# NAD+ in DNA repair

> DNA repair and its energy cost | NAD+ — NAD+ supports DNA-repair chemistry and is consumed by it. Follow purified-enzyme experiments, mouse immune cells and the cost of keeping repair going.

**Repair**

The awkward finding is that repair needs NAD+ and can help exhaust it. I became much less comfortable with the word “more” after following that argument.

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## PARP1 and the mitochondrial repair enzyme

Mitochondria have DNA of their own. In [the purified-enzyme repair experiments](https://pubmed.ncbi.nlm.nih.gov/33482196/), PARP1 inhibited Pol γ, the enzyme involved in repairing mitochondrial DNA, when NAD+ was absent. Raising NAD+ to physiological levels allowed maximal repair activity to resume.

The experiment was reconstructed from biochemical components outside a body. Calling these “human findings” merely because the proteins were human or mammalian would give the result the wrong scale. This was an enzyme experiment, with no treatment outcome in a person.

The sharply bounded question is attractive: how does the availability of NAD+ affect this repair machinery? The answer gives a reason to investigate metabolism alongside DNA damage. It leaves the supplement question for later.

![Gloved hands arrange clear reaction tubes beside a chilled metal block.](images/plate-enzyme-preparation.webp)

*Imagined laboratory still life for the purified-enzyme experiment; this does not depict the study itself.*

## The energy cost of repair

PARP1 responds to damaged DNA and uses NAD+ as a substrate, material consumed during its work. The [review of PARP1 activation and cell death](https://pubmed.ncbi.nlm.nih.gov/25283336/) describes a consequential limit: excessive activation can deplete NAD+ and cellular energy. A process involved in repair can then contribute to a cell's death.

That does not tell us that repair is undesirable, or that more NAD+ always prevents the problem. It puts metabolism inside the explanation. The state of the cell and the extent of the damage matter.

I had been treating “supports DNA repair” as an uncomplicated compliment. The mechanism deserves a longer description. Supporting a process, controlling it and improving the fate of the whole cell are different experimental questions.

## Mouse immune cells under inflammatory stress

In [mouse bone-marrow-derived macrophages studied in the laboratory](https://pubmed.ncbi.nlm.nih.gov/30858618/), an inflammatory stimulus led to reactive oxygen species, DNA damage and PARP-mediated NAD+ consumption. The cells became dependent on NAMPT-driven salvage, the recycling route that rebuilds NAD+, to sustain their altered metabolism.

The recycling demand is the revealing part for me. An immune cell dealing with inflammation also has to pay for the consequences of DNA damage. NAD+ consumption and replacement help connect those jobs.

This was work on mouse cells outside a body. It cannot establish that a precursor supplement treats inflammation in humans. The [review of NAD+ metabolism in immune responses](https://pubmed.ncbi.nlm.nih.gov/33817782/) describes the broader research interest, including autoimmunity and inflammation associated with ageing.

## CD38 in ageing mice

PARP1 is one consumer among several. In [the mouse study of CD38 and age-related NAD+ decline](https://pubmed.ncbi.nlm.nih.gov/27304511/), CD38 activity rose with age. Mice lacking CD38 preserved NAD+ and had better mitochondrial function and metabolic health as they aged.

Deleting a gene in a mouse changes the question substantially. It helps identify a cause of NAD+ loss in that model; an oral precursor in a human leaves that gene in place. An oral human trial still has to succeed on its own terms.

Together, these experiments make a more demanding story than replenishment alone. NAD+ availability depends on production, recycling and consumption. The [clinical findings](/research) show how much of that story has reached people so far.

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