Phytochelatins, a class of heavy-metal-binding peptides from plants, are functionally analogous to metallothioneins

Phytochelatins are a class of heavy-metal-binding peptides previously isolated from cell suspension cultures of several dicotyledonous and monocotyledonous plants. These peptides consist of repetitive gamma-glutamylcysteine units with a carboxyl-terminal glycine and range from 5 to 17 amino acids in length. In the present paper we show that all plants tested synthesized phytochelatins upon exposure to heavy metal ions. No evidence for the occurrence of metallothionein-like proteins was found. All data so far obtained indicate that phytochelatins are involved in detoxification and homeostasis of heavy metals in plants and thus serve functions analogous to those of metallothioneins in animals and some fungi. Phytochelatins are induced by a wide range of metal anions and cations. Phytochelatin synthesis in suspension cultures was inhibited by buthionine sulfoximine, a specific inhibitor of gamma-glutamylcysteine synthetase (EC 6.3.2.2). This finding and kinetic studies of phytochelatin induction point to a synthesis from glutathione or its precursor, gamma-glutamylcysteine, in a sequential manner, thereby generating the set of homologous peptides.