title: L-selenomethionine reduces platinum(IV) anticancer model compounds at strikingly faster rates than L-methionine.
abstract: L-Selenomethionine (SeMet), the predominant form of selenium acquired from the diet by humans, has been used as a supplement, and exhibit some important functions like cancer prevention and antioxidative defense. Its interactions with Pt(II) anticancer drugs have been characterized, but its redox reactions with platinum(IV) anticancer prodrugs have not been exploited. In this work, the oxidation of SeMet by Pt(IV) anticancer model compounds trans-[PtX2(CN)4](2-) (X = Cl, Br) was characterized. A stopped-flow spectrometer was used to record the rapid scan spectra and to follow the reaction kinetics over a wide pH range. An overall second-order rate law was derived: -d[Pt(IV)]/dt = k'[Pt(IV)][SeMet], where k' pertains to the observed second-order rate constants. The k'-pH profiles showed that k' increased only about 6 times even though the solution pH was varied from 0.25 to 10.5. The redox stoichiometry was determined as Δ[Pt(IV)]/Δ[SeMet] = 1 : (1.07 ± 0.07), suggesting that SeMet was oxidized to selenomethionine selenoxide. The selenoxide together with its hydrated form was identified explicitly by high resolution mass spectral analysis. A reaction mechanism was proposed which encompassed three parallel rate-determining steps relying on the protolytic species of SeMet. Rate constants of the rate-determining steps were obtained from the simulations of the k'-pH profiles. Activation parameters were determined for the reactions of the zwitterionic form of SeMet with the Pt(IV) complexes. A bridged electron transfer process is delineated in the rate-determining steps and several lines of evidence support the bridged electron transfer mode. Strikingly, reduction of [PtX2(CN)4](2-) by SeMet is 3.7 × 10(3)-5.7 × 10(4) times faster than that by L-methionine. Some potential biological consequences resulting from the strikingly fast reduction are discussed.
authors: College of Chemistry and Environmental Science, and the MOE Key Laboratory of Medicinal Chemistry and Molecular Diagnostics, Hebei University, Baoding 071002, Hebei Province, P. R. China. shensg@hbu.edu.cn rock@hbu.edu.cn.
authors: Shuying
authors: S
authors: Huo
authors: Jingran
authors: J
authors: Dong
authors: Shigang
authors: S
authors: Shen
authors: Yanli
authors: Y
authors: Ren
authors: Changying
authors: C
authors: Song
authors: Jianzhong
authors: J
authors: Xu
authors: Tiesheng
authors: T
authors: Shi
mesh_terms: D000970:Antineoplastic Agents / Q000737:chemistry* / Q000494:pharmacology; D004347:Drug Interactions; D008715:Methionine / Q000737:chemistry* / Q000494:pharmacology; D008958:Models, Molecular; D009944:Organoplatinum Compounds / Q000737:chemistry* / Q000494:pharmacology; D010084:Oxidation-Reduction; D012645:Selenomethionine / Q000737:chemistry* / Q000494:pharmacology; D013053:Spectrophotometry
chemical_list: D000970:Antineoplastic Agents; D009944:Organoplatinum Compounds; D012645:Selenomethionine; D008715:Methionine