Tryptophan niacin

Horwitt, M. K., Harper, A. E. and Henderson, L. M. 1981. Niacin-tryptophan relationships for evaluating niacin equivalents. Am. J. Clin. Nutr. 34, 423-427. 

L12. Ludovici, P. P., and Axelrod, A. E., Circulating antibodies in vitamin-deficiency states. Fteroylglutamic acid, niacin-tryptophan, vitamins B12, A, and D deficiencies. Proc. Soc. Exp. Biol. Med. 77, 526-530 (1951). 

Singh, M., Effect of niacin and niacin-tryptophan deficiency on pancreatic acinar cell function in rats in vitro, Am.. Clin. Nutr., 44, 512, 1986. 

Niacin Tryptophan, glutamine Liver NAD, NADP coenzymes for oxidation reactions... 

Provitamins. These are nonvitamin substances that occur in foods which can be converted into vitamins in the body. Well-known examples of provitamins are (1) betacarotene, which is converted to vitamin A in the intestinal wall (2) 7-dehydrocholesterol in the skin, which is converted to vitamin Dj, by ultraviolet light (sunlight) (3) ergosterol of plants, which is converted to vitamin Dj, by ultraviolet light and (4) the amino acid tryptophan, which can be converted to niacin. (Because of the poor efficiency—60 mg of tryptophan is required to produce 1 mg of niacin—tryptophan is not utilized for its provitamin value.)... 

Thiamine Biotin Riboflavin Niacin-tryptophane Vitamin A... 

Other Additives. Cats cannot convert tryptophan to niacin (22), or carotene to vitamin A in sufficient amounts to meet thein needs (23). These deviations, as compared with other animals, need not produce problems because added dietary sources of niacin and vitamin A provide the needs of cats. 

Bioassays procedures have been developed in species such as chicks which have been fed a niacin-deficient diet. Due to the fact that, for example, tryptophan is a biological precursor of niacin, niacin can be produced from other sources (55). As a result, the tryptophan content of the diet has to be monitored carefully for accurate results. 

In this regard, tryptophan is considered a provitamin and is assigned a niacin equivalent of 1/60. The following fists the vitamin content of many common foodstuffs and in Table 3, values of vitamin B content are compared to niacin potential from tryptophan. 

The RDA for niacin is based on the concept that niacin coen2ymes participate in respiratory en2yme function and 6.6 niacin equivalents (NE) are needed per intake of 239 kj (1000 kcal). One NE is equivalent to 1 mg of niacin. Signs of niacin deficiency have been observed when less than 4.9 NE/239 kj or less than 8.8 NE per day were consumed. Dietary tryptophan is a rich source of niacin and the average diet in the United States contains 500—1000 mg of tryptophan. In addition, the average diet contains approximately 8—17 mg of niacin. In total, these two quantities total 16—34 NE daily. Table 5 Hsts the RDA and U.S. RDA for niacin (69). 

Most foods of animal origin contain nicotinamide in the coenzyme form (high bioavialability). Liver and meat are particularly rich in highly bioavailable niacin. Most of the niacin in plants, however, occurs as nicotinic acid in overall lower concentrations and with a lower bioavailability. The major portion of niacin in cereals is found in the outer layer and its bioavailability is as low as 30% because it is bound to protein (niacytin). If the diet contains a surplus of L-tryptophan (Ttp), e.g., more than is necessary for protein synthesis, the liver can synthesize NAD from Trp. Niacin requirements are therefore declared as niacin equivalents (1 NE = 1 mg niacin = 60 mg Trp). 

A vitamin is defined as an organic compound that is required in the diet in small amounts for the maintenance of normal metabofic integrity. Deficiency causes a specific disease, which is cured or prevented only by restoring the vitamin to the diet (Table 45-1). However, vitamin D, which can be made in the skin after exposure to sunhght, and niacin, which can be formed from the essential amino acid tryptophan, do not stricdy conform to this definition. 

Niacin was discovered as a nutrient during studies of pellagra. It is not strictly a vitamin since it can be synthesized in the body from the essential amino acid tryptophan. Two compounds, nicotinic acid and nicotinamide, have the biologic activity of niacin its metabolic function is as the nicotinamide ring of the coenzymes NAD and NADP in oxidation-reduction reactions (Figure 45-11). About 60 mg of tryptophan is equivalent to 1 mg of dietary niacin. The niacin content of foods is expressed as mg niacin equivalents = mg preformed niacin + 1/60 X mg tryptophan. Because most of the niacin in cereals is biologically unavailable, this is discounted. 

