Niacin (vitamin B3) is unique among water-soluble vitamins in having an alternative biosynthetic route. The amino acid precursor and the limiting enzyme for endogenous niacin synthesis in the liver is:

• A Phenylalanine; phenylalanine hydroxylase converts it to tyrosine, which enters the niacin pathway
• B Tryptophan; tryptophan 2,3-dioxygenase (TDO) and the kynurenine pathway, requiring vitamin B6 at multiple steps, ultimately yielding quinolinic acid converted to nicotinic acid mononucleotide ✓
• C Methionine; S-adenosylmethionine donates a methyl group to nicotinamide in the final step
• D Glutamine; glutamine amidotransferase incorporates the amide nitrogen into the niacin pyridine ring

Explanation

Approximately 60 mg of dietary tryptophan can yield 1 mg of niacin (60:1 ratio). Tryptophan 2,3-dioxygenase (TDO, hepatic; or IDO, extrahepatic) is the rate-limiting enzyme catalysing the first committed step of tryptophan catabolism through the kynurenine pathway. Multiple downstream steps require pyridoxal phosphate (vitamin B6) as cofactor; thus B6 deficiency worsens niacin adequacy. The pathway ultimately yields quinolinic acid, which is converted to nicotinate mononucleotide (a direct precursor of NAD+) by QPRT. This explains why pellagra is common in regions with corn (maize) diets — corn is low in tryptophan as well as bound niacin (niacytin not bioavailable without alkali treatment/nixtamalization).

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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Written and medically reviewed by the StethoPrep medical team.