Abstract: Lead(Ⅱ)binding behaviors of rabbit liver thionein₂(apo-MT2) has been studied by monitoring the changes of their electronic absorption(UV) and circular dichroism(CD) spectra. Two MT species, Pb₇-MT and Pb_7-MT, are observed upon the binding of Pb to apoMTat different pH. The formation of Pb₇-MT -or Pb₇-MT is closely realted to the amount of deprotoned sulfhydryl groups. Pb₇-MT is formed only at high pHcharacterized by UV 340 nm shoulder and CD 245 nm band. Then the formation of a new leadmetallothionein complex, namely PbMT, is observed at low pH (pH<6). This new species is characterized by a strong circular dichroism(CD) cotton effect at 395 nm(-), 310 nm(-) peaks and 350 nm(+), 250 nm(+) doublehump and a unique electronic absorption(UV) spectruMAt 325 nm, 375 nMAnd 400 nm shoulder. From their characteristic peaks in UV and CD spectra, it is deduced that Pb²⁺in Pb₇-MT is coordinated by four sulfhydryl groups in Td microsymmetry, while multiple Pb²⁺coordination including twocoordination mayb.

Key words: Metallothionein, Lead(Ⅱ) ion, UVVis spectrum, Circular dichroism, Novel structure