Photometric determination of methyl parathion GSH S-methyl transferase

Two methods are described for the assay of the enzyme that transfers a methyl group from methyl parathion (O,O-dimethyl-O, p-nitrophenyl phosphorothionate) to glutathione. A colorimetric assay depends on the measurement of alkali-soluble, chloroform-insoluble aromatic nitro compounds in the reaction mixture and a more rapid and sensitive direct spectrophotometric assay is described that uses the double wavelength Perkin-Elmer 356 Spectrophotometer. The utility of both methods has been demonstrated by the measurement of some kinetic constants and the distribution of the enzyme among some insects and vertebrates.