The inhibitory effects of some drugs on 6-phosphogluconate dehydrogenase from human erythrocytes have been investigated. For this purpose, initially, erythrocyte 6-phosphogluconate dehydrogenase was purified 3364 times in a yield of 58% by using ammonium sulfate precipitation and 2',5'-ADP Sepharose 4B affinity gel. A temperature of +4 degrees C was maintained during the purification process. Enzyme activity was determined with the Beutler method by using a spectrophotometer at 340 nm. This method was utilized for all kinetic studies. Many commonly used drugs were investigated in this study. Some drugs (ketotifen (K(i): 8.3 +/- 1.7 microM), dacarbazine (K(i): 10.1 +/- 0.7 microM), meloxicam (K(i): 50.9 +/- 13.2 microM), furosemide (K(i): 127 +/- 37.8 microM), methotrexate (K(i): 136.7 +/- 25.3 microM), metochloropramide hydrochloride (K(i): 2.1113 +/- 0.6979 mM), ritodrine hydrochloride (K(i): 6.0353 +/- 1.2783 mM), and gadopentetic acid (K(i): 73.4 +/- 21.9 mM)) inhibited enzyme activity in vitro. K(i) constants for the enzyme were found by means of Lineweaver-Burk graphs. All drugs showed non-competitive inhibition. In addition, IC(50) values of the drugs were determined by plotting activity percent vs [I].