Development of a high throughput yeast-based screening assay for human carbonic anhydrase isozyme II inhibitors

Carbonic anhydrase (CA; EC 4.2.1.1) catalyzes the reversible hydration of carbon dioxide (CO₂) to bicarbonate and proton. There are 16 known isozymes of α-CA in humans, which differ widely in their kinetics, subcellular localization and tissue-specific distribution. Several disorders are associated with abnormal levels of CA, and so the inhibition of CA has pharmacological application in the treatment of many diseases. Currently, searching for novel CA inhibitors (CAI) has been performed using in vitro methods, and so their toxicity remains unknown at the time of screening. To obtain potentially safer CAIs, a screening procedure using an in vivo assay seems to have more advantages. Here, we developed a yeast-based in vivo assay for the detection of inhibitors of the human CA isozyme II (hCAII). The yeast Saccharomyces cerevisiae mutant deprived of its own CA (Δnce103 strain) can grow under a high CO₂ condition (5% (v/v) CO₂) but not at an ambient level. We constructed Δnce103 strains expressing various levels of hCAII from a plasmid harboring the hCAII gene arranged under the control of variously modified GAL1 promoter and relying on the expression of hCAII for growth under low CO₂ condition. Using a multidrug-sensitive derivative of the Δnce103 strain expressing a low level of hCAII, we finally established a high throughput in vivo assay for hCAII inhibitors under a low CO₂ condition. Cytotoxicity of the candidates obtained could be simultaneously determined under a high CO₂ condition. However, their inhibitory activities against other CA isozymes remains to be established by further investigation.