Fundamentals of Biochemistry - A. A. Anisimov 1986

Vitamins
Vitamin-like substances, vitamin interactions, antivitamins

10.4.1. Vitamin-like substances. This group typically encompasses a diverse array of chemical compounds that exhibit vitamin-like properties, though they are partially synthesized within the body and sometimes incorporated into tissue structures.

Vitamin B₁₃ (orotic acid). This substance is classified as vitamin-like because it has The ability to promote the growth of microorganisms and higher animals; it is a pyrimidine derivative (see Section 4.2.1). Sources of vitamin B₁₃ include Liver, milk, and Yeast. In birds and mammals, orotic acid is synthesized from aspartic acid and carbamoyl phosphate, and it participates in The formation of Nucleic Acids. Orotic acid is the only cyclic compound incorporated into pyrimidine NUCLEOTIDES after exogenous administration, which allows it to stimulate PLANT AND ANIMAL growth as well as anabolic processes. The potassium salt of vitamin B₁₃ (potassium orotate) is used in feeding premature infants, in treating liver and Heart diseases, and for stimulating red Blood Cell production in certain forms of anemia.

Vitamin B₁₅ (pangamic acid). This is an ester of gluconic acid and dimethylglycine. Pangamic acid has been isolated from yeast, liver, bovine blood, and rice germ.

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It contains two labile methyl groups, making vitamin B₁₅ a lipotropic factor that improves Lipid METABOLISM and prevents fatty liver infiltration. Vitamin B₁₅ promotes the synthesis of creatine phosphate (see Section 7.4.2) and activates oxidative processes, which leads to a reduction in Muscle fatigue. In addition, the vitamin exerts a detoxifying effect in acute and chronic poisoning caused by drugs, alcohol, organochlorine compounds, and tetracycline Antibiotics. The human physiological requirement for vitamin B₁₅ is unknown; for therapeutic purposes, pangamic acid is used in doses of 0.1–0.3 g.

As a component of Folic acid, this compound activates the Synthesis of Purines and Pyrimidines, thereby participating in nucleic acid synthesis; it influences thyroid function, increases tolerance to oxygen deprivation, inhibits The Development of experimental atherosclerosis, and acts as a GROWTH AND REPRODUCTION factor for microorganisms. Structural analogues of Para-aminobenzoic Acid—sulfonamides—are used as bacteriostatic agents in infectious diseases because, as structural analogues of para-aminobenzoic acid, they act as its antagonists and can competitively replace it in the enzyme systems of pathogenic microbes. Derivatives of para-aminobenzoic acid such as novocaine and anesthesine possess local anesthetic activity. Para-aminobenzoic acid is widely distributed in foods: yeast, liver, Kidneys, heart, and mushrooms, and in somewhat smaller amounts in milk, eggs, potatoes, carrots, spinach, and wheat.

Vitamin N (Lipoic Acid). Lipoic acid is an organic acid containing a disulfide bridge. The five-membered ring Structure OF THE molecule, which includes the disulfide bond, is essential for the biological activity of vitamin N. Lipoic acid is widely distributed in plants and microorganisms, with the highest concentrations found in Mitochondria and Chloroplasts. The estimated human daily requirement for lipoic acid is 1–2 mg. Lipoic acid performs a specific coenzyme role in The oxidative decarboxylation of a-keto acids, in The transfer of acyl residues, and in the coupled oxidation-reduction transformations (see Section 6.9.5). Consequently, lipoic acid participates in energy release and biosynthetic processes. It is used in the Treatment of atherosclerosis, certain liver diseases, Diabetes Mellitus, and Various Forms of intoxication.

It is found in raw vegetable juices, especially cabbage juice. It serves as an active form of The amino acid Methionine. A crystalline preparation isolated from asparagus and fresh tomatoes has been shown to be 1,000 times more active than cabbage juice. The stimulating effect of methylmethionine in repairing damaged gastrointestinal mucosa is attributed to its role as an active methyl group donor. In addition, it participates in the synthesis of choline and creatine.

Vitamin F (essential polyunsaturated Fatty acids, or EPUFAs). Essential Fatty Acids (EFAs) are fatty acids that cannot be synthesized by the animal body in quantities sufficient for growth, development, and the normal progression of various physiological processes. A deficiency of vitamin F primarily results in growth arrest. Severe Skin lesions occur, characterized by ulceration, focal necrosis, and altered pigmentation due to impaired melanin metabolism. Hair becomes brittle and falls out, and Nails become coarse and prone to splitting. It is believed that The Role of EFAs in metabolic processes is mediated by Prostaglandins, which are EFA derivatives. Because prostaglandins actively influence cAMP metabolism—and consequently a wide range of reactions associated with the metabolism and Functions of most Hormones—the symptoms of EFA deficiency are highly diverse (see Sections 12.3 and 12.4).

10.4.2. Vitamin Interactions. A substantial body of data on Vitamins indicates that specific interactions exist among them. Some vitamins act as antagonists to one another (e.g., thiamine and nicotinic acid), others exhibit synergy (e.g., ascorbic acid and bioflavonoids), and still others are capable of mutual substitution (e.g., vitamins K and E, or K and A, in certain aspects of their action). The phenomenon of vitamin interaction is crucial both for understanding the mechanisms of vitamin action and for their proper combined use in clinical practice.

10.4.3. Antivitamins. This group comprises substances that interfere with the utilization of vitamins by living Cells through various mechanisms, thereby inducing a state of vitamin deficiency. Based on their MECHANISM OF ACTION, all antivitamins can be divided into two groups.

1. Substances that interact directly with a vitamin, causing it to lose its biological activity. A prime example is Avidin, an egg protein that binds to biotin to form an insoluble avidin-biotin complex that is not absorbed in the intestine, leading to vitamin H deficiency. Other Examples in this group include ascorbate oxidase, which oxidizes ascorbic acid, and thiaminase, which degrades thiamine.

2. Structural analogs of vitamins in which a specific functional group is replaced, resulting in the loss of vitamin activity by the molecule. This represents a specific case of typical antimetabolites. Examples include sulfonamides, which act as competitors of para-aminobenzoic acid, and coumarin, a competitor of vitamin K.