Functions of vitamin B
The metabolic significance of the Bs vitamers depends on their conversion to pyridoxal phosphate (P ALP), which functions as a coenzyme in numerous biochemical reactions, most of which in some way involve amino acids. PALP-dependent aminotransferases (transaminases) carry the
found in phosphorylase, the enzyme that initiates the mobiliza tion of glycogen stores by forma tion of glucose-lphosphate. Presence of PALP appears to be essential for .be activity ofthe enzyme, but its exact catalytic function ill the reaction is not known. Because administration of excess vitamin Bs increases the level of phosphorylase parallel to the increase in tissue Bs retention, phosphorylase, in turn, may serve as a tissue reservoir of ,itamin BS. Recent research indicates that PALP also may serve as a modulator of steroid hormone action by reacting with the activated hormone-receptor complex to inhibit its binding to DNA and cell nuclei.
Cellular uptake of amino acids is impaired in vitamin 6s deficiency. Results from recent studies suggest that the defect in amino acid transport is related to the reduced levels of growth hormone and, perhaps, insulin, instead of the direct effect by the lack of vitamin BS. The mecharrism by which vitamin Bs influences the secretion of these tlormones is not known.
Several drugs act as vitamin Bs antagonists. These clude isoniazid and cycloserine, which may be used in tuberculosis therapy, and penicillamine, which is used as 3. copper-chelator in Wilson's disease. De;oxypyridoxine has been used in the production of experinental vitamin Bs deficiency. Inadequate intake of vitamin B by infants may result n growth retardation or weight loss, vomiting and ablominal stress, hyperirritability, convulsive seizures, and memia. Vitamin B deficiency in adults may cause periOnality changes, such as depression and confusion, ablOrmal changes in electroencephalograms, convulsions, ieborrheic dermatitis, and oral lesions resembling those used by riboflavin and niacin deficiency. Biochemical Llterations usually precede the clinical manifestations.
