Vitamin 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, ... 

Riboflavin was first isolated from whey in 1879 by Blyth, and the structure was determined by Kuhn and coworkers in 1933. For the structure determination, this group isolated 30 mg of pure riboflavin from the whites of about 10,000 eggs. The discovery of the actions of riboflavin in biological systems arose from the work of Otto Warburg in Germany and Hugo Theorell in Sweden, both of whom identified yellow substances bound to a yeast enzyme involved in the oxidation of pyridine nucleotides. Theorell showed that riboflavin 5 -phosphate was the source of the yellow color in this old yellow enzyme. By 1938, Warburg had identified FAD, the second common form of riboflavin, as the coenzyme in D-amino acid oxidase, another yellow protein. Riboflavin deficiencies are not at all common. Humans require only about 2 mg per day, and the vitamin is prevalent in many foods. This vitamin... 

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 ... 

Flavins can serve as either one-electron or two-electron transfer reagents, acting at the interface between the cytochromes and NADH. The vitamin, riboflavin, is converted by the body to the catalytically active coenzyme forms, FMN or FAD. Flavoproteins are involved in oxidation reactions with substrates which include the pyridine nucleotides, a-amino acids and a-hydroxy acids, and compounds containing saturated carbon-carbon bonds convertible into olefins. When oxidation occurs, the isoalloxazine ring of FMN (Fig. 8) or FAD becomes reduced. 

Several classes of vitamins are related to, or are precursors of, coenzymes that contain adenine nucleotides as part of their structure. These coenzymes include the flavin dinucleotides, the pyridine dinucleotides, and coenzyme A. The adenine nucleotide portion of these coenzymes does not participate actively in the reactions of these coenzymes rather, it enables the proper enzymes to recognize the coenzyme. Specifically, the adenine nucleotide greatly increases both the affinity and the speeifieity of the coenzyme for its site on the enzyme, owing to its numerous sites for hydrogen bonding, and also the hydrophobic and ionic bonding possibilities it brings to the coenzyme structure. 

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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