Niacin pyridine nucleotide coenzymes

Preformed niacin occurs in foods either as nicotinamide (niacinamide) or as the pyridine nucleotide coenzymes derived from it, or as nicotinic acid, without the amide nitrogen, which is the form known as niacin in North America. Both nicotinamide and nicotinic acid are equally effective as the vitamin, but in large doses they exert markedly different pharmacological effects, so it is important, at least in that context, to make and maintain the distinction. In addition to the preformed vitamin, an important in vivo precursor is the amino acid L-tryptophan, obtained from dietary protein. Because the human total niacin supply, and hence niacin status, depends on the dietary tryptophan supply as well as on the amount of preformed dietary niacin and its bioavailability, it has become the accepted practice to express niacin intakes as niacin equivalents, ... 

Of the two pyridine nucleotide coenzymes, NAD is present mainly as the oxidized form in the tissues, whereas NADP is principally present in the reduced form, NADPH2. There are important homeostatic regulation mechanisms which ensure and maintain an appropriate ratio of these coenzymes in then-respective oxidized or reduced forms in healthy tissues. Once converted to coenzymes within the cells, the niacin therein is effectively trapped, and can only diffuse out again after degradation to smaller molecules. This implies, of course, that the synthesis of the essential coenzyme nucleotides must occur within each tissue and cell type, each of which must possess the enzymatic apparatus for their synthesis from the precursor niacin. Loss of nicotinamide and nicotinic acid into the urine is minimized (except when the intake exceeds requirements) by means of an efficient reabsorption from the glomerular filtrate. 

Niacin is found in the body tissues largely as part of two important coenzymes, nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) together, NAD and NADP are known as the pyridine nucleotides. The structure of NAD is given in Fig. N-4. 

Through the study of Roth el al. with carboxyl C -labeled niacin, it has been estimated that the turnover time of niacin in tissues is from 4 to 8 days 19ff). More recent data, however, indicate that in liver the half-life of the pyridine nucleotides may be considerably shorter 197). The observation that the injection of nicotinamide into mice can produce large increases in the levels of liver DPN has provided a useful means of studying the in vivo metabolism of the P3uidine coenzymes. The results obtained with this method of approach are presented in this section. 

Pyridine nucleotides are intimately associated with niacin, another B vitamin (the pellagra-preventive factor). Niacin is a simple pyridine derivative, pyridine-3-carboxylic acid or nicotinic acid. The amide (niacinamide, nicotinamide) also prevents pellagra. The vitamin appears in the coenzyme in this form ... 

Two vitamins, nicotinamide and pyridoxine (vitamin B6), are pyridine derivatives. Nicotinamide participates in two coenzymes, coenzyme I (65 R = H) which is known variously as nicotinamide adenine dinucleotide (NAD) or diphosphopyridine nucleotide (DPN), and coenzyme II (65 R = P03H2) also called triphosphopyridine nucleotide (TPN) or nicotinamide adenine dinucleotide phosphate (NADP). These are involved in many oxidation-reduction processes, the quaternized pyridine system acting as a hydrogen acceptor and hydrogen donor. Deficiency of nicotinamide causes pellagra, a disease associated with an inadequately supplemented maize diet. Nicotinic acid (niacin) and its amide are... 

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