3 FAMILIES OF THIOL PEPTIDES ARE INDUCED BY CADMIUM IN MAIZE

Phytochelatins ((gamma GluCys)(n)Gly) are synthesized from glutathione by plants exposed to metals like Cd2+, Cu2+ and Zn2+. An intracellular complex formed by phytochelatins with Cd2+ and sulfide is thought to detoxify the metal possibly by sequestration in the vacuole. It was found that maize seedlings exposed to Cd2+ produced phytochelatins and two additional families of cysteine-containing peptides, (gamma GluCys)(n) and (gamma GluCys)(n)Glu. All thiol peptides with n = 2 and 3 were purified and their structure characterized by tandem mass spectrometry. For maize plants exposed to Cd2+ for 7 days, phytochelatins were synthesized preferentially in the first 24 h whereas the amounts of (gamma GluCys)(n) and (gamma GluCys)(n)Glu were the highest thereafter. This was probably due to an initial large pool of glutathione available in control plants compared with a dearth of gamma GluCys and no detectable gamma GluCysGlu. The (gamma GluCys)(n)Glu peptides were induced exclusively by Cd2+ as they were below the detection limit in control seedlings that contained low amounts of phytochelatins and (gamma GluCys)(n). Since the Cys moiety of the peptides is essential for binding Cd2+, a role for accumulated (gamma GluCys)(n) and (gamma GluCys)(n)Glu in detoxifying Cd2+ in plants must be considered.