Biological Chemistry - Berezov, T. T., Korovkin, B. F. 1998

Vitamins
Water-Soluble Vitamins
Vitamin B6

Vitamin B₆ (pyridoxine, anti-dermatitic factor) was discovered as an independent, standalone dietary factor by P. György in 1934. Unlike the Water-Soluble Vitamins B₁, B₂, and PP known at the time, it cured a specific form of limb dermatitis in rats known as acrodynia. Vitamin B₆ was first isolated from Yeast and Liver in 1938 and was chemically synthesized shortly thereafter. It proved to be a derivative of 3-hydroxypyridine, specifically 2-methyl-3-hydroxy-4,5-dihydroxymethylpyridine. According to the recommendations of the International Commission for Biochemical Nomenclature, the term “vitamin B₆” refers to all three 3-hydroxypyridine derivatives exhibiting equal vitamin activity: pyridoxine (pyridoxol), pyridoxal, and pyridoxamine:

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As can be seen, these 3-hydroxypyridine derivatives differ from one another in The Nature of the substituent group at position 4 of the pyridine ring. Vitamin B₆ is readily soluble in water and ethanol. Its aqueous solutions are quite stable in acidic and alkaline environments, yet they are sensitive to light in the neutral pH range.

Vitamin B₆ deficiency has been studied in greatest detail in rats, where the most characteristic sign is acrodynia—a specific dermatitis predominantly affecting the Skin of the paws, tail, Nose, and ears. Manifestations include increased skin scaling, Hair loss, ulceration of the extremities ending in Gangrene of the digits. These conditions do not respond to Treatment with Vitamin PP, but resolve rapidly upon the administration of pyridoxine. In cases of more severe B₆ avitaminosis in dogs, pigs, rats, and chickens, epileptiform seizures accompanied by degenerative Changes in the Central Nervous system are observed.

In humans, vitamin B₆ deficiency is less common, although certain pellagra-like dermatoses that fail to respond to nicotinic acid are easily cured by the administration of pyridoxine. In infants, dermatitis and nervous system disorders (including epileptiform seizures) caused by an inadequate pyridoxine content in artificial formula have been described. Pyridoxine deficiency is frequently observed in tuberculosis patients undergoing treatment with isonicotinic acid hydrazide (isoniazid), which, much like deoxypyridoxine, acts as an antagonist of vitamin B₆.

Biochemical abnormalities associated with vitamin B₆ deficiency include homocystinuria and cystathioninuria, as well as impaired Tryptophan METABOLISM, which manifests as increased urinary excretion of xanthurenic acid and a decreased level of excreted kynurenic acid (see Chapter 12).

Biological role. It turned out that while all three 3-hydroxypyridine derivatives possess vitamin properties, only the phosphorylated derivatives of pyridoxal and pyridoxamine perform coenzyme Functions.

The phosphorylation of pyridoxal and pyridoxamine is an enzymatic reaction that proceeds with the participation of specific Kinases. The synthesis of Pyridoxal phosphate, for instance, is catalyzed by pyridoxal kinase, which exhibits its highest activity in Brain tissue. This reaction can be represented by the following equation:

It has been established that the interconversion of pyridoxal phosphate and pyridoxamine phosphate occurs in animal Tissues, particularly in AMINO ACID Transamination and decarboxylation reactions (see Chapter 12).

It should be noted that significant contributions to elucidating the Biological Role of vitamin B₆ and pyridoxal phosphate in Nitrogen metabolism were made by A.E. Braunstein, S.R. Mardashev, E. Snell, D. Metzler, A. Meister, and others. More than 20 pyridoxal Enzymes are known to catalyze key reactions of nitrogen metabolism across All living organisms. For instance, it has been proven that pyridoxal phosphate serves as the prosthetic group for aminotransferases, which catalyze the reversible transfer of an amino group (NH₂ group) from Amino Acids to an α-keto acid, and for amino acid Decarboxylases, which effect the irreversible removal of CO₂ from the carboxyl group of amino acids to yield biogenic amines. The coenzyme role of pyridoxal phosphate has also been established in Enzymatic reactions involving the Non-Oxidative Deamination of Serine and Threonine, The oxidation of tryptophan and kynurenine, the metabolism of Sulfur-Containing Amino Acids, the interconversion of Serine and Glycine (see Chapter 12), as well as in the synthesis of δ-aminolevulinic acid (the precursor of the Hemoglobin heme molecule), and others. Pyridoxine is among the vitamins whose coenzyme role has been studied in the greatest detail. In recent years, the number of newly discovered pyridoxal enzymes has grown rapidly. Thus, for Glycogen phosphorylase activity, the phosphoryl group of pyridoxal phosphate proved essential rather than the aldehyde group. Because of the extensive involvement of pyridoxal phosphate in metabolic processes, vitamin B₆ deficiency results in a diverse range of Amino acid metabolism disorders.

Natural occurrence and daily requirement. Vitamin B₆ is widely distributed in foods of PLANT AND ANIMAL origin. The primary dietary sources of vitamin B₆ for humans include bread, peas, beans, potatoes, meat, Kidneys, liver, and others. In many animal-derived foods, pyridoxine is chemically bound to protein, but it is readily released in the digestive tract through the action of enzymes. The precise daily requirement for pyridoxine in humans has not been definitively established, as it is synthesized by the intestinal microflora in quantities that partially meet the body's needs. Indirect calculations indicate that an adult human should ingest approximately 2 mg of vitamin B₆ per day.