In aqueous media, muscle pyruvate kinase is highly selective for K + over Na + . We now studied the selec-tivity of pyruvate kinase in water/dimethylsulfoxide mix-tures by measuring the activation and inhibition constants of K + and Na + , . their binding to the monovalent and divalent cation binding sites of pyruvate kinase, respect-ively [Melchoir . (1965) Biochemistry4, 1518–1525]. In 40% dimethylsulfoxide appfor K + and Na + were 190 and 64-fold lower than in K + and Na +decreased 116 and 135-fold between 20 and 40% dimethylsulfoxide. The ratios of Kiapp/ app for K + and Na + were 34– and –, respectively