Pellagra Can Occur as a Result of Disease Despite an Adequate Intake of Tryptophan Niacin... 

A number of genetic diseases that result in defects of tryptophan metabolism are associated with the development of pellagra despite an apparently adequate intake of both tryptophan and niacin. Hartnup disease is a rare genetic condition in which there is a defect of the membrane transport mechanism for tryptophan, resulting in large losses due to intestinal malabsorption and failure of the renal resorption mechanism. In carcinoid syndrome there is metastasis of a primary liver tumor of enterochromaffin cells which synthesize 5-hydroxy-tryptamine. Overproduction of 5-hydroxytryptamine may account for as much as 60% of the body s tryptophan metabolism, causing pellagra because of the diversion away from NAD synthesis. 

Shibata, K. and K. Iwai. 1988. Effect of dietary paraquat on the enzymes involved in tryptophan-niacin metabolism in rats. Agric. Biol. Chem. 52 1857-1858. 

Niacin requirements are dependent on the tryptophane supply and the ease with which the conversion of tryptophane to niacin can be made. Chickens and rats carry out the conversion readily. Dogs do so less readily. Monkeys and human beings carry out the process relatively ineffectively. Since this conversion involves several enzymatic steps, it is clear on the basis of gene-enzyme relationships why species differences exist. On the same basis inter-individual differences may be presumed to exist also. 

That individual monkeys have distinctive niacin needs was shown by recent work of Tappan and co-workers.59 One animal, for example, required only 11 weeks to show niacin deficiency weight loss by this animal was halted only when 30 mg. of niacin was given. Another animal required 9 months to show a niacin deficiency and then grew adequately when only 6 mg. of niacin per week was furnished. This seems to show a several-fold range in niacin needs within a small group of fine animals. The tryptophane needs of the different monkeys, as judged by growth responses, were found to vary under comparable conditions from 1 to about 3.5 gm. per week. 

NADP(H) of tryptophan (corn major dietary staple), which supplies a portion of the niacin requirement. 

NAD(H) from niacin (some may be synthesized from tryptophan)... 

Fukawatari, T., Suigimoto, E. and Shibata, K. (2001). Effect of feeding with a poisonous mushroom Clitocybe acromelalga on the metabolism of tryptophan-niacin in rats. Shokuhin Eiseigaku Zasshi, 42, 190-196. 

In contrast to most of the vitamins encountered so far, here we have simple structures. Humans are able to synthesize these molecules from the amino acid tryptophan but not in quantities adequate to meet physiological needs. Consequently, we need to find adequate amounts in our diet. The UL for niacin is 35 mg/day for adult men and women. 

A deficiency of niacin in the diet results in the disease known as pellagra, characterized by the four D s diarrhea, dermatitis, dementia, and death. In the early years of the twentieth century in the United States, pellagra was common among poor tenant farmers and mill workers in the rural South. The diet there at that time was rich in com that contained little niacin and little available tryptophan from which to synthesize it. 

Nicotinate and nicotinamide, together referred to as niacin, are required for biosynthesis of the coenzymes nicotinamide adenine dinucleotide (NAD"") and nicotinamide adenine dinucleotide phosphate (NADP" ). These both serve in energy and nutrient metabolism as carriers of hydride ions (see pp. 32, 104). The animal organism is able to convert tryptophan into nicotinate, but only with a poor yield. Vitamin deficiency therefore only occurs when nicotinate, nicotinamide, and tryptophan are all simultaneously are lacking in the diet. It manifests in the form of skin damage (pellagra), digestive disturbances, and depression. 

Niacin is also known as vitamin PP or vitamin Bj. The term niacin describes two related compounds, nicotinic acid and nicotinamide (Figure 19.18), both with biological activity. Niacin is formed from the metabolism of tryptophan, and therefore it is not strictly a vitamin. It is a precursor of two cofactors nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP), which are essential for the functioning of a wide range of enzymes involved in redox reactions. 

Riboflavin (vitamin Bj) is chemically specified as a 7,8-dimethyl-10-(T-D-ribityl) isoalloxazine (Eignre 19.22). It is a precnrsor of certain essential coenzymes, such as flavin mononucleotide (FMN) and flavin-adenine dinucleotide (FAD) in these forms vitamin Bj is involved in redox reactions, such as hydroxylations, oxidative carboxylations, dioxygenations, and the reduction of oxygen to hydrogen peroxide. It is also involved in the biosynthesis of niacin-containing coenzymes from tryptophan. 

---