Kinetic analysis of the cis-diamminedichloroplatinum(II)--cysteine reaction: implications to the extent of platinum--DNA binding.

The reaction between cis-diamminedichloroplatinum(II) (cis-DDP) and L-cysteine was examined at neutral pH at 37 degrees C. The reaction proceeds through a Pt(NH3)2 (cys)Cl intermediate which undergoes parallel reactions with a second molecule of cysteine to form a bis(cysteine) complex, Pt(NH3)2(cys)2 and with the starting platinum complex to form a cysteine-bridged dinuclear complex. In the presence of excess cysteine, the product is predominantly the bis(cysteine) complex. The intermediate is formed by the direct reaction of the platinum complex with cysteine with a bimolecular rate constant 2.2 +/- 0.2 x 10(-2) M-1.s-1 at 37 degrees C as well as through a rapid reaction with the mono aqua-platinum complex. The rate constant for the formation of the dimer was evaluated to be 0.24 +/- 0.4 M-1.s-1, an order of magnitude higher than that for the mononuclear complex formation. The intermediate reacts with a second cysteine molecule with a bimolecular rate constant, 5.6 +/- 0.4 x 10(-2) M-1.s-1. The rate constant for the equation of Pt(NH3)2(cys)Cl was evaluated to be 1.8 +/- 0.2 10(-4) s-1. The Pt-195 chemical shifts for the mono(cysteine), bis(cysteine), and cysteine bridged dimer were found to be -3308, -3705, and -3104 ppm. The bis(cysteine) complex at neutral pH undergoes slow reaction (t1/2 approximately equal to four days) to form a secondary product, presumably Pt(NH3)(cys)2, in which one cysteine acts a bidentate chelating agent. In acidic solution, with equimolar concentrations of cysteine and diaqua-platinum complex, the reaction predominantly yielded a cysteine bridged dimeric complex. When cysteine concentration was increased fourfold over the platinum complex, the bis(cysteine) chelate with complete removal of coordinated ammonia appeared as the dominant product. The platinum-195 chemical shift for this chelate was found to be -3290 ppm. Considering the abundance of thiols in amino acids/peptides and replication enzymes in the cellular milieu, it remains to be seen how platinum complexes react with DNA. Direct platination to replication enzymes as a possible mechanism for antineoplactic activity is yet to be ruled out.