The Nature of Arsenic-Phytochelatin Complexes in Holcus lanatus and Pteris cretica

We have developed a method to extract and separate phytochelatins (PCs)-metal(loid) complexes using parallel metal(loid)-specific (inductively coupled plasma-mass spectrometry) and organic-specific (electrospray ionization-mass spectrometry) detection systems-and use it here to ascertain the nature of arsenic (As)-PC complexes in plant extracts. This study is the first unequivocal report, to our knowledge, of PC complex coordination chemistry in plant extracts for any metal or metalloid ion. The As-tolerant grass Holcus lanatus and the As hyperaccumulator Pteris cretica were used as model plants. In an in vitro experiment using a mixture of reduced glutathione (GS), PC2, and PC3, As preferred the formation of the arsenite [As-(III)]-PC3 complex over GS-As-(III)-PC2, As-(III)-(GS)(3), As-(III)-PC2, or As-(III)-(PC2)(2) (GS: glutathione bound to arsenic via sulphur of cysteine). In H. lanatus, the As-(III)-PC3 complex was the dominant complex, although reduced glutathione, PC2, and PC3 were found in the extract. P. cretica only synthesizes PC2 and forms dominantly the GS-As-(III)-PC2 complex. This is the first evidence, to our knowledge, for the existence of mixed glutathione-PC-metal(loid) complexes in plant tissues or in vitro. In both plant species, As is dominantly in non-bound inorganic forms, with 13% being present in PC complexes for H. lanatus and 1% in P. cretica.