# How Is NAD+ Made? The Three Pathways Cells Use

Updated 2026-11-06.

Cells make NAD+ by three routes. The de novo pathway builds NAD+ from the amino acid tryptophan. The Preiss-Handler pathway starts from nicotinic acid. The salvage pathway recycles nicotinamide and also uses NMN and nicotinamide riboside. Key enzymes include NAMPT, NMNAT and NRK (Covarrubias et al., 2021; Reactome; KEGG).

| | |
| --- | --- |
| Three pathways | De novo (from tryptophan), Preiss-Handler (from nicotinic acid) and salvage (from nicotinamide, NMN and nicotinamide riboside) (Covarrubias et al., 2021). |
| De novo start | The amino acid tryptophan. IDO or TDO make the first product, N-formylkynurenine ([KEGG module M00912](https://www.kegg.jp/entry/M00912)). |
| Preiss-Handler start | Nicotinic acid. The enzyme NAPRT turns it into NAMN (Covarrubias et al., 2021). |
| Salvage start | Nicotinamide. The enzyme NAMPT turns it into NMN (Covarrubias et al., 2021). |
| Last step | NMNAT enzymes make NAD+ from NMN. NAD synthetase makes NAD+ from NAAD (Covarrubias et al., 2021). |
| Where the de novo route runs | Mostly in the liver. Most other cells lack the full set of enzymes (Covarrubias et al., 2021). |
| KEGG records | [Pathway map00760](https://www.kegg.jp/entry/map00760), [module M00912](https://www.kegg.jp/entry/M00912) and [module M00115](https://www.kegg.jp/entry/M00115). |
| Reactome record | [Nicotinate metabolism, R-HSA-196807](https://reactome.org/content/detail/R-HSA-196807). |
| Records checked | 6 November 2026 |

Your cells need a steady supply of NAD+. They make it in three ways. The de novo pathway starts from tryptophan. The Preiss-Handler pathway starts from nicotinic acid. The salvage pathway recycles nicotinamide, NMN and nicotinamide riboside. This page follows each route step by step and names the enzyme at each step.

This page is part of [the NAD+ hub](/learn/nad-plus). The molecule itself is described on the [what NAD+ is page](/learn/nad-plus/what-is-nad-plus). NMN, NADH and NADP+ are set side by side on the [NAD+ vs NMN, NADH and NADP+ page](/learn/nad-plus/nad-plus-vs-nmn-nadh-and-nadp). This page covers cell chemistry only.

## How is NAD+ made in cells?

Cells make NAD+ by three pathways: the de novo pathway from tryptophan, the Preiss-Handler pathway from nicotinic acid, and the salvage pathway from nicotinamide, NMN and nicotinamide riboside. Covarrubias et al. say three independent routes keep NAD+ levels steady (2021). Reactome sorts the same reactions into the same three groups (Nicotinate metabolism, R-HSA-196807). A pathway is a chain of reactions. An enzyme is a protein that speeds up one reaction in the chain.

The routes share steps. The de novo and Preiss-Handler routes both pass through nicotinic acid mononucleotide (NAMN). The salvage route passes through nicotinamide mononucleotide (NMN). NMNAT enzymes act in all three routes (Covarrubias et al., 2021).

Cells do not only make NAD+. Covarrubias et al. describe NAD+ as made, used up and recycled all the time. Sirtuins, PARPs and CD38 use up NAD+ and leave nicotinamide behind. The salvage pathway turns that nicotinamide back into NAD+ (2021). Which enzymes use NAD+ and which make it is set out on [the substrate or product page](/learn/nad-plus/is-nad-plus-a-substrate-or-product).

## How does the de novo pathway make NAD+ from tryptophan?

The de novo pathway turns the amino acid tryptophan into NAD+ through the kynurenine pathway. IDO or TDO first turn tryptophan into N-formylkynurenine. A formamidase turns that into kynurenine. KMO makes 3-hydroxykynurenine, and kynureninase (KYNU) makes 3-hydroxyanthranilic acid. The enzyme 3HAO then makes a molecule called ACMS, short for α-amino-β-carboxymuconate ε-semialdehyde (Covarrubias et al., 2021; KEGG module M00912).

ACMS can fold up by itself into quinolinic acid, with no enzyme needed. The enzyme QPRT turns quinolinic acid into NAMN. NMNAT turns NAMN into NAAD, and NAD synthetase (NADSYN1) turns NAAD into NAD+. Reactome lists these last steps as QPRT, NMNAT2 and NADSYN1 (Covarrubias et al., 2021; Reactome R-HSA-196807).

