https://www.liebertp...9/rej.2018.2077
Abstract
Nicotinamide adenine dinucleotide (NAD+) is an essential pyridine nucleotide that serves as an electron carrier in cellular metabolism and plays a crucial role in the maintenance of balanced redox homeostasis. Quantification of NAD+:NADH and NADP+:NADPH ratios are pivotal to a wide variety of cellular processes, including intracellular secondary messenger signaling by CD38 glycohydrolases, DNA repair by poly(adenosine diphosphate ribose) polymerase (PARP), epigenetic regulation of gene expression by NAD-dependent histone deacetylase enzymes known as sirtuins, and regulation of the oxidative pentose phosphate pathway. We quantified changes in the NAD+ metabolome in plasma samples collected from consenting healthy human subjects across a wide age range (20–87 years) using liquid chromatography coupled to tandem mass spectrometry. Our data show a significant decline in the plasma levels of NAD+, NADP+, and other important metabolites such as nicotinic acid adenine dinucleotide (NAAD) with age. However, an age-related increase in the reduced form of NAD+ and NADP+—NADH and NADPH—and nicotinamide (NAM), N-methyl-nicotinamide (MeNAM), and the products of adenosine diphosphoribosylation, including adenosine diphosphate ribose (ADPR) was also reported. Whereas, plasma levels of nicotinic acid (NA), nicotinamide mononucleotide (NMN), and nicotinic acid mononucleotide (NAMN) showed no statistically significant changes across age groups. Taken together, our data cumulatively suggest that age-related impairments are associated with corresponding alterations in the extracellular plasma NAD+ metabolome. Our future research will seek to elucidate the role of modulating NAD+ metabolites in the treatment and prevention of age-related diseases.
Introduction
In the last decade, there has been growing interest in the role of redox active nucleotides in the metabolism.1 The significance of pyridine nucleotide coenzymes, such as nicotinamide adenine dinucleotide (NAD+) and its phosphorylated form NADP+, as main electron transfer molecules and substrates for over 700 oxidoreductase enzymes is undebated.2 We and others have previously demonstrated that disturbances in the redox balance, for example, following exposure to chronic oxidative stress, often represents an important component of the pathobiology of cell loss in cardiovascular and neurodegenerative diseases.3,4 Exogenous stressors, such as overfeeding, starvation, alcohol ingestion, or drug treatment can alter the intracellular redox status of these coenzymes.5
NAD+ represents one of the most important coenzymes in the hydride transfer reactions.6 NAD+ is the precursor of the pyridine nucleotide family, including NADH, NADP+, and NADPH, and is the end product of tryptophan metabolism via the kynurenine pathway.7 It has been well established that NAD+ is a substrate for major dehydrogenase enzymes involved in nutrient catabolism, including alcohol and lactate dehydrogenase reactions.8 As well, NADH, which is the reduced form of NAD+, preferentially provides electrons to power mitochondrial oxidative phosphorylation. Apart from its roles in fuel utilization, NAD+ also serves as an exclusive substrate for the nuclear repair enzymes poly(adenosine diphosphate [ADP] ribose) polymerases (PARP). PARPs are a family of enzymes that are activated by double- or single-stranded DNA breaks in DNA, and are thought to promote DNA repair by the ADP-ribosylation of histones and other nuclear proteins.9 NAD+ is also a substrate for the enzyme NAD+ glycohydrolases (CD38) that leads to the production of cyclic ADP-ribose, a calcium efflux effector.10 NAD+ has also been shown to be the sole substrate for a new class of NAD-dependent histone deacetylase (“HDAC”) enzymes known as sirtuins.11 Increasing histone acetylation is associated with age-related pathologies, whereas gene silencing by upregulation of sirtuins has been shown to extend lifespan in yeast and small organisms.12 HDACs are also being found to interact with a variety of nonhistone proteins and to thereby change their function, activity, and stability by post-translational modifications.
Accurate determination of the NAD+ metabolome is of major interest due to its potential association with cognitive decline, including AIDS dementia complex,13–15 cancer,16–18 aging, and a plethora of age-related disorders. Recently, nicotinic acid adenine dinucleotide (NAAD), an intermediate of NAD+ synthesis from nicotinic acid (NA) via the NAD+ salvage pathway, has been shown to increase following ingestion with niacin.19 This finding suggests that increased NAD+ anabolism by supplementation with NAD+ precursors not only increases the accumulation of by-products of NAD+ catabolism (such as ADP-ribose and N-methyl-nicotinamide [MeNAM]), but also stimulates retrograde synthesis of NAAD and nicotinic acid mononucleotide (NAMN). However, the mechanism responsible for this elusive biochemical reaction is yet to be identified.
Given the significance of the NAD+ metabolome in a multitude of biological processes, accurate quantification of its concentration and redox state in plasma and tissue is essential for better understanding of important biochemical processes, and determining the metabolic state of organisms in response to treatment with various compounds and disease states. We and others have shown that the NAD+:NADH ratio varies between 1 and 10 in catabolic tissue of “physiologically” aged female Wistar rats, and human subjects.3,4,20 As NAD+ also serves as an oxidative agent in some biochemical processes such as fatty acid oxidation, glycolysis, and citrate cycle, changes to the NAD+:NADH ratio may also represent an indicator of alterations in metabolic processes and several diseases including multiple sclerosis. In 2011, we were the first to prove that NAD+ is an essential factor in the aging process in major declining levels of catabolic tissue such as the brain, heart, lung, liver and kidney of rats, and in human pelvic tissue.3,4,20 Increased NAD+ anabolism has been shown to ameliorate mitochondrial dysfunction in a mechanism dependent on SIRT1, a nuclear sirtuin.
While it is thought that NAD+ is predominantly an intracellular nucleotide, emerging evidence suggests that extracellular NAD+ crosses the plasma membrane and replenishes intracellular NAD+.21 Intracellular NAD+ concentrations have been shown to range between 10 and 1000 μM, and are much higher than the levels reported in the extracellular space.22 This is because (1) NAD+ is released from cells at low amounts; (2) NAD+ catabolism is rapid leading to biologically active products; and (3) NAD+ directly interacts with cell surface receptors such as connexion 43 channels and several subtypes of purinergic P2 receptors.23
Therefore, accurate monitoring of the plasma NAD+ metabolome is necessary and may provide valuable information regarding the effect of various lifestyle and dietary factors, pharmacological and nutraceutical supplementation of NAD+ and/or its metabolites. Monitoring the plasma NAD+ metabolome levels will also allow drug candidates to be screened for a new type of potentially adverse effect—the depletion of NAD+ and/or other desirable metabolites. Moreover, the ratio (e.g. the NAD:NADH ratio) of oxidized and reduced forms of pyridine dinucleotides provides important information regarding redox metabolism disorders or alterations to cellular bioenergetics and may become important biomarkers for the early detection of pathological states.