Coenzymes, niacinamide

Vitamin B5 Niacin (= nicotinic acid) and niacinamide (= nicotinamide) are supplied in the diet and converted in the body into the coenzymes nicotinamide-adenine dinucleotide (NAD) and nicotinamide-adenine dinucleotide phosphate (NADP). These coenzymes are important in tissue respiration. Nicotinic acid can also be formed from tryptophan via kynolin acid. 

The two commercial forms of the vitamin, niacin and niacinamide, are rapidly absorbed from both the stomach and intestine. As the dose increases, absorption decreases. It is not clear whether there is a feedback mechanism operating or the transport system becomes saturated. Conversion to the coenzyme forms occurs in the cells where NAD and NADP are needed. 

The vitamin B complex contains a number of factors which are closely associated in their distribution in nature and have related functions in intermediate metabolism. Of the eleven factors which are available in pure form, five have been shown to be constituents of coenzymes, namely, thiamine, riboflavin, niacinamide, pyridoxine, and pantothenic acid. It seems likely that other B vitamins may be found to function in a similar manner. Two members of the B complex, choline and inositol, appear to have lipotropic activity, and two others, folic acid and vitamin B12, have antianemic properties. Deficiency of vitamins of the B complex is one of the most frequently encountered syndromes of malnutrition in man. 

Niacin, the pellagra-preventive factor, is a unique vitamin in that one of the amino acids, tryptophan, serves as a precursor of this vitamin in many animal and plant species. It is interesting to note that niacinamide was shown to be part of two coenzymes prior to the discovery of its nutritional importance (Chapter 19). 

Two compounds, nicotinic acid and nicotinamide, have the biological activity of niacin. When nicotinic acid was discovered to be a curative and preventive factor for pellagra, it was already known as a chemical compound, and was therefore never assigned a number among the B vitamins. The name niacin was coined in the USA when it was decided to enrich maize meal with the vitamin to prevent pellagra - it was considered that the name nicotinic acid was not desirable because of the similarity to nicotine. In USA, the term niacin is commonly used to mean specifically nicotinic acid, and nicotinamide is known as niacinamide elsewhere niacin is used as a generic descriptor for both vitamers. Figure 2.16 shows the structures of nicotinic acid, niacin and the nicotinamide nucleotide coenzymes, NAD and NADE... 

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 ... 

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, ... 

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