Synthesis of phytochelatins and homo-phytochelatins in Pisum sativum L

Abstract

In the roots of pea plants (Pisum sativum L) cultivated with 20 μM CdCl2 for 3 d, synthesis of phytochelatins [PCs or (γEC)nG, where γEC is yglutamylcysteine and G is glycine] and homophytochelatins [h-PCs, (γEC)nβ-alanine] is accompanied by a drastic decrease in glutathione (GSH) content, but an increase in homoglutathione (h-GSH) content. In contrast, the in vitro activity of GSH synthetase increases 5-fold, whereas h-GSH synthetase activity increases regardless of Cd exposure. The constitutive enzyme PC synthase, which catalyzes the transfer of the γ-EC moiety of GSH to an acceptor GSH molecule thus producing (γEC)2G, is activated by heavy metals, with Cd and Cu being strong activators and Zn being a very poor activator. Using h-GSH or hm-GSH for substrate, the synthesis rate of (γEC)2β-alanine and (γEC)2-serine is only 2.4 and 0.3%, respectively, of the synthesis rate of (γEC)2G with GSH as substrate. However, in the presence of a constant GSH level, increasing the concentration of h-GSH or hm-GSH results in increased synthesis of (γEC)2β-alanine or (γEC)2-serine, respectively; simultaneously, the synthesis of (γEC)2G is inhibited. γEC is not a substrate of PC synthase. These results are best explained by assuming that PC synthase has a γEC donor binding site, which is very specific for GSH, and a γEC acceptor binding site, which is less specific and accepts several tripeptides, namely GSH, h-GSH, and hm-GSH.