NAD+ and PARPs

What PARPs are

PARPs (poly-ADP-ribose polymerases) are a family of enzymes that use NAD+ as a substrate to attach chains of ADP-ribose units onto target proteins — including onto themselves — a modification called poly-ADP-ribosylation.[1] The best-studied member, PARP1, acts as one of the cell's DNA damage sensors: it is rapidly activated at sites of DNA damage, where its activity helps organize chromatin and recruit the machinery involved in DNA repair.[1] Each time PARP1 adds an ADP-ribose unit, it consumes one molecule of NAD+ — this is a direct, stoichiometric consumption of NAD+, not an indirect or metaphorical one.[1]

Why PARP activity can meaningfully deplete NAD+

Under conditions of extensive DNA damage, PARP activation can consume NAD+ fast enough to measurably lower a cell's available NAD+ pool.[1] This is why PARP hyperactivation — for instance under significant oxidative stress or genotoxic injury — is studied as one contributor to cellular NAD+ decline, alongside other NAD+-consuming enzymes such as CD38, covered on NAD+ and CD38.

PARPs and sirtuins compete for the same NAD+ pool

Because PARPs and sirtuins both require NAD+ to function, and often operate in the same cellular compartment (the nucleus), they can effectively compete for a shared, limited resource. A direct experimental demonstration of this: genetically removing PARP1 in mice increased NAD+ availability and SIRT1 activity in certain tissues, showing that reducing PARP-driven NAD+ consumption left more NAD+ available for sirtuin activity.[2] This is a real, demonstrated mechanistic relationship — evidence of NAD+-pool competition between these two enzyme families in that specific experimental system, not a claim about what happens with NMN supplementation in a person.

Why "NMN repairs DNA" is not a valid claim

PARP1's role in the DNA damage response is established biochemistry, and PARP1 does require NAD+ to do that job. It does not follow that supplying more NAD+ precursor from an NMN supplement causes more DNA repair, faster DNA repair, or any specific improvement in genomic stability in a healthy person. PARP1 activity is triggered by actual DNA damage — it is not a dial that turns up DNA repair activity in proportion to available NAD+ in an otherwise undamaged cell. No human NMN trial has measured a DNA repair outcome, a genomic stability marker, or any endpoint that would support a claim that NMN supplementation repairs DNA; see NMN Dosage for what those trials actually measured.

Related reading

For the enzymes that also compete with PARPs for NAD+, see NAD+ and Sirtuins and NAD+ and CD38. For how these mechanisms connect to broader cellular stress responses, see NAD+, DNA Repair, and Cellular Stress Responses.