CISPLATIN BINDING-SITES ON HUMAN ALBUMIN

Reactions of cisplatin (cis-[PtCl2(NH3)(2)]) with albumin are thought to play an important role in the metabolism of this anticancer drug. T hey are investigated here via (i) labeling of cisplatin with N-15 and use of two-dimensional H-1,N-15 NMR spectroscopy, (ii) comparison of n atural human serum albumin with recombinant human albumin (higher homo geneity and SH content), (iii) chemical modification of Cys, Met, and His residues, (iv) reactions of bound platinum with thiourea, and (v) gel filtration chromatography, In contrast to previous reports, it is shown that the major sulfur-containing binding site involves Met and n ot Cys-34, and also a N ligand, in the form of an S,N macrochelate. Ad ditional monofunctional adducts involving other Met residues and Cys-3 4 are also observed. During the later stages of reactions of cisplatin with albumin, release of NH3 occurs due to the strong trans influence of Met sulfur, which weakens the Pt-NH3 bonds, and protein cross-link ing is observed. The consequences of these findings for the biological activity of cisplatin-albumin complexes are discussed.