Species differences in the inhibition of glutathione S-aryltransferase by phthaleins and dicarboxylic acids

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Abstract
Glutathione-S-aryltransferase activity from grass grubs (Costelytra zealandica) was inhibited by phthaleins, sulphonphthaleins and some dicarboxylic acids. These compounds had no detectable action on the enzyme from sheep liver. In insect enzyme the inhibition was competitive with respect to glutathione and noncompetitive with respect to the aromatic substrate. Michaelis constants and inhibitor constants were measured for sheep-liver or grass-grub enzyme between pH5 and pH 10 and evidence was obtained for the presence of 2 groups with pK 9.2 in the glutathione-binding site of the insect enzyme. Only 1 such group was detected in the sheep-liver enzyme.