Niacin, 264,

Niacin has been shown to work on occasion. Byrd Labs tests conclude that niacin doesn t work at all. In other words, something else probably caused a negative, not the niacin. 

Niacin is unusual among the vitamins in that it was discovered as a chemical compound, nicotinic acid produced by the oxidation of nicotine, in 1867 -long before there was any suspicion that it might have a role in nutrition. Its metabolic function as part of what was then called coenzyme II [nicotinamide adenine dinucleotide phosphate (NADP)] was discovered in 1935, again before its nutritional significance was known. 

It is not strictly correct to regard niacin as a vitamin. Its metabolic role is as the precursor of the nicotinamide moiety of the nicotinamide nucleotide coenzymes, nicotinamide adenine dinucleotide (NAD) and NADP, and this can also be synthesized in vivo from the essential amino acid tryptophan. At least in developed countries, average intakes of protein provide more than enough tryptophan to meet requirements for NAD synthesis without any need for preformed niacin. It is only when tryptophan metabolism is disturbed, or intake of the amino acid is inadequate, that niacin becomes a dietary essential. 

The nicotinamide nucleotide coenzymes function as electron carriers in a wide variety of redox reactions. In addition, NAD is the precursor of adenine dinucleotide phosphate (ADP)-ribose for ADP-ribosylation and poly(ADP-ribosylation) of proteins and cADP-ribose and nicotinic acid adenine dinucleotide phosphate (NAADP). They act as second messengers and stimulate increases in intracellular calcium concentrations. 

Pellagra was first described as mal de la rosa in Asturias in central Spain by Casal in 1735. He observed that the condition was apparently related to diet and was distinct from scurvy and other then known causes of superficially similar dermatitis. The name pellagra was coined by the Italian physician Frapolli in 1771 to describe the most striking feature of the disease the roughened, sunburn-like appearance of the skin. Pellagra became common in Europe 

After it had been established that pellagra was a nutritional deficiency disease, the next problem was to discover the missing nutrient. Additional dietary protein was shown to be beneficial, thus it was concluded that pellagra was because of a protein deficiency. This view, and later that it was more specifically from a deficiency of tryptophan, was held for some time. In 1938, Spies and coworkers showed that nicotinic acid would cure pellagra thereafter it was gradually accepted that it was a niacin deficiency disease. 

Niacin is a generic term which refers to two related chemical compounds, nicotinic acid (6.22) and its amide, nicotinamide (6.23) both are derivatives of pyridine. Nicotinic acid is synthesized chemically and can be easily converted to the amide in which form it is found in the body. Niacin is obtained from food or can be synthesized from tryptophan (60 mg of dietary tryptophan has the same metabolic effect as 1 mg niacin). Niacin forms part of two important co-enzymes, nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP), which are co-factors for many enzymes that participate in various metabolic pathways and function in electron transport. 

The classical niacin deficiency disease is pellagra, which is characterized by symptoms including diarrhoea, dermatitis, dementia and eventually death. High-protein diets are rarely deficient in niacin since, in addition to the preformed vitamin, such diets supply sufficient tryptophan to meet dietary requirements. Large doses of niacin can cause the dilation of capillaries, resulting in a painful tingling sensation. 

The RDA for niacin is expressed in terms of energy intake 6.6 mg niacin equivalent (NE, 1 mg niacin or 60 mg tryptophan) per 1000 kcal (4186kJ) per day is recommended (13NEday minimum). This is approximately equivalent to 19 and 15 mg NE day for men and women, respectively. The UK RNI value for niacin is 6.6 mg NE per 1000 kcal (4186 kJ) per day for adults. The richest dietary sources of niacin are meat, poultry, fish and whole-grain cereals. 

Milk contains about 0.1 mg niacin per 100 g and thus is not a rich source of the preformed vitamin. Tryptophan contributes roughly 0.7 mg NE per 100 g milk. In milk, niacin exists primarily as nicotinamide and its concentration does not appear to be affected greatly by breed of cow, feed, season or stage of lactation. Pasteurized goats (0.3 mg niacin and 0.7 mg NE from tryptophan per 100 g) and raw sheep s (0.4 mg niacin and 1.3 mg NE from tryptophan per 100 g) milk are somewhat richer than cows milk. Niacin levels in human milk are 0.2 mg niacin and 0.5 mg NE from tryptophan per 100 g. The concentration of niacin in most dairy products is low (Appendix 6A) but is compensated somewhat by tryptophan released on hydrolysis of the proteins. 

Niacin is relatively stable to most food-processing operations. It is stable to exposure to air and resistant to autoclaving (and is therefore stable to pasteurization and UHT treatments). The amide linkage of nicotinamide can be hydrolysed to the free carboxylic acid (nicotinic acid) by treament with acid but the vitamin activity is unaffected. Like other water-soluble vitamins, niacin can be lost by leaching. 

Niacin is not strictly a vitamin, as it can be synthesized in the body from the essential amino acid tryptophan. Indeed, it is only when tryptophan metabolism is deranged that dietary preformed niacin becomes important. Nevertheless, niacin was discovered as a nutrient during studies of the deficiency disease pellagra, which was a major public health problem in the southern USA throughout the first half of the twentieth century, and continued to be a problem in parts of India and sub-Saharan Africa until the 1990s. 

A number of studies have investigated the equivalence of dietary tryptophan and preformed niacin as precursors of the nicotinamide nucleotides, generally by determining the excretion of niacin metabolites in response to test doses of the precursors in subjects maintained on deficient diets. There is a considerable variation between subjects in the response to tryptophan and niacin, and in order to allow for this it is generally assumed that 60 mg of tryptophan is equivalent to 1 mg of preformed 

Changes in hormonal status may result in considerable changes in this ratio, with between 7 and 30 mg of dietary tryptophan equivalent to 1 mg of preformed niacin in late pregnancy. The intake of tryptophan also affects the ratio, and at low intakes 1 mg of tryptophan may be equivalent to only 1/125 mg preformed niacin. 

