in sirtuin and polymerase activity assays, and in oxidised-to-reduced ratio determinations.

A specification note for anyone reconciling a certificate. The public databases carry separate records for the cation and for the free acid, whose masses differ by one dalton, and the familiar registry number is attached to the free-acid record rather than the cation. A formula, a mass and a registry number quoted from different records will not agree with one another.