Alterations in the binding of [Cl(NH3)(5)Ru-III to DNA by glutathione: Reduction, autoxidation, coordination, and decomposition

The autoxidation of glutathione (GSH) is catalyzed by [Cl(NH3)(5)Ru-III](2) yielding only [OH(NH3)(5)Ru-III](2+) and GSSG according to the rate law d [GSSG]/dt = k[Ru][GSH], where k = 3 M-1 s(-1). The anaerobic reaction of GS H with [Cl(NH3)(5)Ru-III](2+) yields first [OH(NH3)(5)Ru-III](2+) and then [GS(NH3)(5)Ru-III](+) at neutral pH, both through redox catalysis. The reac tion appears to proceed through reduction of Rum by GSH to give [H2O(NH3)(5 )Ru-II](2+), followed by coordination to produce [GSH(NH3)(5)Ru-II](2+) and then oxidation of the latter ion by [OH(NH3)(5)Ru-III](2+) Or GSSG to yiel d [GS(NH3)(5)Ru-III](+). [GS(NH3)(5)Ru-III](+) is also produced by the reac tion of GSH with [(NH3)(6)Ru](3+) or [py(NH3)(5)Ru](3+). Glutathione reduce s [OH(NH3)(5)Ru-III](2+) through a pre-equilibrium mechanism according to t he following rate law: d[Ru-II]/dt = k[Ru-III][GSH]/(K-i + [GSH]), where K- i = 2.0 x 10(-3) M-1 and k = 2.3 x 10(-3) s(-1). The reduction potential of [(GS)(NH3)(5)Ru-III] is pH-dependent according to the Nernstian equation: E = E degrees - 0.59 log {K-a/([H+] + K-a)}, where E degrees = -440 mV, pK( a) = 7.1. While [GS(NH3)(5)Ru-III] is stable for extended periods under ine rt atmosphere, it changes in air, eventually yielding [HO(NH3)(5)Ru-III] am ong other products at high pH with k(obs) (s(-1)) = (k(1)K(a) + k(2)[H+])/( [H+] + K-a), where k(1) = 9 x 10(-6) s(-1), k(2) = 1.2 x 10(-4) s(-1) M-1, and pK(a) = 12. At [GSH]/[Ru-III] less than or equal to 1, the coordination of [Cl(NH3)(5)Ru-III](2+) to DNA. is facilitated by GSH reduction to the m ore substitution-labile [H2O(NH3)(5)Ru-II](2+). However, at [GSH]/[Ru-III] greater than or equal to 1, guanine binding on DNA is inhibited by GSH, whi ch coordinates Ru-II and facilitates oxidation back to Ru-III because of th e low E degrees of [GS(NH3)(5)Ru-III](+). Consistent with this is the incre ased toxicity of [Cl(NH3)(5)Ru-III](2+) to Jurkat T-cells, when GSH levels are suppressed. High [GSH]/[Ru] alters the DNA binding of [H2O(NH3)(5)Ru-II ](2+) to essentially eliminate G(7) coordination and lower C-4 binding, but leaving A(6) binding relatively unaffected, which may have implications fo r the mechanism of ruthenium antitumor agents.