Influence of dietary pyridoxine on glutamic decarboxylase activity of brain.

y-Aminobutyric acid is present in the free form in large amounts in brain (l-3), where it is formed from glutamic acid by a L-glutamic acid decarboxylase (2, 4, 5). The enzyme requires pyridoxal phosphate as coenzyme (5), which can be synthesized in brain from adenosinetriphosphate and pyridoxal (6). This decarboxylase has a high degree of substrate and coenzyme specificity (6). The concentration of vitamin Be was decreased greatly in the tissues of vitamin BG-deficient animals (7) and the activity of the glutamic-aspartic transaminase, a pyridoxal phosphate-requiring system, also was lowered markedly (8, 9). Refeeding of pyridoxine to the deficient animals was found to return the transaminase activity to an almost normal level (9). When a bacterial tyrosine decarboxylase apoenzyme was employed for assay, it was shown that the tissues of rats which received pyridoxine contained several times more codecarboxylase (or pyridoxal phosphate) than did those of animals on a pyridoxine-deficient diet (10). The chief purpose of the present investigation was to determine whether the brain glutamic acid decarboxylase responds to different levels of pyridoxine nutrition in the manner found for transaminase. Additional observations were made on the effects of the feeding of excess pyridoxine to animals on a normal laboratory diet and the administration of desoxypyridoxine, a vitamin Bs antagonist (ll), to rats on a pyridoxine-deficient diet.