BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 8. VITAMINS

8.3. Water-soluble vitamins

8.3.4. Vitamin PP — Niacin, Nicotinic Acid, Nicotinamide

Chemical Structure and physicochemical properties. Vitamin PP is a pyridine derivative containing a carboxyl or amide group (Fig. 8.15). Nicotinic acid was first obtained by The oxidation of nicotine, a tobacco alkaloid, which gave the compound its name. The Biological Significance of this substance was elucidated in 1935 following the ISOLATION OF PURE NADP from erythrocytes. Nicotinamide forms colorless crystals with a melting point of 131-132 °C and is highly soluble in Water.

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Fig. 8.15. Vitamin PP

Biochemical Functions. Nicotinic acid and nicotinamide are essential for The Biosynthesis of two Coenzymes: nicotinamide adenine dinucleotide (NAD+, NAD⁺) and nicotinamide adenine dinucleotide phosphate (NADP⁺, NADP⁺), which are components of A large number of dehydrogenases catalyzing oxidation-reduction reactions (Fig. 8.16).

Unlike flavin NUCLEOTIDES, NAD and NADP do not form covalent bonds with the apoenzyme, act as stronger reducing agents, and are incapable of direct oxidation by molecular oxygen. The biosynthesis of NAD in the body occurs in two steps:

Fig. 8.16. Structure of NAD and NADP.

First, nicotinamide attaches to phosphoribosyl pyrophosphate (PRPP) via a specific phosphorylase, after which AMP (donated by ATP) is added to the resulting nicotinamide mononucleotide.

NADP is formed through the phosphorylation of NAD by NAD kinase (Fig. 8.16). Examples of NAD-dependent Enzymes include Alcohol dehydrogenase, Lactate dehydrogenase, and malate dehydrogenase.

Daily requirement and natural occurrence. Niacin deficiency leads to pellagra, a disease characterized by three classic symptoms: Skin lesions (dermatitis), gastrointestinal tract disorders (diarrhea), and neurological impairment (dementia). Specific forms of dermatitis develop symmetrically on sun-exposed areas of the skin. Gastrointestinal involvement is accompanied by nausea, abdominal pain, as well as inflammation of the Gums and Tongue; cerebral symptoms include headaches, dizziness, irritability, and depression. The primary dietary sources of the vitamin include Liver, meat, and buckwheat. The daily requirement is 15-25 mg. Excessive intake from food or injections causes cutaneous hyperemia, pruritus, rashes, dyspepsia, and impaired liver function. Vitamin PP can also be synthesized from Tryptophan, with 1 molecule of the vitamin being produced from every 60 molecules of this amino acid.