Vitamin B3 Functions

Oxidation-reduction (redox) reactions

Living organisms derive most of their energy from oxidation-reduction (redox) reactions, which are processes involving the transfer of electrons. As many as 200 enzymes require the niacin coenzymes, NAD and NADP, mainly to accept or donate electrons for redox reactions. NAD functions most often in reactions involving the degradation (catabolism) of carbohydrates, fats, proteins, and alcohol to produce energy. NADP functions more often in biosynthetic (anabolic) reactions, such as in the synthesis of fatty acids and cholesterol.

Non-redox reactions

The niacin coenzyme, NAD, is the substrate (reactant) for two classes of enzymes (mono-ADP-ribosyltransferases and poly-ADP-ribose polymerase) that separate the niacin moiety from NAD and transfer ADP-ribose to proteins. Mono-ADP-ribosyltransferase enzymes were first discovered in certain bacteria where they were found to produce toxins such as those of cholera and diptheria. These enzymes and their products, ADP-ribosylated proteins, have also been found in the cells of mammals and are thought to play a role in cell signaling by affecting G-protein activity. G-proteins are proteins that bind guanosine-5'-triphosphate (GTP) and act as intermediaries in a number of cell-signaling pathways. Poly-ADP-ribose polymerases (PARPs) are enzymes that catalyze the transfer of many ADP-ribose units from NAD to acceptor proteins. PARPs appear to function in DNA replication and repair, as well as cell differentiation, suggesting a possible role for NAD in cancer prevention. At least 5 different PARPs have been identified, and although their functions are not yet well understood, their existence indicates a potential for considerable consumption of NAD. A third class of enzyme (ADP-ribosyl cyclase) catalyzes the formation of cyclic ADP-ribose, a molecule that works within cells to provoke the release of calcium ions from internal storage sites, and probably also plays a role in cell signaling.