[KEGG module M00912](https://www.kegg.jp/entry/M00912) gives the same chain with enzyme numbers. It lists tryptophan 2,3-dioxygenase (EC 1.13.11.11), indoleamine 2,3-dioxygenase (EC 1.13.11.52), kynurenine formamidase (EC 3.5.1.9), kynurenine 3-monooxygenase (EC 1.14.13.9), kynureninase (EC 3.7.1.3), 3-hydroxyanthranilate 3,4-dioxygenase (EC 1.13.11.6), QPRT (EC 2.4.2.19), nicotinate-nucleotide adenylyltransferase (EC 2.7.7.18) and NAD+ synthase (EC 6.3.5.1). KEGG marks the module as complete in Homo sapiens.

Side branches of the same pathway also make kynurenic acid and picolinic acid. Serotonin is made from tryptophan by a separate route. Outside the liver, most cells lack the full set of enzymes for this route (Covarrubias et al., 2021).

## What is the Preiss-Handler pathway?

The Preiss-Handler pathway makes NAD+ from nicotinic acid in three steps. The enzyme NAPRT turns nicotinic acid into NAMN. NMNAT enzymes (NMNAT1, NMNAT2 and NMNAT3) turn NAMN into NAAD. NAD synthetase (NADS) then turns NAAD into NAD+ (Covarrubias et al., 2021).

Reactome gives the same route. Nicotinic acid, also called niacin, enters the cell and becomes nicotinate D-ribonucleotide (NAMN). That molecule can then join the de novo route and become NAD+ (Reactome R-HSA-196807).

[KEGG](https://www.kegg.jp/entry/map00760) lists the NAPRT reaction (EC 6.3.4.21) as nicotinate plus 5-phosphoribosyl diphosphate plus ATP plus water, giving nicotinate D-ribonucleotide, diphosphate, ADP and phosphate.

The pathway carries the names of Jack Preiss and Philip Handler. They published a paper on the making of NAD+ in 1958. NAD+ was then called diphosphopyridine nucleotide (Preiss and Handler, 1958). Nicotinic acid and nicotinamide are both forms of vitamin B3 (Bogan and Brenner, 2008).

## What is the NAD+ salvage pathway?

The salvage pathway recycles nicotinamide (NAM) back into NAD+. NAM is left over when enzymes such as sirtuins, PARPs and CD38 use up NAD+. The enzyme NAMPT turns NAM into NMN. NMNAT enzymes then turn NMN into NAD+ (Covarrubias et al., 2021).

Covarrubias et al. call NAM recycling a basic step in restoring NAD+ after it is used up (2021). Reactome names NAMPT as a central enzyme in how cells use NAM (R-HSA-196807).

[KEGG](https://www.genome.jp/dbget-bin/www_bget?ec:2.4.2.12) lists NAMPT as EC 2.4.2.12. Its entry links nicotinamide, 5-phosphoribosyl diphosphate and nicotinamide D-ribonucleotide (NMN). KEGG lists NMNAT as EC 2.7.7.1, with the reaction ATP plus nicotinamide ribonucleotide giving diphosphate plus NAD+.

NAM does not always return to NAD+. The enzyme NNMT can add a methyl group to NAM, and the product leaves the body in urine (Covarrubias et al., 2021).

## How do NMN and nicotinamide riboside become NAD+?

Both molecules join the salvage pathway and end up as NAD+ through NMNAT enzymes. Inside the cell, nicotinamide riboside (NR) becomes NMN through nicotinamide riboside kinases 1 and 2 (NRK1 and NRK2). NMNAT1, NMNAT2 and NMNAT3 then turn NMN into NAD+ (Covarrubias et al., 2021).

Outside the cell, the enzyme CD73 removes a phosphate group from NMN and makes NR. NR then enters the cell and NRK1 turns it back into NMN. Covarrubias et al. also describe a specific NMN transporter, Slc12a8, in the mouse small intestine. They say more work is needed to learn how much it matters (2021).

[KEGG](https://www.kegg.jp/entry/map00760) lists the NRK reaction (EC 2.7.1.22) as ATP plus nicotinamide riboside giving ADP plus NMN. The [NAD+ vs NMN page](/learn/nad-plus/nad-plus-vs-nmn-nadh-and-nadp) compares the two molecules.

## Where in the cell is NAD+ made?

Cells make NAD+ in more than one place, because the NMNAT enzyme comes in three forms. NMNAT1 works in the nucleus, and NMNAT2 is the form found in the cytosol, the fluid inside the cell (Covarrubias et al., 2021). Covarrubias et al. say the enzymes that make and break NAD+ are highly compartmentalised, so each cell part keeps its own pool.

NMNAT acts in all three routes. In the salvage pathway, it turns NMN into NAD+. In the de novo and Preiss-Handler pathways, it turns NAMN into NAAD, and NAD synthetase finishes the job (Covarrubias et al., 2021).

