Carbonic anhydrases (CAs) catalyse the reversible hydration of carbon dioxide to bicarbonate, and therefore produce as well as consume the substrate of bicarbonate transporters. In mammals, 16 different CA isoforms are described. The isoforms used in this study, CA I, II and III, are all localised in the cytosol. They show differences in their catalytic activity and their affinity to sulfonamides. Whereas CA II is highly active due to an effective proton transfer from the active site to the environment, CA I shows only moderate and CA III low activity. An important family of acid/base transporters is the solute carrier family 4 (SLC4), which transports HCO₃^- across the cell membrane. The sodium-bicarbonate cotransporter (NBCe1) belongs to this family, and is an electrogenic transporter which cotransports Na⁺ together with HCO₃^-. Heterologously expressed in Xenopus oocytes, it shows a stoichiometry of 2 HCO₃^- : 1 Na⁺. Some bicarbonate transporters are able to bind carbonic anhydrase, forming a complex called 'transport metabolon'. Referring to the interaction between the bicarbonate transporter NBCe1 and carbonic anhydrase, Alvarez et al. (2003, Biochemistry 42: 12321–12329) showed a direct interaction between NBCe1 and the extracellular CA IV. A transport metabolon with CA II was also suggested (Gross et al., 2002, J Physiol 544: 679–685; Pushkin et al., 2004, J Physiol 559: 55–65; Becker & Deitmer, 2007, J Biol Chem 282: 13508–13521).

We have expressed human kidney NBCe1 in Xenopus laevis oocytes and determined its transport activity by measuring the membrane current in two-electrode voltage-clamp, and the cytosolic proton and sodium concentrations using ion-selective microelectrodes. After co-expression of NBCe1 together with CA I, II or III, we could observe an increase in NBCe1 activity. As control, we injected CA I protein directly into NBCe1-expressing oocytes. Again, we were able to detect an enhancing effect on NBCe1 activity. The increase of NBCe1 activity mediated by the CA isoforms I and II could be reversed by the CA inhibitor ethoxyzolamide, while CA III was largely insensitive to sulfonamides.

Our results show that the transport rate of NBCe1 is enhanced by the catalytic activity of all three isoforms CA I, II and III.

Supported by the GRK 845 of the DFG, and the LSP 'Membrantransport'.