Reduction of ormaplatin by a dithiol model compound for the active site of thioredoxin: stopped-flow kinetic analysis

Abstract

Ormaplatin ([Pt(dach)Cl4]) is a prototypical of Pt(IV) anticancer prodrug; its reduction by 3,6-dioxa-1,8-octanedithiol (DODT, a model compound for the active site of thioredoxin), has been characterized at 25.0 °C and in a wide pH range by rapid scan spectral and kinetic analysis. The reduction follows overall second-order kinetics, being first-order each in [Pt(IV)] and in [DODT]; the observed second-order constant k′ versus pH profile has been established. A 1:1 reaction stoichiometry is obtained, indicating that DODT is oxidized to its intramolecular disulfide; these characteristics are consistent with the redox properties of thioredoxin. A reaction mechanism has been proposed which involves the three protolytic species of DODT attacking [Pt(dach)Cl4] in parallel as rate-determining steps. The rate equation is derived from the reaction mechanism and rate constants of the rate-determining steps have been evaluated. The thiol–thiolate and dithiolate forms of DODT are 105–106 times more reactive than the dithiol form. Comparisons of the rate constants highlight that the electron density of the sulfur atom plays a key role in determining the reactivity of DODT toward ormaplatin and similar Pt(IV) compounds. It is concluded that the reduced thioredoxin is a strong competitor to other thiol-containing compounds in the reduction processes of Pt(IV) anticancer prodrugs.