HOMOGLUTATHIONE SELECTIVITY BY SOYBEAN GLUTATHIONE S-TRANSFERASES

Soybeans (Glycine max) have nearly undetectable levels of glutathione, substituting the tripeptide homoglutathione for the same functions. S ome herbicides are detoxified in soybeans by homoglutathione conjugati on catalyzed by glutathione S-transferase (GST) enzyme(s). We have clo ned and overexpressed a new soybean GST (GSTa), the previously describ ed soybean GST (GH2/4), and two maize GSTs. Their ability to utilize h omoglutathione and glutathione in several nucleophilic substitution re actions was measured. In most cases conjugation to subsaturating conce ntrations of electrophilic substrate was faster with glutathione. Howe ver homoglutathione conjugation was faster with some combinations of e nzyme and substrate, notably, GH2/4 and the herbicide chlorimuron ethy l. Steady-state kinetic evaluations revealed that a ternary complex is part of the reaction mechanism, and the binding of substrates lakes p lace in random order. A random order rapid equilibrium model was used to compare the GH2/4-catalyzed reaction of both thiols with chlorimuro n ethyl and alachlor. This revealed that catalytic rate constants do n ot differ significantly between the thiols. Conjugation rates with hom oglutathione exceed those with glutathione when a high dissociation co nstant for second substrate makes formation of a glutathione containin g ternary complex unfavorable, in GH2/4 this occurs with chlorimuron e thyl but not with alachlor. (C) 1998 Academic Press.