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De novo NAD+ synthesis: how cells build NAD+ from tryptophan
Prepared by: Research Desk
Prepared by: Research Desk. Sources are linked for verification.
Cells have three main ways to make NAD+. They can build it from scratch out of the amino acid tryptophan, which is the de novo or kynurenine pathway. They can convert nicotinic acid, one form of niacin, through the Preiss-Handler pathway. Or they can recycle nicotinamide through the salvage pathway [1]. Most consumer writing about NAD+ focuses on salvage, because NMN and NR enter there. This explainer covers the other two routes: their enzymes, where in the body they run, and what lab and human studies have reported about them.
For the broader picture of what NAD+ does, see our NAD+ guide. For the recycling route in detail, see the NAD+ salvage pathway.
The three routes at a glance
Route | Starting material | Key entry enzyme | Intermediate that reaches NMNAT |
|---|---|---|---|
De novo (kynurenine) | L-tryptophan | IDO1 / TDO, later QPRT | Nicotinic acid mononucleotide (NaMN) |
Preiss-Handler | Nicotinic acid | NAPRT | NaMN |
Salvage | Nicotinamide | NAMPT | Nicotinamide mononucleotide (NMN) |
Two features stand out. First, the de novo and Preiss-Handler routes merge at the same intermediate, NaMN. They therefore share their final two steps. Second, the salvage route skips those steps entirely and makes NMN directly [1][7].
Step by step: the kynurenine pathway
Step 1: opening the indole ring (IDO1 and TDO)
The first and committed step converts L-tryptophan to N-formylkynurenine. Two kinds of enzyme can do this. Tryptophan 2,3-dioxygenase (TDO) is expressed mainly in the liver. Indoleamine 2,3-dioxygenase (IDO1) is found more widely and can be induced by inflammatory signals [2][3]. Much of the research interest in IDO1 comes from immunology and cancer biology, not from NAD+ itself. Breaking down tryptophan in a local tissue environment affects immune-cell signalling, and kynurenine acts as a signalling molecule in its own right [3][4]. A 2023 review also describes IL4i1, a separate oxidase. It sits beside IDO1 at what the authors call a "tryptophan metabolic nexus", so tryptophan has more than one route out [4].
Step 2: kynurenine to 3-hydroxyanthranilic acid
N-formylkynurenine is converted to kynurenine. Kynurenine then passes through several enzymatic steps, including hydroxylation and cleavage, to give 3-hydroxyanthranilic acid. Each step is a branch point. Kynurenine can also be diverted to other metabolites, so only part of the tryptophan that enters the pathway ends up as NAD+ [2].
Step 3: the ACMS branch point
3-hydroxyanthranilic acid is oxidised to an unstable compound called ACMS (α-amino-β-carboxymuconate-ε-semialdehyde). This is the key fork in the pathway:
ACMS decarboxylase (ACMSD) sends ACMS toward complete oxidation. This route takes carbon away from NAD+.
If ACMS is not decarboxylated, it closes into a ring on its own to form quinolinic acid, the direct precursor for NAD+.
Because ACMSD competes with this spontaneous ring closure, its activity helps set how much tryptophan becomes NAD+. Reviews of tryptophan metabolism and ageing describe ACMSD as a candidate control point that animal studies have examined for this reason [2].
Step 4: QPRT, the gateway to NAD+
Quinolinate phosphoribosyltransferase (QPRT) converts quinolinic acid to NaMN. This step commits the molecule to NAD+ synthesis. Mouse genetics illustrate how much the step matters. Shibata and colleagues reported that mice lacking QPRT developed what the authors termed "true niacin deficiency" when fed a niacin-free diet. Without QPRT, the animals could not make NAD+ precursors from tryptophan and depended entirely on dietary niacin [5].
Steps 5 and 6: NMNAT and NADSYN1
From NaMN, the de novo and Preiss-Handler pathways share two final steps:
NMNAT (nicotinamide mononucleotide adenylyltransferase) adds an adenylyl group from ATP, turning NaMN into nicotinic acid adenine dinucleotide (NaAD). The same NMNAT enzymes also convert NMN to NAD+ in the salvage route, so NMNAT is where all three pathways meet [1][7].
NADSYN1 (NAD synthetase) swaps the acid group on NaAD for an amide group, using glutamine as the nitrogen source, to give NAD+.
Salvage does not need NADSYN1. The de novo and Preiss-Handler routes do. A cell study of NADSYN1-deficient cells reported broad metabolic changes when this step was missing. This supports the idea that NADSYN1 is a required step for NAD+ made from tryptophan or nicotinic acid [6].
