In this study, the in vitro effects of some sulfonamide derivatives, which are carbonic anhydrase inhibitors, on the enzymes activities of glucose-6-phosphate dehydrogenase, 6-phospho gluconate dehydrogenase and glutathione reductase were investigated. For this purpose, these three enzymes were purified from human erythrocytes. Purification procedure composed of four steps; preparation of the hemolysate, ammonium sulfate precipitation, 2',5'-ADP Sepharose 4B affinity chromatography, and gel filtration chromatography on Sephadex G-200. 5-(3alpha-Hydroxy-5-beta-cholanamido)-1,3,4-thiadiazole-2-sulfonamide (1), 5-(3alpha,12alpha-Dihydroxy-5-beta-cholanamido)-1,3,4-thiadiazole-2-sulfonamide (2), 5-(3alpha,7alpha,12alpha-Trihydroxy-5-beta-cholanamido)-1,3,4-thiadiazole-2-sulfonamide (3), 5-(3alpha,Acetoxy-5-beta-cholanamido)-1,3,4-thiadiazole-2-sulfonamide (4), 5-(3alpha,7alpha,12alpha-Triacetoxy-5-beta-cholanamido)-1,3,4-thiadiazole-2-sulfonamide (5), 5-(3,7,12-Trioxo-5-beta-cholanamido)-1,3,4-thiadiazole-2-sulfonamide (6), acetazolamide, and dorzolamide were tested in this experiment. Compounds 3, 5, and dorzolamide showed inhibitory effects on the activity of 6-phosphogluconate dehydrogenase, and I(50) values and K(i) constants were calculated as 0.0601 mM, 0.00253 mM, and 1.41 mM and 0.0878 +/- 0.0274 mM, 0.0042 +/- 0.0009 mM, and 3.1446 +/- 0.2081 mM, respectively. Glutathione reductase was also inhibited by 1 and 2. I(50) values and K(i) constants were 0.0471 mM and 0.0723 +/- 0.0388 mM for 1 and 0.0045 mM and 0.0061 +/- 0.0014 mM, for 2. If these sulfonamide derivatives are proposed as drugs, some of which are being used in glaucoma treatment such as acetazolamide and dorzolamide, these results should be taken into consideration concerning via these enzymes.