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Metabolism and Enzymology

Proton/L-Glutamate Symport and the Regulation of Intracellular pH in Isolated Mesophyll Cells ¹

Department of Biological Sciences, Brock University, St. Catharines, Ontario, Canada L2S 3A1

Addition of L-[U-¹⁴C]glutamate to a suspension of mechanically isolated asparagus (Asparagus sprengeri Regel) mesophyll cells results in (a) alkalinization of the medium, (b) uptake of L-[U-¹⁴C]glutamate, and (c) efflux of [¹⁴C]4-aminobutyrate, a product of glutamate decarboxylation. All three phenomena were eliminated by treatment with 1 millimolar aminooxyacetate. In vitro glutamate decarboxylase (GAD) assays showed that (a) 2 millimolar aminooxyacetate eliminated enzyme activity, (b) activity was pyridoxal phosphate-dependent, and (c) activity exhibited a sharp pH optimum at 6.0 that decreased to 20% of optimal activity at pH 5.0 and 7.0. Addition of 1.5 millimolar sodium butyrate or sodium acetate to cell suspensions caused immediate alkalinization of the medium followed by a resumption of acidification of the medium at a rate approximately double the initial rate. The data indicate that (a) continued H⁺/L-glutamate contransport is dependent upon GAD activity, (b) the pH-dependent properties of GAD are consistent with a role in a metabolic pH-stat, and (c) the regulation of intracellular pH during H⁺/L-Glu symport may involve both H⁺ consumption during 4-aminobutyrate production and ATP-driven H⁺ efflux.

³ Present address: Department of Physiology, University of Toronto, Toronto, Ontario, M5S 1A1.

⁴ Present address: School of Optometry, University of Waterloo, Waterloo, Ontario, N2L 3G1.

¹ Supported by grant No. A4453 from the Natural Sciences and Engineering Research Council of Canada to A.W.B.

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