Carbonic anhydrase is a zinc metalloenzyme that is found wherever bicarbonate (HCO₃^-) is absorbed or secreted. Wherever it occurs, in the kidney or the salivary glands or indeed anywhere else, the enzyme either catalyses the conversion of carbonic acid (H₂CO₃) into CO₂ and water OR the reverse, the conversion of CO₂ and water into H₂CO₃.

Which way the reaction goes depends on the relative amounts of substrate and product. If, for example [CO₂] is high, the reaction will proceed towards HCO₃^- production. If [HCO₃^-] is high, the reaction heads the other way and CO₂ is produced

The enzyme gets its name from the process of converting H₂CO₃ into CO₂ and water because this process is in effect removing water from H₂CO₃. An (without) hydr (water) ase (enzyme) could be translated as "Without water enzyme", a name given to water removing enzymes in general.

To date (October 2010), 16 members of the αCA gene family have been discovered and there are at least 13 functional isoforms of the enzyme. The vast majority of these are cytosolic, that is within the cytoplasm of cells. In the kidney; CAII is the most widely distributed and active cytosolic form of the enzyme, CAIV is the most extensively expressed membrane bound form, particularly in the proximal tubule (S2 segment) and the distal tubule intercalated cells.

The enzyme is tethered to the apical membrane of the tubule cells by a glycosylphosphatidylinositol (GPI) lipid anchor. Most GPI linked proteins are targeted exclusively to the apical membrane of polarised epithelial cells, CAIV is a bit odd because it is also expressed at the basolateral membrane of proximal tubules. I really don't know why, but this basolateral expression does seem somehow to facilitate HCO₃^- reabsorption.

The lumenal, membrane bound isoform is most important in HCO₃^- absorption because it catalyses the production of CO₂ in the urine, which is central to bicarbonate reabsorbtion.

If you want to know more, you could try the rather chaotic and slightly out-of-date (likely less so than your textbooks) Wikipedia entry on carbonic anhydrase. Alternatively, try the introduction of the paper in PLOS ONE about the most recently discovered isoforms of carbonic anhydrase by Saari et al. The paper by by Purkerson et al in Kidney International about GPI anchors is also worth a look, but be warned, this is a harder read.