Interactions between cadmium and several heavy metals in the activation of the catalytic activity of phytochelatin synthase

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Abstract
Plant cells exposed to excess heavy metals such as Cd2+ accumulate heavy metal-binding peptides, called phytochelatins [PCs; (λ-Glu-Cys)n-Gly with n = 2-11] (Rauser 1995). It is considered that PCs are involved in the detoxification of heavy metals and heavy metal tolerance by scavenging these ions in cells. PC synthesis uses glutathione (GSH) as a substrate and this reaction is catalyzed by phytochelatin synthase. The catalytic activity is dependent on heavy metals such as Cd2+. Therefore, it is important for understanding the response of plants to heavy metal stress to analyze the effects of heavy metals on the catalytic activity of the enzyme. The catalytic properties of this enzyme have been investigated for Silene cucubalus cell cultures (Grill et al. 1989; Loeffler et al. 1989) and Pisum sativum L. (Klapheck et al. 1995). However, the effect of the coexistence of several heavy metals on the catalytic activity of PC synthase is unclear. In this study, we analyzed the activation of the catalytic activities of the enzyme by several heavy metals in tobacco cell extracts, and then investigated the interactions between Cd2+ and the other heavy metals in the activation of the catalytic activity of the enzyme.