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Niacin, serotonin (5-hydroxytryptamine), and melatonin biosynthesis from tryptophan. In addition to absorbing niacin from diet, niacin can be synthesized from the essential amino acid tryptophan, a five-step process with the penultimate compound being quinolinic acid (see figure).
Tryptophan contains an α-amino group, an α-carboxylic acid group, and a side chain indole, making it a polar molecule with a non-polar aromatic beta carbon substituent. Tryptophan is also a precursor to the neurotransmitter serotonin, the hormone melatonin, and vitamin B 3 (niacin). [4] It is encoded by the codon UGG.
Example of a label showing the amount of niacin (Vitamin B3), and specifying to be niacinamide in the ingredient section.. The United States adopted in 1942 the terms niacin to nicotinic acid and niacinamide to nicotinamide to avoid references to nicotine, [12] [13] the terms were incorporated into the United States Adopted Name [14] that was created later in 1961.
Also known as kynureninase deficiency, this extremely rare inherited disorder is caused by the defective enzyme kynureninase which leads to a block in the pathway from tryptophan to niacin (nicotinic acid). As a result, tryptophan is no longer a source of niacin, hence leading to pellagra (niacin deficiency).
Niacin deficiency is a consequence of a diet low in both niacin and the amino acid tryptophan, a precursor for the vitamin. Low plasma tryptophan is a non-specific indicator, meaning it can have other causes. The signs and symptoms of niacin deficiency start to revert within days of oral supplementation with large amounts of the vitamin. [23] [24]
Humans can produce some vitamins from precursors they consume: for example, vitamin A is synthesized from beta carotene; and niacin is synthesized from the amino acid tryptophan. [54] Vitamin C can be synthesized by some species but not by others. Vitamin B 12 is the only vitamin or nutrient not available from plant sources.
Niacin: Niacin deficiency, along with a deficiency of tryptophan, causes pellagra. Symptoms include aggression, dermatitis, insomnia, weakness, mental confusion, and diarrhea. In advanced cases, pellagra may lead to dementia and death (the 3(+1) D's: dermatitis, diarrhea, dementia, and death). Vitamin B 5: Pantothenic acid
Synthesis of B 3 from tryptophan involves vitamin B 2 and B 6, so deficiencies in either of these nutrients can lead to niacin deficiency. An excess of leucine, an essential amino acid, in the diet can also interfere with tryptophan conversion and subsequently result in a B 3 deficiency. [114]