The niacin content of foods is generally expressed as mg niacin equivalents 1 mg niacin equivalent = mg preformed niacin + 1/60 X mg tryptophan. Because most of the niacin in cereals is biologically unavailable (section 11.8.1.1), it is conventional to ignore preformed niacin in cereal products. 

Chemical analysis reveals niacin in cereals (largely in the bran), but this is biologically unavailable, as it is bound as niacytin - nicotinoyl esters to polysaccharides, polypeptides and glycopeptides. 

Pellagra was originally thought to be due to inadequate dietary niacin and tryptophan. In some parts of the world, it is associated with consumption of diets high in maize (American corn), which, like other cereal grains, is relatively deficient in tryptophan and niacin. In addition, about 20% of the niacin in maize is protein-bound and not biologically available. Several bound forms of niacin have been characterized. They include niacinogens (peptides of M.W. 12,000-13,000) and niacytin, isolated from wheat (M.W. 2370). 

Pellagra is currently thought to be due to imbalance of dietary amino acids and deficiency of niacin. The common variety of maize is rich in leucine, which inhibits synthesis of nicotinic acid mononucleotide and causes deficiency of NAD+ and NADP+. A strain of maize known as opaque 2 contains less leucine and does not cause pellagra unless excess leucine is added to the diet. 

How do the special transcription factors regulate the activities of the basal transcription factors Information on this issue is only beginning to become available. One might expect, for example, to find a scenario where RXR/VDR directly contacts TFIIA, to stimulate the activity of the initiation complex. 

The researcher always needs to take care in the quest for discovering new response elements and new transcription factors. Just because a sequence in DNA resembles the order of bases in an established response element does not mean that it actually binds any transcription factor. Just because a specific DNA sequence binds a transcription factor does not mean that it actually functions to regulate any gene in the living cell. 

Niacin is a water-soluble vitamin. The RDA of niacin for the adult man is 19 mg. Niacin is converted in the body to the cofactor nicotinamide adenine dinucleotide (NAD). NAD also exists in a phosphorylated form, NADR The phosphate group occurs on the 2-hydroxyl group of e AMP half of the coenzyme. NAD and NADP are used in the catalysis of oxidation and reduction reactions. These reactions are called redox reactions. NAD cycles between the oxidized form, NAD, and the reduced form, NADH + H . The coenzyme functions to accept and donate electrons. NADP behaves in a similar fashion. It occurs as NADP and NADPH + 

The utilization of NAD is illustrated in the sections on glycolysis, the malate-aspartate shuttle, ketone body metabolism, and fatty acid oxidation. The utilization of NADP is illustrated in the sections concerning fatty acid synthesis and the pentose phosphate pathway. 

NAD tends to be an electron acceptor in catabolic reactions involving the degradation of carbohydrates, fatty acids, ketone bodies, amino acids, and alcohol. NAD is used in energy-producing reactions. NADP, which is cytosolic, tends to be involved in biosynthetic reactions. Reduced NADP is generated by the pentose phosphate pathway (cytosolic) and used by cytosolic pathways, such as fatty acid biosynthesis and cholesterol synthesis, and by ribonucleotide reductase. The niacin coenzymes are used for two-electron transfer reactions. The oxidized form of NAD is NAD . There is a positive charge on the cofactor because the aromatic amino group is a quaternary amine. A quaternary amine participates in four 

FIGURI 9,64 NAD accepts two electrons and ore proton. When used as a sub.slratc by enzymes, NAD is a 2-eleclron acceptor and a 2-electron donor. However, when acting purely as a chemical, in the absence of enzymes, NAD (or NADH + H ) can accept or donate a single electron. 

FIGURE 9.65 A (facing page). InvolvcinctU of NAD for ADP-ribosylation reactions. Four molecules of NAD arc shown for the synthesis of a polymer consisting uf four residues of ADP-ribose. P (above). Cyclic ADP-ribose, a molecule thought to be used for cell-signaling, is made from NAD. 

Nicotinic acid was first isolated by the Polish-American biochemist Casimir Funk (1884-1967) in 1912. At the time, 

Niacin. Red atoms are oxygen white atoms are hydrogen black atoms are carbon blue atom is introgen. Gray sticks indicate double bonds. 

Niacin is synthesized naturally in the human body beginning with the amino acid tryptophan. Tryptophan occurs naturally in a number of foods, including dairy products, 

Niacin plays a number of essential roles in the body. It is necessary for cell respiration metabolism of proteins, fats, and carbohydrates the release of energy from foods the secretion of digestive enzymes the synthesis of sex hormones and the proper functioning of the nervous system. It is also involved in the production of serotonin, an essential 

METABOLISM Process that includes all of the chemical reactions that occur in cells by which fats, carbohydrates, and other compounds are broken down to produce energy and the compounds needed to build new cells and tissues. 

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Pellagra is a disease caused by a deficiency of niacm (C6FI5NO2) in the diet Niacin can be synthesized in the laboratory by the side chain oxidation of 3 methylpyndine with chromic acid or potassium permanganate Suggest a reasonable structure for niacin... 

VITAT S - NIACINE,NICOTINAMIDEAND NICOTINIC ACID] (Vol25)... 

Vitamin B3. See Vitamins, Niacin, Nicotinamide, and Nicotinic acid. 

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