SYNTHESIS OF PHYTOCHELATINS AND HOMO-PHYTOCHELATINS IN PISUM-SATIVUM L

In the roots of pea plants (Pisum sativum L.) cultivated with 20 mu M CdCl2 for 3 d, synthesis of phytochelatins [PCs or (gamma EC)(n)G, where gamma EC is gamma glutamylcysteine and G is glycine] and homophytochelatins [h-PCs, (gamma EC)(n) beta-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 gamma-EC moiety of GSH to an acceptor GSH molecule thus producing (gamma EC)(2)G, 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 (gamma EC)(2) beta-alanine and (gamma EC)(2)-serine is only 2.4 and 0.3%, respectively, of the synthesis rate of (gamma EC)(2)G 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 (gamma EC)(2) beta-alanine or (gamma EC)(2)-serine, respectively; simultaneously, the synthesis of (gamma EC)(2)G is inhibited. gamma EC is not a substrate of PC synthase. These results are best explained by assuming that PC synthase has a gamma EC donor binding site, which is very specific for GSH, and a gamma EC acceptor binding site, which is less specific and accepts several tripeptides, namely GSH, h-GSH, and hm-GSH.