Bismuth(III) complexes of the tripeptide glutathione (γ-L-Glu-L-Cys- Gly)

Abstract

The tripeptide glutathione (γ-L-Glu-L-Cys-Gly, GSH) is thought to play an important role in the pharmacology of bismuth drugs, but to our knowledge no chemical studies of bismuth glutathione complexes have been reported. We report here studies of interactions of the antiulcer compound ranitidine bismuth citrate (1) and [Bi(edta)]- with glutathione in aqueous solution and in intact red blood cells by NMR spectroscopy. The deprotonated thiol group is shown to be the strongest binding site for Bi III, and a complex with the stoichiometry [Bi(GS)3] is formed, as determined by 13C NMR titrations. A remarkably large low-field shift of approximately 1.37 ppm for the β-CH21H NMR resonances of GSH was observed on binding to BiIII. The complex [Bi(GS)3] is stable over the pH* range 2-10 (pH* = pH meter reading in D2O solution). A formation constant log K of 29.6 ± 0.4 (I = 0.1 M, 298 K) for [Bi(GS)3] was determined by displacement of edta by GSH. The rate of exchange of GSH between free and bound forms is pH-dependent, ranging from slow exchange (on the 1H NMR timescale) at low pH (ca. 3 s-1 at pH 4.0) to intermediate exchange at biological pH (ca. 1500 s-1). Such facile exchange may be important in the transport and delivery of BiIII in vivo. Spin-echo 1H NMR showed that 1 reacts with GSH in red cells both in vivo and in vitro. A first-order reaction of 1 with red blood cells was observed in vitro (k = 0.20 ± 0.04 h-1, t1/2 = 3 h, 310 K), and the rate-determining step appeared to involve the passage of BiIII through the cell membrane. © VCH Verlagsgesellschaft mbH, 1996.