Herbicide metabolism in plants: specificity of peroxidases for aniline substrates.

Studies on the metabolism in plants (4, 5, 6, 8) aInd in soil (1, 2) of the herbicide propanil (3,4-dichloropropionanilide) have showin that the parent compound is rapidly hydrolyzed to release 3,4-dichloroaniline (DCA). In plants, the aniline moiety is recovered in several compounds including sugar and lign,in conjugates (6, i7), while in soil the major product is an azo compound identified as 3,3',4,4'tetrachloroazobenzene (TCAB) (1). This unusual conversion, although catalyzed by soil peroxidases and by crystalline horseradish peroxidase, could not be detected in plants (6, 7). We have compared the peroxidase activities in crude extracts of barnyard grass, Echinocloa crusgalli, (a plant which is readily killed by propanil and might be expected to contribute to the peroxidase activity of the s!oil), fresh horseradish root, rice, and the commercially purified crystalline horseradish enzYme. We infer that the failure of some plants to accomplish the aniline to azo conversion (DCA -* TCAB) may be due to differences in substrate specificity among the plant peroxidases. Peroxidase activity was measured essentially as described by De JoIng et al. (3). The assay mixture containied 0.2 ml of 1 M p-anisidine in methanol, 1 nml of 0.2 M acetate buffer pH 5, 0.3 ml of a 0.1 % (v/v) H12O., solution, in a final volume of 3 ml. The reaction was started by the addition of 0.1 ml of enzyme preparation. Activity was measured as the change in optical density at 460 m, determined on a Beckmain DU spectrophotometer equipped with a Gilford Model 220 optical density converter and a potentiometric recorder. The cuvette compartment was maintained at 25°. There was no detectable activitv in the absence of enzyme. A crude enzyme preparation from barnyard grass was prepared from 100 g of grass at the 3-leaf stage, grown in the greenhouse. Whole plants were washed carefully and ground in a cold mortar and pestle. The expressed juice was filtered through 8