Niacine (Figure 9), a pyridine, contains a positively charged nitrogen at position 1 in the ring. The biologically active forms of niacin are Nicotinamide Adenine Dinucleotide (NAD+) and its phosphorylated derivative, Nicotinamide Adenine Dinucleotide Phosphate (NADP+). In these two forms, an amide is present rather than a carboxyl group that is found on niacin. The carbon in the nicotinamide make it possible to accept a negatively charged hydride ion (colored violet; H: an H atom with 2 electrons) from a certain substrate, like lactate catalyzed by lactate dehydrogenase (Madarcos, 2013).
Figure 9. Structure and biosynthesis of NAD+ and NADP+. Note that a metabolite of tryptophan (quinolinate) can also be used in the synthesis of NAD+. Lippincott’s Illustrated Reviews: Biochemistry 5th ed. (p. 377), by R. A. Harvey & D. R. Ferrier, 2011, Philadelphia: Lippincott Williams & Wilkins, a Wolters Kluwer business. Copyright 201 by Lippincott Williams & Wilkins, a Wolters Kluwer business.
According to Murray et. al. (2009) niacin is not strictly a vitamin since it can be synthesized in the body from the essential amino acid tryptophan. Some 60 mg of tryptophan is equivalent to 1 mg of dietary niacin. Niacin is found in unrefined and enriched grains and cereal, milk, and lean meats, especially liver. Corn-based diets, which are low in both niacin and tryptophan, can cause pellagra, a disease due to the deficiency of niacin in the body. This disease involves the skin, gastrointestinal tract, and CNS. The symptoms of pellagra (Figure 10) progress through the three Ds: dermatitis, diarrhea, dementia—and, if untreated, death (Harvey & Ferrier, 2011).
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Niacin also strongly inhibits lipolysis in adipose tissue, the primary producer of circulating free fatty acids. The liver normally uses these circulating fatty acids as a major precursor for triacylglycerol synthesis. Thus, niacin causes a decrease in liver triacylglycerol synthesis, which is required for very-low density lipoprotein production. Low-density lipoprotein (LDL, the cholesterolrich lipoprotein) is derived from VLDL in the plasma. Thus, both plasma triacylglycerol (in VLDL) and cholesterol (in VLDL and LDL) are lowered. Therefore, niacin is particularly useful in the treatment of Type IIb hyperlipoproteinemia, in which both VLDL and LDL are elevated (Harvey & Ferrier, 2011).