Glutathione S-transferase (GST) is a multifunctional enzyme that is thought to plays a role in the start of xenobiotic detoxification process. This enzyme catalyzes the reaction between xenobiotic compounds and the -SH groups. This reaction leads to reduced biological activity and increased solubility. So, the resulting compounds are more polar and therefore easier to remove. Activity of this enzyme is investigated in a large group of invertebrate species which indicate the relationship between the GST enzyme activity and tolerance of these organisms to foreign compounds. Gammarus species are sensitive to a wide range of contaminants and are suitable for the detection of environmental pollutants therefore they can be used as a model organism in the study of environmental pollution. The partial purification of GST was the main purpose of this study. In order to reduce the volume, concentration and purification of proteins, various precipitation methods can be used. Salting out with appropriate concentration of ammonium sulfate, which is a highly soluble salt, separate proteins with different solubility. In our study solid ammonium sulfate with final concentration of 60% and 90%was added to Gammarus homogeneous and the mixture was stirred until dissolving of the salt. The suspension was centrifuged at 12000×g, then the supernatant was dialyzed overnight at 4̊ C against 100 mM phosphate buffer, pH=7 containing 9g NaCl per liter. Protein concentration was estimated by Bradford method and GST activity was assayed by Habig method. Protein concentration was decreased in supernatant of both 60%and 90saturations. Enzyme activity had a 3-fold decrease in 90%, but on the contrary, a4-fold increase was observed in enzyme activity in 60% saturated solution. In conclusion, the present findings suggest precipitation with ammonium sulfate affords a procedure for concentrating as well as purifying the enzyme.
