Fluorescent sensor for thiols: Deprotection of nonfluorescent 1 by thiols (R′SH) proceeds rapidly and near‐quantitatively in aqueous solution (pH 7.4) to produce highly fluorescent 2. Assays performed in the presence of 1 provide a rapid and simple method for the determination of inhibitory constants for inhibitors such as donepezil toward acetyl‐ and butyrylcholinesterases.
Reactive oxygen species (ROS) such as superoxide (O 2 ÀC ), hydrogen peroxide (H 2 O 2 ), and the hydroxyl radical (HOC) are important mediators of pathological processes in various diseases.[1] Detection by fluorescent probes is one of the most useful methods for evaluating the roles of ROS in pathological processes. 2',7'-Dichlorofluorescin (DCFH) and its diacetyl derivative (DCFH-DA) [2] have been widely used as fluorescent probes for measuring cell-derived H 2 O 2 , [3] but these compounds suffer from the major drawback that they are poorly selective toward H 2 O 2 . Researchers have demonstrated that oxidation of DCFH to dichlorofluorescein is also induced by peroxidase [4] and other hemoproteins [5] as well as by hydroperoxides in the presence of peroxidase, [6] nitric oxide, [7] and peroxynitrite. [8] Therefore, the fluorescent response based on the oxidation of DCFH provides an index, not for cell-derived H 2 O 2 , but for the total oxidants present in biological systems. This limitation stems from its mechanism of fluorescence, which is based on oxidation. Dihydro derivatives of fluorescent compounds such as dihydrorhodamine 123 [3c,g] and N-acetyl-3,7-dihydroxyphenoxazine (Amplex Red) [9] have been shown to function as probes for detecting H 2 O 2 . However, their mechanism of action is similar to that of DCFH, which implies that low selectivity toward H 2 O 2 is a shortcoming that must be accepted when utilizing these probes. In fact, dihydrorhodamine 123 was shown to react with various ROS, [3c, 7b] and although Amplex Red seems to have high selectivity toward H 2 O 2 , peroxidase is essential for its fluorescence, similar to the case of DCFH. Thus, developing probes for H 2 O 2 based on a non-oxidative fluorescence mechanism, which would allow the highly specific and peroxidase-independent detection of H 2 O 2 under the complicated oxidative circumstances found in biological systems, is a worthwhile goal.Recently, we found that perhydrolysis of acyl resorufins is a useful reaction that acts as a fluorescent indicator for H 2 O 2 assays.[10] The method is based on simple deprotection, not on oxidation, thus allowing acyl derivatives of fluorescent compounds such as resorufin and fluorescein to work as probes for detecting cell-derived H 2 O 2 with higher selectively than that provided by DCFH and its analogues. Unfortunately, the competition between perhydrolysis and hydrolysis of acyl resorufins and fluoresceins in biological systems was not altered in a manner favorable towards H 2 O 2 -based deacylation.We thus designed pentafluorobenzenesulfonyl fluoresceins (1 a-c, Scheme 1) as selective fluorescent probes for H 2 O 2 but would eliminate, or at least significantly reduce, competition from hydrolysis reactions of the acetyl derivatives. These compounds were chosen for the following reasons: sulfonates are more stable to hydrolysis than are esters; fluoresceins have high fluorescence quantum yields in aqueous solution; and the pentafluorobenzene ring enhances the reactivity of the sulfonates tow...
Fluorometric detection of O2-* is performed based on desulfonylation of 3 to the corresponding fluoresceins 4 through nucleophilic substitution, and this fluorescing process is quite specific toward O2-* over H2O2, t-BuOOH, NaOCl, 1O2, HO*, NO*, and ONOO-. Furthermore, effects of glutathione, cytochrome P450 reductase/NADPH, and diaphorase/NADH are relatively small on the fluorescing process of probe 3 with X = Y = F, which is useful to detect O2-* released from neutrophils stimulated by phorbol myristate acetate with satisfactory sensitivity.
Clinical application of anticancer agents has been often hampered by toxicity against normal cells, so the achievement of their cancer-specific action is still one of the major challenges to be addressed. Previously, we reported that arsenic trioxide (As 2 O 3 ) could be a promising new drug against not only leukemia but also solid tumors. The cytotoxicity of As 2 O 3 occurred through the generation of reactive oxygen species (ROS), thus inhibiting radical scavenging systems would enhance the therapeutic efficacy of As 2 O 3 provided that normal cells were relatively resistant to such a measure. Here, we report that the combination therapy of As 2 O 3 with L-buthionine-sulfoximine (BSO), which inhibits a critical step in glutathione synthesis, effectively enhanced in vitro growth inhibition effect of As 2 O 3 on all 11 investigated cell lines arising from prostate, breast, lung, colon, cervix, bladder, and kidney cancers, compared with As 2 O 3 treatment alone. Furthermore, this combination enhanced cytotoxicity to cell lines from prostate cancer with less toxicity to those from normal prostate. In vitro cytotoxic assay using ROSrelated compounds demonstrated that hydrogen peroxide (H 2 O 2 ) is a major cytotoxic mediator among ROS molecules. Biochemical analysis showed that combined use of As 2 O 3 and BSO blocked H 2 O 2 -scavenging systems including glutathione, catalase, and glutathione peroxidase, and that the degree of this blockade was well correlated with intracellular ROS levels and sensitivity to this treatment. Finally, the effectiveness of the combination therapy of As 2 O 3 with BSO was demonstrated with an orthotopic model of prostate cancer metastasis. We propose that the combination therapy of As 2 O 3 with BSO is a valid means of blockade of H 2 O 2 -scavenging system, and that the combination of a ROSgenerating agent with an inhibitor of major scavenging systems is effective in terms of both efficacy and selectivity. Furthermore, because the effective doses of both compounds are within clinically achievable range, this report will lead to immediate benefit for the development of a new cancer therapy.
Simply the BESThio: The fluorescein derivative BESThio (see scheme) operates as a thiol probe at pH 7.4 and functions as a selenol probe at pH 5.8, and can thus be selectively applied to the rapid quantification of selenocysteine and the determination of the selenocysteine content in selenoproteins such as glutathione peroxidase and thioredoxin reductase with high sensitivity.
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