The Preiss-Handler pathway: niacin's short route
Nicotinic acid enters through NAPRT (nicotinic acid phosphoribosyltransferase), which makes NaMN in a single step. From there it follows the same NMNAT and NADSYN1 steps described above [1]. In practice, the Preiss-Handler route is a short alternative entry into the second half of the de novo pathway. This overlap is why dietary niacin and dietary tryptophan can partly stand in for each other in nutrition science. The QPRT knockout mice described above show the relationship clearly [5].
Where de novo synthesis happens: liver and kidney
Not every tissue runs the full kynurenine pathway. Reviews describe the liver as the main site of tryptophan breakdown through TDO and of de novo NAD+ production [2]. Many peripheral tissues lack parts of the enzyme set and rely mainly on recycling nicotinamide through NAMPT [1][7]. Isotope-tracing experiments in mice are often cited for this tissue map. The sources linked on this page directly support the liver and kidney part of it.
The kidney is the best-studied example outside the liver:
Human data. Poyan Mehr and colleagues reported that adults with acute kidney injury had markers consistent with reduced de novo flux, including a raised ratio of urinary quinolinic acid to tryptophan. They also reported reduced QPRT expression in kidney tissue. Mouse experiments in the same paper pointed to QPRT as a bottleneck [8].
Mouse and cell data. Bignon and colleagues reported that a cellular stress response in kidney tubule cells suppressed de novo NAD+ synthesis. This links injury signalling to this specific pathway [9].
Neither paper shows that changing the pathway produces any health outcome in people. They describe measurements and mechanisms.
The pathway in cancer and immune research
QPRT and the tryptophan entry steps also appear in cancer biology:
In triple-negative breast cancer models, Fedoroff and colleagues reported that the amino-acid transporter LAT1 increased tryptophan uptake. This fed a tryptophan-to-QPRT-to-NAD+ route that the authors linked to metabolic rewiring (cell and animal study) [10].
In leukemic cell lines, QPRT was identified as a target gene of the WT1 protein and showed anti-apoptotic activity (cell study) [11].
Reviews of NAD+ deficits in ageing and cancer place these findings in a wider context in which tumours may use several NAD+ sources at once [7].
These are preclinical observations about how cancer cells use the pathway. They say nothing about any product or intervention.
How this relates to NMN and NR research
Human precursor trials have mostly used compounds that bypass the de novo pathway. Nicotinamide riboside is converted to NMN. NMN is then converted to NAD+ by NMNAT. Neither compound needs IDO, TDO, QPRT or NADSYN1 [1]. A randomized crossover trial of NR in healthy middle-aged and older adults reported that NAD+ measured in blood cells rose and that the compound was well tolerated over the study period [12]. A randomized trial in 25 postmenopausal women with prediabetes measured the effects of NMN on muscle insulin sensitivity and NAD+ metabolites [13]. Our pages on NMN human trials and nicotinamide riboside research cover what those studies did and did not measure. How these compounds are classified under U.S. rules is covered in our legal-status explainer.
Further reading
Pathway diagrams in general textbooks are the fastest way to check the step order described here. Useful comparison criteria for a biochemistry or metabolism textbook:
Edition date. Older editions may present the ACMS branch point and NMNAT isoforms in less detail.
Pathway coverage. Look for a chapter that draws de novo, Preiss-Handler and salvage on one figure, not just the redox role of NAD+.
Citations. Chapter references to primary literature let readers trace any claim.
Format and price. Ebook, print and used editions vary widely in price. Checking which edition a course lists avoids buying the wrong one.
Our evidence-graded reading list applies similar criteria to books on NAD+ and ageing.
Selection for this slot follows the criteria above, not commission rates.
Frequently asked questions
What is de novo NAD synthesis?
It is the process of building NAD+ from scratch out of the amino acid L-tryptophan, through the kynurenine pathway. Tryptophan is converted step by step to quinolinic acid. QPRT then turns quinolinic acid into NaMN, which NMNAT and NADSYN1 convert to NAD+ [2][5].
How is the Preiss-Handler pathway different from de novo synthesis?
The Preiss-Handler pathway starts from nicotinic acid instead of tryptophan. It reaches NaMN in a single step through NAPRT. After that, both routes share the same final enzymes [1].
Why do most tissues rely on the salvage pathway?
Many tissues do not express the full set of kynurenine-pathway enzymes, while NAMPT-driven recycling of nicotinamide is widespread. The published reviews cited here describe de novo synthesis as concentrated mainly in the liver, with the kidney also using it [1][2][8].
Do NMN and NR use the de novo pathway?
No. Both enter the salvage side of NAD+ metabolism and reach NAD+ through NMNAT. They bypass the tryptophan steps, QPRT and NADSYN1 [1].
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Last updated: 23 October 2026.
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