Vitamin B
Pyridoxine was identified in 1938 as a separate fraction of the B complex. Subsequently, vitamin B proved to be a complex of three closely related chemical compounds-pyridoxine (PN), pyridoxal (PL), and pyridoxamine (PM) all of which are active physiologically.The need for this factor was first demonstrated in rats but it is now established that it is required by most s. A deficiency is associated vvith a peculiar type of anemia in some species and extreme muscular weakness, titis, and nervous disorders in others. Vitamin Bs be synthesized by intestinal organisms in the rat, but whether or not this is true in humans has not been established.
The need for and the function of vitamin Bs in humans has been demonstrated conclusively in both adults and infants. The accidental destruction of this factor in a canned-milk formula resulted in the occurrence of ner-
amino group (NH2) from one amino acid to an acceptor keto acid to produce a different amino and keto acid, in a process called transamination. Transamination is the first step in the utilization of most amino acids for energy, and also in the synthesis of nonessential amino acids. Aminotransferases are mostly cellular enzymes, but small amounts of some are normally present in the blood serum. Elevation in serum aminotransferase activity is associated with recent tissue damage. Aspartate aminotransferase (also known as serum glutamate oxaloacetate transaminase or SGOT) is especially active in the cardiac muscle. Its activity in serum rises sharply following myocardial infarction and is widely used as a diagnostic test to confirm infarction. In liver damage, elevation of serum alanine aminotransferase (serum glutamate pyruvate transaminase or SGPT) generally is more pronounced than that of SGOT. Both of these aminotransferases are found in the blood cells, and their activity in the erythrocytes has been employed as a measure of vitamin Be status, especially in metabolic research.
PALP-containing enzymes are also involved in decarboxylation and transuifUration (removal of CO2 and H2S groups) of amino acids. Chemical changes in the central nervous system, that is, formation of serotonin from tryptophan and gamma aminobutyric acid (GABA) from glutamic acid, require vitamin Be-dependent decarboxylases, as does forma hon of norepinephrine from tyrosine (through dopa and dopamine). Several P ALP-dependent enzymes are involved in the metabolic pa th ways of sulfurcontaining amino acids, which result in the conversion of methionine to cysteine and cysteine to taurine. Two of these enzymes are affected in genetic Be-dependency syndromes in which defective enzyme proteins are unable to function "vith physiologic levels of PALP; however, they respond to pharmacologic doses of the vitamin, varying from 10 mg to 1000 mg, in contrast to a normal requirement of approximately 2 mg.
Several other reactions in the metabolism of tryptophan require PALP, including its conversion to niacin (NAD). In Be deficiency, formation of niacin is reduced, and more tryptophan is metabolized by alternate pathways, which are normally insignificant. If a large dose of tryptophan (tryptophan load test) is administered to a person with a vitamin Be deficiency, various intermediate and alternate products-including xanthurenic acidaccumulate in the urine. Excretion of xanthurenic acid after a tryptophan load is, therefore, an indirect measure of vitamin B status, which is used for clinical and research purposes.
Vitamin Be is required also for the formation of deltaamino-levulinic acid in the synthesis of heme and in the metabolism of folic acid. A significant fraction of vitamin Be in tissues, especially the liver and muscle, is vous irritability and convulsive seizures in young infants. Rapid recovery followed injection of the vitamin, proving conclusively that the symptoms noted were the result of a deficiency
