What is NAD (Nicotinamide Adenine Dinucleotide)?
NAD (nicotinamide adenine dinucleotide) is a critical coenzyme found in all living cells that serves dual essential functions: as an electron carrier in energy-producing redox reactions and as a substrate for enzymes involved in DNA repair, gene expression, and cellular signaling. 1
Structural Composition
- NAD consists of two nucleotides joined by their phosphate groups—one containing adenine and the other containing nicotinamide 2
- It exists in two forms: NAD+ (oxidized) and NADH (reduced), which interconvert during metabolic reactions 1
- The phosphorylated forms NADP+ and NADPH also exist and serve distinct cellular functions 1
Primary Metabolic Functions
Energy Metabolism
- NADH serves as the principal electron donor in the respiratory chain, being oxidized to NAD+ at complex I to drive mitochondrial oxidative phosphorylation 1
- NAD+ is reduced to NADH in the TCA cycle, maintaining the cellular energy production cycle 1
- The NAD+/NADH ratio provides a measurement of cellular oxidation-reduction state 1
Redox Reactions
- NAD participates in over 400 enzymatic reactions throughout the body, acting as a cofactor for reduction-oxidation processes 2, 3, 4
- NAD(P)H functions as an electron donor in hundreds of metabolic reactions within both cytosol and mitochondria 1
Non-Redox Functions
- NAD+ serves as an essential cofactor for sirtuins, poly(ADP-ribose) polymerases (PARP), and CD38—enzymes critical for DNA repair, chromatin remodeling, and cellular signaling 3, 4
- NAD+ is consumed by PARP isozymes during DNA repair processes 1
- NADP+/NADPH primarily supports maintenance of reduced glutathione and thioredoxin pools, protecting against oxidative stress 1
Clinical Significance
Age-Related Decline
- Cellular NAD+ levels decline significantly with age in both humans and animal models, with correlation coefficients of -0.706 in men and -0.537 in women 5, 3
- This decline is causally linked to numerous age-related diseases including cognitive decline, metabolic disease, and sarcopenia 3, 6
Measurement Methods
- NAD(P)H can be detected through endogenous autofluorescence using specialized microscopy (excitation 340-360 nm, emission ~450 nm) 1
- Both NADH and NADPH autofluoresce in their reduced state but not in their oxidized state 1
- Biochemical assays including high-performance liquid chromatography and enzymatic assays are available for quantification 1
Important Caveats
- The cellular NAD(P)H autofluorescence signal is dominated by protein-bound mitochondrial NAD(P)H, as the signal co-localizes with mitochondrial markers 1
- NADP+/NADPH does not contribute directly to mitochondrial oxidative phosphorylation, distinguishing it from NAD+/NADH 1
- Excessive UV exposure during NAD(P)H fluorescence detection can be highly phototoxic to cells, requiring careful optimization of imaging parameters 1