A detailed inhibition study of five carbonic anhydrase (CA, EC 4.2.1.1) isozymes with carboxylates including aliphatic (formate, acetate), dicarboxylic (oxalate, malonate), hydroxy/keto acids (l-lactate, l-malate, pyruvate), tricarboxylic (citrate), or aromatic (benzoate, tetrafluorobenzoate) representatives, some of which are important intermediates in the Krebs cycle, is presented. The cytosolic isozyme hCA I was strongly activated by acetate, oxalate, pyruvate, l-lactate, and citrate (K(A) around 0.1 microM), whereas formate, malonate, malate, and benzoate were weaker activators (K(A) in the range 0.1-1mM). The cytosolic isozyme hCA II was weakly inhibited by all the investigated anions, with inhibition constants in the range of 0.03-24 mM. The membrane-associated isozyme hCA IV was the most sensitive to inhibition by carboxylates, showing a K(I) of 99 nM for citrate and oxalate, of 2.8 microM for malonate and of 14.5 microM for pyruvate among others. The mitochondrial isozyme hCA V was weakly inhibited by all these carboxylates (K(I)s in the range of 1.67-25.9 mM), with the best inhibitor being citrate (K(I) of 1.67 mM), whereas this is the most resistant CA isozyme to pyruvate inhibition (K(I) of 5.5mM), which may be another proof that CA V is the isozyme involved in the transfer of acetyl groups from the mitochondrion to the cytosol for the provision of substrate(s) for de novo lipogenesis. Furthermore, the relative resistance of CA V to inhibition by pyruvate may be an evolutionary adaptation of this mitochondrial isozyme to the presence of high concentrations of this anion within this organelle. The transmembrane, tumor-associated isozyme hCA IX was similar to isozyme II in its slight inhibition by all these anions (K(I) in the range of 1.12-7.42 mM), except acetate, lactate, and benzoate, which showed a K(I)>150 mM. The lactate insensitivity of CA IX also represents an interesting finding, since it is presumed that this isozyme evolved in such a way as to show a high catalytic activity in hypoxic tumors rich in lactate, and suggests a possible metabolon in which CA IX participates together with the monocarboxylate/H(+) co-transporter in dealing with the high amounts of lactate/H(+) present in tumors.