A calorimetric study of the binding of S-alkylglutathiones to glutathione S-transferase

The binding of three competitive glutathione analogue inhibitors (S-alkylgl utathione derivatives) to glutathione S-transferase from Schistosoma japoni cum, SjGST, has been investigated by isothermal titration microcalorimetry at pH 6.5 over a temperature range of 15-30 degreesC. Calorimetric measurem ents in various buffer systems with different ionization heats suggest that no protons are exchanged during the binding of S-alkylglutathione derivati ves. Thus, at pH 6.5, the protons released during the binding of substrate may be from its thiol group. Calorimetric analyses show that S-methyl-, S-b utyl-, and S-octylglutathione bind to two equal and independent sites in th e dimer of SjGST. The affinity of these inhibitors to SjCST is greater as t he number of methylene groups in the hydrocarbon side chain increases. In a ll cases studied, AGO remains invariant as a function of temperature, while DeltaH(b) and DeltaS(0) both decrease as the temperature increases. The bi nding of three S-alkylglutathione derivatives to the enzyme is enthalpicall y favourable at all temperatures studied. The temperature dependence of the enthalpy change yields negative heat capacity changes, which become less n egative as the length of the side chain increases. (C) 2001 Elsevier Scienc e B.V. All rights reserved.