## Do all living things make NAD+ the same way?

No. Some living things start from a different molecule. KEGG module M00115 starts from the amino acid aspartate and passes through quinolinate. KEGG marks it as complete in plants such as Arabidopsis thaliana. KEGG module M00912, from tryptophan, is marked as complete in Homo sapiens.

Both modules share the last steps. Quinolinate becomes nicotinate D-ribonucleotide through QPRT (EC 2.4.2.19), then nicotinate-nucleotide adenylyltransferase (EC 2.7.7.18) and NAD+ synthase (EC 6.3.1.5 or EC 6.3.5.1) finish the job. [KEGG module M00115](https://www.kegg.jp/entry/M00115) and [module M00912](https://www.kegg.jp/entry/M00912) list the full steps.

## Comparison

Criteria: the molecule each pathway starts from, the enzymes named for each pathway, and the source. All data are from the cited review and database records.

| Pathway | Starts from | Key enzymes | Source |
| --- | --- | --- | --- |
| De novo (kynurenine) | Tryptophan | IDO or TDO, KMO, KYNU, 3HAO, QPRT, NMNAT, NADS | Covarrubias et al., 2021; KEGG M00912 |
| Preiss-Handler | Nicotinic acid | NAPRT, NMNAT, NADS | Covarrubias et al., 2021; Reactome R-HSA-196807 |
| Salvage from NAM | Nicotinamide | NAMPT, NMNAT | Covarrubias et al., 2021; KEGG EC 2.4.2.12 |
| Salvage from NR or NMN | Nicotinamide riboside or NMN | NRK1 and NRK2 (for NR), NMNAT | Covarrubias et al., 2021; KEGG EC 2.7.1.22 |

## Frequently asked questions

### Does the body make NAD+?

Yes. Cells make NAD+ by the de novo, Preiss-Handler and salvage pathways, and they recycle nicotinamide left by enzymes that use NAD+ up (Covarrubias et al., 2021). The de novo route runs mostly in the liver. Most other cells lack the full set of enzymes for it.

### Is NAD+ made from tryptophan?

Yes, in the de novo pathway. Tryptophan passes through the kynurenine pathway to quinolinic acid. QPRT then turns it into NAMN, and further steps give NAD+ (Covarrubias et al., 2021; KEGG module M00912).

### Is NAD+ made from niacin?

Yes, in the Preiss-Handler pathway. Niacin is another name for nicotinic acid. The enzyme NAPRT turns it into NAMN, and two more steps give NAD+ (Reactome R-HSA-196807; Covarrubias et al., 2021).

### Which enzyme makes NAD+ in the last step?

It depends on the route. In the salvage pathway, NMNAT enzymes turn NMN into NAD+. In the de novo and Preiss-Handler pathways, NAD synthetase turns NAAD into NAD+ (Covarrubias et al., 2021).

### What does NAMPT do?

NAMPT is the enzyme that turns nicotinamide into NMN in the salvage pathway. NMNAT enzymes then turn NMN into NAD+ (Covarrubias et al., 2021). KEGG lists NAMPT as EC 2.4.2.12.

## Citations

- Covarrubias AJ et al.. Nature Reviews Molecular Cell Biology. NAD+ metabolism and its roles in cellular processes during ageing, 2021;22(2):119-141.. DOI: 10.1038/s41580-020-00313-x
- Reactome Knowledgebase.. Reactome. Nicotinate metabolism, pathway R-HSA-196807.. DOI: 10.3180/R-HSA-196807.7
- Preiss J, Handler P.. Journal of Biological Chemistry. Biosynthesis of diphosphopyridine nucleotide, 1958;233(2):488-492.. DOI: 10.1016/S0021-9258(18)64789-1
- Bogan KL, Brenner C.. Annual Review of Nutrition. Nicotinic acid, nicotinamide, and nicotinamide riboside: a molecular evaluation of NAD+ precursor vitamins in human nutrition, 2008;28:115-130.. DOI: 10.1146/annurev.nutr.28.061807.155443

[NAD+ reference material](/shop/nad-plus)

Research use only. Not for human or animal use.

[Read the NAD+ research guide](/research/nad-plus)

---

Canonical page: https://www.formlaboratories.com/learn/nad-plus/how-is-nad-plus-made

Form Laboratories - Perth-based research-use-only peptide supplier, shipping Australia-wide.

- Site guide for agents: https://www.formlaboratories.com/llms.txt
- Sitemap: https://www.formlaboratories.com/sitemap.xml
- Full catalogue: https://www.formlaboratories.com/shop
- Contact: hello@formlaboratories.com

All materials are supplied for laboratory research use only - not for human or veterinary use, and nothing on this site is medical advice.