Complexes of serum albumin and cis-dichlorodiammineplatinum (II). The role of cysteine 34 as a nucleophilic entering group and evidence for reaction between bound platinum and a second macromolecule.

Human and bovine serum albumin (38-377 p ~) bound approximately 1 mol of platinum per mol of protein when incubated with 300 t o 600 PM cis-dichlorodiammineplatinum ( 1 1 ) (cis-DDP) for 6 h at 37 "C.Significantly increased binding was not demonstrated with higher concentrations of cis-DDP or longer incubation periods.Bovine albumin that was carboxamidomethylated retained 0.03-0.06mol of sulfhydryl group/mol, compared with 0.62 mol/mol of unmodified bovine albumin, and bound 6 5 4 0 % less platinum when reacted with cis-DDP.Competition experiments were performed in which bovine or human albumin were incubated with cis-DDP and the plasma protease inhibitor, a2-macroglobulin (a2M).The albumins failed to protect a2M from the previously described inactivation by cis-DDP (Gonias, S. L., and Pizzo, S. V. (1981) J. Biol.Chem 256,12478-12484), even when present at concentrations 270 times that of the protease inhibitor.Equivalent results were obtained when competition experiments were performed with cis-DDP that was preincubated in a manner that yielded large amounts of the more reactive teaquo" forms of the drug.Platinum-albumin complex was resolved from unreacted drug and incubated with a2M.Partial loss of the protease inhibitor activity was observed.Dialysis experiments showed that the complexes formed between albumin and cis-DDP do not dissociate to a significant extent.It is suggested that the inactivation of azM by the platinumalbumin complex may involve direct reaction of the protease inhibitor with the complex.