the sirtuin deacetylases, the adenosine diphosphate (ADP)-ribose transferases/poly(ADP-ribose) polymerases, and the cyclic ADP-ribose (cADPR) synthases. The ensuing protein modification and messenger generation play a spectrum of roles in gene-regulatory, signaling, metabolic and cellular homeostasis pathways, and also in aging and diseases. Unlike the reversible shuttling between oxidized and reduced forms, which do not change the overall coenzyme concentration, the activity of NAD+ consuming enzymes requires the replenishing of the NAD+ pool. This heightens the importance of NAD+ biosynthesis and of its regulation. Several routes exist to synthesize NAD+ from various dietary sources. A de novo pathway, downstream of kynurenine metabolism of tryptophan degradation, uses quinolinic acid (QUIN) as the starting material. Others, described as salvage pathways and the primary source of NAD+, use dietary niacin (vitamin B3) - nicotinic acid (NA) and its pyridine-nucleoside or amide forms of nicotinamide riboside (NR) and nicotinamide (NAM), respectively. NAM is also the product of the reactions carried out by the enzymes that use NAD+ as a substrate and which split the molecule at its N-glycosidic bond. The uptake of NAD precursors into the cell is not yet well characterized but several transporters for tryptophan and nicotinic acid have been reported. Transport of NAD and/or metabolites across the plasma membrane is also not well understood. Connexin 43 (Gja1) hemichannels may mediate flux of NAD while the purinergic P2x7 receptor may mediate NADH transport.

Glucose metabolism and reactions in other metabolic networks generate NADH from NAD. NADHs (and FADH2) formed during the tricarboxylic acid cycle will donate their electrons to fuel the electrochemical gradient the electron transport chain (ETC) generates which drives ATP synthesis. NADP can be formed via two routes: a de novo route whereby NAD is phosphorylated by NAD kinases, and via the reactions carried out by NADPH-dependent enzymes, converting the reduced molecule to its oxidized form. NADPH can also be formed in two ways: conversion of the oxidized molecule to its reduced form by NADP-dependent enzymes or via the reaction carried out by the mitochondrial transhydrogenase. Two enzymes in the pentose phosphate pathway, and the NADP-dependent cytosolic and mitochondrial isocitrate dehydrogenases and malic enzymes generate NADPH. The mitochondrial nicotinamide nucleotide transhydrogenase NNT carries out the conversion NADP + NADH <-> NAD + NADPH. As the reaction is proton-driven, the equilibrium is shifted far to the right, making the reaction almost unidirectional. The mitochondrial membrane is rather impermeable to NAD/NADH. The reducing equivalents of cytosolic NADH are shuttled into the mitochondria by NADH shuttles (redox shuttles): the malate-aspartate shuttle (MAS) and the glycerol-3-phosphate shuttle, and MAS plays the prominent role. Cytosolic and mitochondrial proteins compose the shuttles.

There are three pathways of NAD utilization: the sirtuin-mediated pathway and the ADP ribosylation pathways - mono and poly-ribosylation, and cyclic ribosylation. Sirtuins belong to class III deacetylases whose targets include histones and other proteins; a few could be involved in mono-ADP-ribosylation. There are 7 sirtuin proteins with various cellular localizations. Mono and poly-ADP-ribosylation are carried out by members of poly(ADP)-ribose polymerases - PARPs. There are 17 members in the mammalian family and apart from the highly-conserved ADP-ribosyl transferase (ART) catalytic domain show great structural domain variability. Most can only mono-ribosylate substrates (MAR), but a few can produce long chains (PAR), up to 200 units, and even introduce branching, occasionally. Cd38 and Cd157 are the enzymes involved in the formation of cyclic ADP-ribose (cADPR) and more is known about Cd38, although overall, less is known about this route of NAD consumption. cADPR prompts calcium release from intracellular stores. Another NAD derivative known to be a very potent calcium release agent is nicotinic acid adenine dinucleotide phosphate (NAADP), but the exact mode of its formation is rather debated.

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