THE COMPLEXATION OF APO-METALLOTHIONEIN WITH CADMIUM ION AND ITS CONFORMATION STUDY

The circular dichroism was used to study the complexation of two isofo rms of rabbit liver apo-metallothioneins (apo-MT1 and apo-MT2) with Cd 2+ ion and the influence of Cd2+ ion on the conformation of reconstitu ted MTs. The stability of sulfhydryls of ape-MT has been investigated at the room temperature in the presence of air. The reconstitutions of apo-MT1 with Cd2+ ion were carried out at pH 4.71 (stable state) and pH 7.9 (with 90% sulfhydryls oxidated) respectively. It was found that the characteristic CD band at 257 nm(+), 238 nm(-), 226 nm(+) of reco nstituted MT with Cd2+ ion was the same as native MT at pH 4.71, howev er only one peak at 243 nm(+) appeared on the CD spectra at pH 7.9 whi ch arose from mononuclear complexes with four separated thiolate ligan ds per Cd2+ ion. The CD spectra of apo-MTs + 7 eq Cd2+ system were mea sured at various pH values. It was found that the peak at 256 nm of ap o-MT1 binding Cd2+ ion split into two small peaks at pH between 2.42 a nd 3.02, and became one peak at pH 3.32, while the shapes of Cd peaks of apo-MT2 binding Cd2+ ion did not change with pH, indicating that th e binding sites and pathway of apo-MT1 binding Cd2+ ion were different from those of apo-MT2. A possible mechanism was suggested.