Sulfur-containing biomolecules, reaction

H. Reaction Products of Pt-Amine Compounds and Sulfur-Containing Biomolecules... 

The reaction of sulfur-containing biomolecules with platinum antitumor compounds, thereby preventing binding to the critical DNA target, is a possible mechanism of inactivation and is supported by numerous studies. Thus, glutathione (GSH, a cysteine-containing tripeptide see also Fig. 6), which is the predominant intracellular thiol and is present in concentrations varying from 0.5 to 10 mM, is able to inhibit the reaction of DNA with [Pt(en)Cl2] (74) and with cis-Pt (75, 76). It has also been observed that the presence of cysteine can inhibit the reaction between cis-Pt and d-Guo (77). Furthermore, the antitumor activity of cis-Pt was proved to be inhibited by coadministered methionine (78, 79) and even a bis-adduct between cis-Pt and methionine has been isolated from the urine of patients (80). 

A second mechanism of inactivation might be the reaction of sulfur-containing biomolecules with the cis-Pt-DNA monoadducts (product 1 in Fig. 4), which prevents those from rearranging to toxic bifunctional adducts. Supportive for such a mechanism is the observation that GSH can be cross-linked to DNA by cis-Pt (41,41a) and [Pt(en)Cl2] (74), and that cysteine can be cross-linked to d-Guo by cis-Pt (77). Furthermore, cis-Pt-DNA monoadducts can be experimentally quenched with thiourea, which reduces drug toxicity (82, 83). trans-Pt also yields monofunctional adducts after reaction with DNA, and these rearrange somewhat slower than does cis-Pt into bifunctional adducts (41,84), clearly for sterical reasons. The relatively long-living monofunctional adducts react efficiently with GSH and proteins (41 a, 84-86). 

The high affinity of many platinum compounds for sulfur and the availability of many sulfur-containing biomolecules have raised the question whether Pt-sulfur biomolecule interactions could serve as a drug reservoir for platination at DNA, necessary for the antitumor activity of cis-Pt. Two reaction paths are possible, i.e., spontaneous release of plantinum from the sulfur, or nucleophilic displacement of platinum from sulfur by guanine (N7), for example. At the moment, there is no real evidence for the existence of such reactivation mechanisms. In fact, it has been reported that Pt-protein interactions in the plasma (albumin) are not reversible under normal conditions (161, 165). Further, a mixture of cis-Pt-methionine products does not show antitumor properties (166), indicating no induced platination of DNA. More research is required to investigate the existence of a reactivation mechanism. However, it is predicted that if such a reactivation phenomenon is operational, the most likely candidate is the labile Pt-methionine bond, as has been shown by its rapid reaction with Naddtc, STS, and thiourea (vide supra) (131). 

Reactions of Ru -pac Complexes with Sulfur-Containing Biomolecules 193... 

Ongoing Pt-binding studies with S-containing biomolecules and derivatives with the purpose to investigate in detail Pt migration reactions and competitions between a sulfur and the reactive guanine N7. 

Sulfur-containing amino acids like cysteine, methionine and tripeptide glutathione are very powerful antioxidants that participate in different stages of free-radical chain reactions of biomolecule oxidation [18]. For example, cysteine participates in the synthesis of taurine, the substance that effectively blocks the peroxide oxidation of lipids by binding hypochlorite anion to form chloramine complex. In any organism, cysteine and glutathione reduces the oxidized form of vitamin C to its initial active form while methionine (being an... 

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