Chaotropic Ions in Activation and Protection of γ-Glutamyltransferase from Fruiting Bodies of Lentinus edodes

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γ-Glutamyltransferase from fruiting bodies of Lentinus edodes was further tested for its activation by chaotropic ions such as SCN-, NO3-, Cl-, Br-, I-, F- and ClO4-. The thiocyanate ion increased the Km value for γ-glutamyl-p-nitroanilide without affecting the Vmax value of the reaction, whereas other anions as represented by NO3- and Br- increased the Vmax without affecting the Km. The inactivation of the enzyme by the SH group-orienting reagents, iodoacetamide and hydrogen peroxide, was stimulated by SCN- but not by the other anions. The activator anions protected the enzyme against its inactivation by chemical modification with 2, 3-butanedione in borate. Their efficiency was parallel to the activator potency of the respective anions, except for SCN- which provided less protection than expected from its activation potency. These dissociable effects of activator anions might be explained by two different mechanisms; binding of SCN- to a basic group to bring about a significant change in protein conformation and binding of other anions by electrostatic and hydrophobic forces to an arginyl residue located near the active site of the enzyme.
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