This is part one of a two-part article where we take a look at NAD+ biology, its history, how it relates to the biology of aging, and the potential for therapeutic interventions via the so-called NAD+ precursors.
What is NAD+?
Nicotinamide adenine dinucleotide (NAD) is a coenzyme found in all living cells. It is a dinucleotide, which means that it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an adenine base, and the other contains nicotinamide.
In metabolism, NAD facilitates redox reactions, carrying electrons from one reaction to another. This means that NAD is found in two forms in the cell; NAD+ is an oxidizing agent that takes electrons from other molecules in order to become its reduced form, NADH. NADH can then become a reducing agent that donates the electrons it carries. The transfer of electrons is one of the main functions of NAD, though it also performs other cellular processes, including acting as a substrate for enzymes that add or remove chemical groups from proteins in post-translational modifications.
NAD+ is created from simple building blocks, such as the amino acid tryptophan, and it is created in a more complex way via the intake of food that contains nicotinic acid (niacin) or other NAD+ precursors. These different pathways ultimately feed into a salvage pathway, which recycles them back into the active NAD+ form.
Nicotinamide adenine dinucleotide (NAD+) biology has seen a great deal of interest in the last few years, partially due to the discovery of two precursors of NAD+ biosynthesis, nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN), which both increase NAD+ in multiple tissues.
Accumulating evidence suggests that NAD+ systemically declines with age in a variety of organisms, including rodents and humans, which contributes to the development of many age-related diseases. For this reason, there is a great deal of interest in creating potential interventions that increase NAD+ levels via precursors, thus delaying or even preventing certain aspects of age-related functional decline and diseases.
History of NAD+ research
Before we talk more about NAD+ precursors, let’s take a look at the history of NAD+ biology in order to get a better idea of the background and research that has taken place.
We have known about NAD+ for over a hundred years. Harden and Young originally discovered NAD+ back in 1906, when they discovered that it increased the rate of fermentation in yeast . In the years following this, Euler-Chelpin, a German-born Swedish biochemist, identified that NAD+ was a nucleotide and, in 1929, received the Nobel Prize in Chemistry for his research on the fermentation of sugar and the enzymes involved in the resulting complex reactions.
In 1937, the American biochemist Conrad Elvehjem identified two vitamins, nicotinic acid and nicotinamide, which were deficient in people suffering from pellagra . In 1949, Elvehjem showed that nicotinamide was able to prevent pellagra in dogs by increasing the production of NAD+ . This discovery ended up with nicotinic acid and nicotinamide being classified as vitamin B3, and they are considered to be the original NAD+ precursors.
The first NAD+ gold rush
In 1963, there was a surge of interest in NAD+, when Chambon, Weill, and Mandel showed that NMN activated a previously unknown DNA-dependent polyadenylic acid-synthesizing nuclear enzyme . This research subsequently led to the discovery of poly-ADP-ribose and poly-ADP-ribose polymerases (PARPs), which are part of a family of proteins involved in a number of cellular processes, including DNA repair, maintaining genomic stability, and programmed cell death .
In 1965, Hayaishi and his team started to reveal the NAD+ pathway, how it was created via tryptophan, and the NAD+ intermediate nicotinic acid mononucleotide (NMN) . In 1966, Gholson suggested that there was an active turnover cycle of NAD+, which, at the time, performed “an important but as yet unknown function” relating to cellular metabolism . A decade later, in 1976, this was confirmed by Rechsteiner and his team when they published evidence for the rapid turnover of NAD+; this further supported the suggestion that NAD+ had another major function in eukaryotic cells in addition to its established role in oxidation and reduction .
In 1989, Lee and his research team incubated NAD+ with extracts from sea urchin eggs, discovering a new NAD+ metabolite, cyclic ADP-ribose . The enzyme ADP-ribosyl cyclase was initially identified in 1991 by Lee and his team in Aplysia, a type of large sea slug . It was later found to exist in mammals as CD38, which was confirmed by Lee, Walseth, and Lee in 1992 .
The second NAD+ gold rush
There was another surge of interest in NAD+ biology that happened in the year 2000 and has lasted until the present. The study that started the renewed interest was published by Guarente in 2000, which showed that yeast SIR2 (silent information regulator 2) and the related mouse ortholog, SIRT1, both had NAD+ dependent protein deacetylase activity . Adding further fuel to the fire, in 2001, human nicotinamide/nicotinic acid mononucleotide adenylyltransferase (NMNAT) was successfully isolated . NMNAT is an important NAD+ biosynthetic enzyme, so its isolation and characterization was an important step.
Next, nicotinamide phosphoribosyltransferase (NAMPT), a rate-limiting enzyme responsible for converting nicotinamide into NAD+, was isolated and characterized between 2002-2004 [14-15].
In 2004, nicotinamide riboside was shown to lead to the creation of NAD+ via nicotinamide riboside kinases (NRKs). This study demonstrated that nicotinic acid mononucleotide was not the only path to NAD+ biosynthesis .
That brings us to the end of our potted history of NAD+; almost half a century after the first surge of interest, we are now enjoying a new era of research and discovery in NAD+ biology. Join us for part two of this article, where we will be delving deeper into the NAD world.
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