Sirtuins and NAD: Influence on Metabolism and Aging

Cells rely on intricate molecular systems to regulate metabolism and maintain function over time. Among these, sirtuins and nicotinamide adenine dinucleotide (NAD) play a crucial role in cellular health, influencing processes such as energy production, DNA repair, and stress resistance. Their activity has drawn attention for its potential links to aging and longevity.

Understanding how sirtuins and NAD interact provides insight into metabolic regulation and age-related decline.

Sirtuin Families In Humans

Sirtuins are a family of NAD-dependent enzymes that regulate metabolic and cellular processes, with seven isoforms (SIRT1–SIRT7) identified in humans. Each exhibits distinct subcellular localization and functional roles, influencing energy homeostasis, genomic stability, and stress adaptation. Their enzymatic activity primarily involves deacetylation, though some also possess ADP-ribosyltransferase functions, modifying proteins based on cellular energy status.

SIRT1, the most extensively studied, resides in the nucleus and cytoplasm, modulating transcription factors such as PGC-1α, FOXO, and p53. Through these interactions, it influences mitochondrial biogenesis, oxidative stress resistance, and apoptosis. Research in Cell Metabolism found that increased SIRT1 expression in mice improved insulin sensitivity and extended lifespan under caloric restriction. SIRT2, primarily cytoplasmic but capable of shuttling to the nucleus, plays a role in cell cycle regulation and neurodegenerative disease pathways.

Mitochondrial sirtuins—SIRT3, SIRT4, and SIRT5—govern metabolic flux by modifying enzymes involved in oxidative phosphorylation, fatty acid oxidation, and amino acid metabolism. SIRT3 enhances mitochondrial function by deacetylating enzymes such as superoxide dismutase 2 (SOD2) and long-chain acyl-CoA dehydrogenase (LCAD). A study in Nature Medicine found that SIRT3-deficient mice exhibited accelerated metabolic decline and increased susceptibility to age-related disorders. SIRT4 inhibits glutamate metabolism and insulin secretion, while SIRT5 regulates ammonia detoxification through desuccinylation of carbamoyl phosphate synthetase 1 (CPS1).

Nuclear sirtuins SIRT6 and SIRT7 contribute to chromatin remodeling and genomic stability. SIRT6 is essential for DNA repair and telomere maintenance, with research in Science showing that SIRT6-deficient mice exhibit premature aging due to impaired double-strand break repair. It also represses glycolytic gene expression, promoting oxidative phosphorylation in low-energy states. SIRT7, localized in the nucleolus, regulates ribosomal RNA transcription and maintains proteostasis under metabolic fluctuations.

NAD Synthesis And Recycling

Nicotinamide adenine dinucleotide (NAD) serves as a central coenzyme in metabolism, facilitating redox reactions and acting as a substrate for NAD-dependent enzymes, including sirtuins. Cells maintain NAD levels through de novo synthesis and salvage pathways. The de novo pathway begins with dietary tryptophan or nicotinic acid, which undergoes enzymatic conversions to generate NAD. Quinolinic acid and nicotinic acid mononucleotide (NaMN) are key intermediates, with quinolinate phosphoribosyltransferase (QPRT) playing a pivotal role in converting quinolinic acid into NaMN. However, this process is limited by dietary intake and enzymatic regulation.

The salvage pathway efficiently recycles NAD precursors, including nicotinamide (NAM), nicotinic acid (NA), and nicotinamide riboside (NR), into active NAD pools. NAM, a byproduct of NAD-consuming reactions, is converted back into NAD through nicotinamide phosphoribosyltransferase (NAMPT), which catalyzes nicotinamide mononucleotide (NMN) formation. NMN is then converted into NAD by nicotinamide mononucleotide adenylyltransferases (NMNATs). NAMPT is a rate-limiting enzyme in this pathway, and its expression declines with age, contributing to reduced NAD levels and metabolic dysfunction, as shown in Nature Communications.

Beyond NAM recycling, NR and nicotinic acid riboside (NAR) bypass NAMPT-dependent steps, offering alternative routes for NAD biosynthesis. NR is phosphorylated by nicotinamide riboside kinases (NRKs) to generate NMN, which then enters the NAD synthesis pathway. This has sparked interest in therapeutic applications, with clinical trials exploring NR supplementation to counteract NAD depletion in aging. A Cell Reports study demonstrated that NR supplementation increased NAD levels in human skeletal muscle, improving mitochondrial function and metabolic health markers. Similarly, nicotinic acid (NA) follows a distinct conversion pathway involving nicotinic acid phosphoribosyltransferase (NAPRT) before merging with the NAD synthesis network.

