Reversible hydration of carbon dioxide. H(+) + hydrogencarbonate = CO2 + H2O Activated by histamine, L-adrenaline, L- and D-histidine, and L- and D-phenylalanine. Inhibited by coumarins, saccharin, sulfonamide derivatives such as acetazolamide (AZA) and Foscarnet (phosphonoformate trisodium salt). High expression in all parts of the central nervous system and lower expression in adult liver, heart, small intestine, colon, kidney, urinary bladder and skeletal muscle. Belongs to the alpha-carbonic anhydrase family.