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J. Biol. Chem., Vol. 282, Issue 18, 13508-13521, May 4, 2007

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Carbonic Anhydrase II Increases the Activity of the Human Electrogenic Na⁺/ Cotransporter^*

From the Abteilung für Allgemeine Zoologie, Fachbereich Biologie, Technische Universität Kaiserslautern, P. O. Box 3049, D-67653 Kaiserslautern, Germany

Several acid/base-coupled membrane transporters, such as the electrogenic sodium-bicarbonate cotransporter (NBCe1), have been shown to bind to different carbonic anhydrase isoforms to create a "transport metabolon." We have expressed NBCe1 derived from human kidney in oocytes of Xenopus leavis and determined its transport activity by recording the membrane current in voltage clamp, and the cytosolic H⁺ and Na⁺ concentrations using ion-selective microelectrodes. When carbonic anhydrase isoform II (CAII) had been injected into oocytes, the membrane current and the rate of cytosolic Na⁺ rise, indicative for NBCe1 activity, increased significantly with the amount of injected CAII (2–200 ng). The CAII inhibitor ethoxyzolamide reversed the effects of CAII on the NBCe1 activity. Co-expressing wild-type CAII or NH₂-terminal mutant CAII together with NBCe1 provided similar results, whereas co-expressing the catalytically inactive CAII mutant V143Y had no effect on NBCe1 activity. Mass spectrometric analysis and the rate of cytosolic H⁺ change following addition of CO₂/ confirmed the catalytic activity of injected and expressed CAII in oocytes. Our results show that the transport capacity of NBCe1 is enhanced by the catalytic activity of CAII, in line with the notion that CAII forms a transport metabolon with NBCe1.

Received for publication, January 3, 2007 , and in revised form, March 6, 2007.

^* This work was supported by Deutsche Forschungsgemeinschaft Grants DE 231/16-4 and GRK 845. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed. Tel.: 49-0-631-205-2491; Fax: 49-0-631-205-3515; E-mail: h.becker{at}biologie.uni-kl.de.

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