Diabetes, Vol 42, Issue 10 1506-1513, Copyright © 1993 by American Diabetes Association

ARTICLES

GABA production in rat islets of Langerhans

M Michalik, J Nelson and M Erecinska
Department of Pharmacology, University of Pennsylvania, School of Medicine, Philadelphia 19104.

Homogenates of pancreatic islets catalyzed breakdown of L-glutamate to GABA with a rate of 0.24 +/- 0.04 nmol.min-1 x mg-1 protein at 37 degrees C. The formation of GABA was stimulated by addition of pyridoxal phosphate in the range 0.05-1 microM (0.97 +/- 0.02 nmol.min-1 x mg protein-1 at a saturating cofactor concentration), which indicates that the process was catalyzed by glutamic acid decarboxylase. The half-maximal effect was obtained with 0.1 microM PLP. Kinetic analyses of the results showed that the Vmax and Km for the reaction were 1.12 nmol.min-1 x mg protein-1 and 0.66 mM, respectively. The pH optimum was 7.0. Subcellular fractionation revealed that 51% of GAD activity was present in the cytosol, 17% in microsomes, 9% in secretory granules, 5% in mitochondria, and 11% in cell debris. Comparison of the kinetic properties of the cytosolic and microsomal forms of the enzyme showed that their Km for glutamate was the same, but that the cytosolic GAD had a lower Km for PLP. GABA synthesis in the nominal absence of PLP was enhanced by malate (twofold increase at 5 mM) and citrate (threefold increase at 5 mM), but was unaffected by ATP and chloride. However, if the islet homogenate was prepared and incubated in the presence of PLP, neither malate nor citrate influenced enzyme activity. Aspartate and AOA were powerful inhibitors of glutamate breakdown. Freshly isolated islets contained approximately 4 mM GABA, whereas the concentration was < 0.1 mM in whole pancreas.(ABSTRACT TRUNCATED AT 250 WORDS)
CiteULike    Del.icio.us    Digg    Reddit    Technorati    What's this?

This article has been cited by other articles:

				A. Wendt, B. Birnir, K. Buschard, J. Gromada, A. Salehi, S. Sewing, P. Rorsman, and M. Braun Glucose Inhibition of Glucagon Secretion From Rat {alpha}-Cells Is Mediated by GABA Released From Neighboring {beta}-Cells Diabetes, April 1, 2004; 53(4): 1038 - 1045. [Abstract] [Full Text] [PDF]

				J. Bustamante, M. V. T. Lobo, F. J. Alonso, N.-T. A. Mukala, E. Gine, J. M. Solis, J. Tamarit-Rodriguez, and R. Martin Del Rio An osmotic-sensitive taurine pool is localized in rat pancreatic islet cells containing glucagon and somatostatin Am J Physiol Endocrinol Metab, December 1, 2001; 281(6): E1275 - E1285. [Abstract] [Full Text] [PDF]

				H. S. Park and H. J. Park Effects of gamma -aminobutyric acid on secretagogue-induced exocrine secretion of isolated, perfused rat pancreas Am J Physiol Gastrointest Liver Physiol, October 1, 2000; 279(4): G677 - G682. [Abstract] [Full Text] [PDF]

				Y. Shi, J. Kanaani, V. Menard-Rose, Y. H. Ma, P.-Y. Chang, D. Hanahan, A. Tobin, G. Grodsky, and S. Baekkeskov Increased expression of GAD65 and GABA in pancreatic beta -cells impairs first-phase insulin secretion Am J Physiol Endocrinol Metab, September 1, 2000; 279(3): E684 - E694. [Abstract] [Full Text] [PDF]

				J. Kanaani, D. Lissin, S. F. Kash, and S. Baekkeskov The Hydrophilic Isoform of Glutamate Decarboxylase, GAD67, Is Targeted to Membranes and Nerve Terminals Independent of Dimerization with the Hydrophobic Membrane-anchored Isoform, GAD65 J. Biol. Chem., December 24, 1999; 274(52): 37200 - 37209. [Abstract] [Full Text] [PDF]

				B. Rubi, H. Ishihara, F. G. Hegardt, C. B. Wollheim, and P. Maechler GAD65-mediated Glutamate Decarboxylation Reduces Glucose-stimulated Insulin Secretion in Pancreatic Beta Cells J. Biol. Chem., September 21, 2001; 276(39): 36391 - 36396. [Abstract] [Full Text] [PDF]