Reversible hydration of carbon dioxide. Its role in saliva is unknown. H(+) + hydrogencarbonate = CO2 + H2O Inhibited by coumarins, sulfonamide derivatives such as acetazolamide (AZA), saccharin and Foscarnet (phosphonoformate trisodium salt). Major constituent of saliva. Belongs to the alpha-carbonic anhydrase family.