PLATINUM(II)-THIOLATE CLUSTER FORMATION IN HEPTAPLATINUM METALLOTHIONEIN

The binding of platinum(II) to metallothionein (MT) has been studied by spectrophotometric titrations of rat liver apometallothionein-1 (apoMT) with platinum(II) complexes at pH 7.4-7.5. A red shift of approximately 6000 cm-1 in the intense absorption maximum in the electronic spectra of the partially platinated apoMT adducts is observed upon the binding of greater-than-or-equal-to 3 molar equiv of platinum(II). This shift is similar to red shifts of the lowest energy LMCT transitions observed for [X2Pt(mu-X)2PtX2]2- ions (X = Br, I) relative to the corresponding PtX4(2-) ions and can be attributed to a lowering in energy of M-sigma* <-- L-pi transitions from the terminal thiolates due to the presence of bridging thiolates. The partially platinated apoMT solutions (O-7 molar equiv of bound platinum (II) ions) were also assayed for free thiolate contents by using 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB). The DTNB studies showed that platinum equivalents 1-3 block 4 +/- 1 cysteines/platinum and that equivalents 5-7 block a total of 3 +/- 1 cysteines (an average of 1 cysteine/Pt). These experiments suggest that the platinum(II)-thiolate clusters in Pt7MT form via the initial production of approximately three Pt(cysteine)4(2-) centers followed by formation of mu-thiolato bridges.