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

1. Glutathione-S-aryltransferase activity from grass grubs (Costelytra zealandica) was inhibited by phthaleins, sulphonphthaleins and some dicarboxylic acids. 2. These compounds had no detectable action on the enzyme from sheep liver. 3. In insect enzyme the inhibition was competitive with respect to glutathione and non-competitive with respect to the aromatic substrate. 4. Michaelis constants and inhibitor constants were measured for sheep-liver or grass-grub enzyme between pH5 and pH10 and evidence was obtained for the presence of two groups with pK9.2 in the glutathione-binding site of the insect enzyme. 5. Only one such group was detected in the sheep-liver enzyme.