The effects of Cue, Zn2+ and Cd2+ ions on the induction of metallothionein (MT) in a cyanobacterium Anacystis nidulans R-2 have been studied by high-performance liquid chromatography (HPLC) with inductively coupled plasma mass spectrometric (ICP/ MS) detection. After growing the cells in media of systematically varied metal concentrations, the contents of each metal in induced MT were determined by HPLC-ICP/ MS. By separate dosing of a metal ion, MT induction was clearly observed with Zn2+ and Cd2+, but not with Cuet However, Cu2+ induced MT in the presence of a high level (0.77 µM) of Zn2t A high level of Zn2+ also significantly promoted MT induction by Cd2+. These synergistic effects have revealed the predominant role of Zn2+ in heavy-metal detoxification by the organism. The partition of Cu, Zn and Cd between the cytoplasm and the cell wall was studied for cells grown in a medium fairly enriched in the three heavy-metal ions.
KeywordsInductively coupled plasma mass spectrometry, high-performance liquid chromatography, metallothionein, cyanobacterium, sulfhydril group, heavy metal Metallothioneins (MTs) are low-molecular weight proteins which are heat-stable and rich in cysteine residues with high affinity for copper, zinc and cadmium ions. ' MTs are apparently involved in the transport of metals2, metal homeostasis3 and metal detoxification processes4, though the principal biological role of MTs still remains to be clarified.MTs are typically induced in living organisms under the stress of heavy-metal ions.' So far, although a number of studies have been published regarding the effect of an individual toxic metal ion on the MT induction, studies concerning possible synergistic effects of different metal ions have been rather scarce.For higher plants, Girling and Peterson5 reported that Zn uptake increases the Cd content in the organism, though at a high Zn dose an inhibitory effect concerning Cd uptake appears.Turner6 observed for several crops that the Zn content in the plant increases upon adding Cd2+ at a concentration below 0.1 g m3, while the total Zn uptake decreases after the Cd2+ concentration exceeds 1 g m3. Wallace' noted a protective effect of Cu2+ on beans under a high dose of Cd2tAll of these studies concern the interactive effects of metal ions from the viewpoint of the total metal uptake in plants; only little attention has been paid to the conversion of their chemical form, particularly MT induction.As an extension of our attempts8'9 to implement highperformance liquid chromatography (HPLC) combined with inductively coupled plasma mass spectrometry (ICP/ MS) for environmental studies, we investigated both individual and synergic effects of Cu2+, Zn2+ and Cd2+ on MT induction in a cyanobacterium, Anacystis nidulans R-2. ICP/ MS features a highly sensitive, metal-specific analysis of metalloproteins without any need for radioactive tracers. The choice of a cyanobacterium is due to its primary importance as an origin of trophic chains in an aquatic system, its rapid growth and ease in controlli...