Enzymatic Mechanisms Linking Sirtuins And NAD

Sirtuins function as NAD-dependent deacetylases, utilizing NAD to remove acetyl groups from lysine residues on target proteins. This reaction modifies protein activity while consuming NAD, generating nicotinamide (NAM) and O-acetyl-ADP-ribose as byproducts. The dependence on NAD links sirtuin activity to cellular energy states, as fluctuations in NAD availability influence their regulatory role in metabolism. Energy abundance can limit sirtuin function due to NAD consumption by poly(ADP-ribose) polymerases (PARPs) and CD38, while fasting or exercise enhances NAD synthesis, promoting sirtuin-mediated adaptations.

Sirtuins’ catalytic domains facilitate the transfer of acetyl groups from substrate proteins to NAD, producing a deacetylated protein and an ADP-ribose-acetyl conjugate. Structural analyses show that NAD binding induces conformational changes in sirtuins, positioning acetylated lysine for catalysis. This process is highly sensitive to intracellular NAD concentrations, making sirtuins responsive to metabolic shifts. For instance, SIRT3 enhances oxidative phosphorylation by deacetylating enzymes such as isocitrate dehydrogenase 2 (IDH2) and LCAD, optimizing energy production when NAD levels are sufficient.

Beyond deacetylation, certain sirtuins exhibit alternative enzymatic activities. SIRT4 has ADP-ribosyltransferase activity, modifying proteins involved in glutamine metabolism and insulin secretion. Research in Molecular Cell demonstrated that SIRT6, which regulates chromatin structure through histone deacetylation, also exhibits mono-ADP-ribosyltransferase activity, influencing DNA repair and genomic stability in an NAD-dependent manner.

Interactions In Cellular Metabolism

Sirtuins and NAD dynamically regulate metabolism by responding to energy availability. When NAD levels are high, such as during fasting or caloric restriction, sirtuins enhance oxidative metabolism by deacetylating mitochondrial enzymes, promoting ATP production through oxidative phosphorylation while suppressing glycolysis. Conversely, low NAD levels favor glycolytic pathways and reduce mitochondrial efficiency.

Sirtuins also regulate lipid metabolism. SIRT1 activates peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), a key regulator of fatty acid oxidation, facilitating lipid breakdown during fasting. Mitochondrial sirtuins like SIRT3 further enhance lipid catabolism by modifying enzymes such as hydroxyacyl-CoA dehydrogenase. These mechanisms ensure metabolic flexibility in response to physiological demands.

Relevance For Age-Associated Processes

Aging is marked by a decline in NAD metabolism and sirtuin activity, contributing to impaired mitochondrial function, oxidative stress, and genomic instability. NAD levels decrease over time, leading to metabolic dysfunction and neurodegeneration. Studies have shown that boosting NAD through precursors like nicotinamide riboside (NR) or nicotinamide mononucleotide (NMN) can restore sirtuin activity and mitigate age-related decline. Research in Cell Metabolism found that NMN supplementation in aged mice improved mitochondrial function and endurance.

Sirtuins also influence genomic maintenance. SIRT6 plays a role in DNA repair and chromatin stability, with reduced activity linked to accelerated aging. Conversely, overexpression in model organisms has been associated with extended lifespan. SIRT1 modulates inflammatory pathways by repressing NF-κB signaling, which becomes dysregulated with age. Chronic low-grade inflammation, or “inflammaging,” contributes to degenerative conditions. Preserving NAD levels and sustaining sirtuin activity may counteract age-related deterioration, supporting interventions aimed at extending healthspan.

Key Factors Regulating NAD Levels

NAD homeostasis depends on synthesis, consumption, and recycling, influenced by diet, metabolism, and environmental factors. Precursors like tryptophan, nicotinic acid, and nicotinamide support NAD biosynthesis, while caloric restriction and intermittent fasting enhance NAD availability by upregulating NAMPT, sustaining sirtuin activity under energy scarcity.

Enzymatic NAD consumption also plays a role, with PARPs and CD38 acting as major consumers. PARPs utilize NAD for DNA repair, while CD38 degrades NAD into cyclic ADP-ribose. CD38 expression increases with age, accelerating NAD depletion and reducing sirtuin activity. Inhibiting CD38 has emerged as a potential strategy to preserve NAD levels, with studies showing that pharmacological blockade restores NAD concentrations and improves metabolic function in aging models. Exercise further supports NAD homeostasis by stimulating mitochondrial biogenesis and enhancing NAD synthesis, reinforcing its role in cellular longevity.