PROPERTIES OF A MAIZE GLUTATHIONE-S-TRANSFERASE THAT CONJUGATES COUMARIC ACID AND OTHER PHENYLPROPANOIDS

A glutathione S-transferase (GST) enzyme from corn (Zea mays L. Pionee r hybrid 3906) that is active with p-coumaric acid and other unsaturat ed phenylpropanoids was purified approximately 97-fold and characteriz ed. The native enzyme appeared to be a monomer with a molecular mass o f approximately 30 kD and an apparent isoelectric point at pH 5.2. The enzyme had a pH optimum between 7.5 and 8.0 and apparent K-m values o f 4.4 and 1.9 mM for reduced glutathione (GSH) and p-coumaric acid, re spectively. In addition to p-coumaric acid, the enzyme was also active with o-coumaric acid, m-coumaric acid, trans-cinnamic acid, ferulic a cid, and coniferyl alcohol. In addition to GSH, the enzyme could also utilize cysteine as a sulfhydryl source. The enzyme activity measured when GSH and trans-cinnamic acid were used as substrates was enhanced 2.6-and 5.2-fold by the addition of 50 mu M p-coumaric acid and 7-hydr oxycoumarin, respectively. H-1- and C-13-nuclear magnetic resonance sp ectroscopic analysis of the conjugate revealed that the enzyme catalyz ed the addition of CSH to the olefinic double bond of p-coumaric acid. Based on the high activity and the substrate specificity of this enzy me, it is possible that this enzyme may be involved in the in vivo con jugation of a number of unsaturated phenylpropanoids.