L‐GLUTAMIC ACID DECARBOXYLASE IN NON‐NEURAL TISSUES OF THE MOUSE

Abstract

Abstract— Low levels of γ‐aminobutyric acid (GABA) and of glutamic acid decarboxylase (GAD) activity have been detected in mouse kidney, liver, spleen and pancreas. Quantitation of both 14 CO 2 and [ 14 C]GABA produced in radiometric assays from [U‐ 14 CJglutamic acid has shown that measurement of 14 CO 2 evolution alone is not, in all cases, a valid estimate of true GAD activity. As evidenced by increased ,14 CO 2 production upon addition of NAD and CoA to assay mixtures, radiometric assay of GAD activity in crude homogenates may yield 14 CO 2 via the coupled reactions of glutamic acid dehydrogenase and a‐ketoglutarate dehydrogenase. The addition of 1 mM aminooxyacetic acid (AOAA) to assays of kidney homogenates inhibited [ ,14 C]GABA production 92 per cent while 14 CO 2 production was inhibited only 53 per cent. No evidence was found to confirm the reported existence of a second form of the enzyme, GAD II. previously described by Haber el al. (H aber B., K uriyama K. & R oberts E. (1970) Biochem. Pharmac. 19, 1119‐1136). Based on sensitivity‐to AOAA and chloride inhibition, the GAD activity in mouse kidney is. apparently, indistinguishable from that of neural origin.