HPLC/ICP Mass-Spectrometric Examination of the Induction and Metal Incorporation of Cyanobacterial Metallothionein through the Uptake of Heavy-Metal Ions

Abstract

The induction of metallothionein (MT) in a cyanobacterium Anacystis nidulans R-2 through the uptake of Zn²⁺, Cd2⁺ and Hg²⁺ was studied by high-performance liquid chromatography (HPLC) with inductively coupled plasma mass-Spectrometric (ICP/MS) detection, under optimized temperature and light conditions for MT biosynthesis. MT was induced immediately after the individual addition of the three metal ions (5μM); its maximum level was reached in 48h (Zn and Cd) or 16h (Hg). Although separate dosing of Cd²⁺ (5μM) and Zn²⁺ (5μM) gave MTs with comparable amounts of the metal incorporated, their simultaneous dosing yielded a MT with a Cd : Zn molar ratio of 2:1. A more pronounced effect was noted for Hg²⁺ in going from single Zn-dosing to simultaneous Hg-plus-Zn dosing. These results suggest that the coexistence of Cd²⁺ or Hg²⁺ suppresses the incorporation of Zn²⁺ into the MT, probably due to inhibition of the uptake of Zn into the organism. The much higher effect of Hg²⁺ than Cd²⁺ is in line with the order of the metal affinity to sulfhydryl (SH) groups, Hg²⁺>Cd²⁺>Zn²⁺.