Kinetic constraints for the thiolysis of 4-methyl-5-(pyrazin-2-yl)-1,2-dithiole-3-thione (oltipraz) and related dithiole-3-thiones in aqueous solution

This report summarizes an investigation of the reactions of biological and other thiols with the cancer chemopreventive oltipraz and other dithiolethiones. Analysis of the kinetics of reaction of 4-methyl-5-(pyrazin-2-yl)-1,2-dithiole-3-thione (oltipraz) 1 with monothiols and dithiols in the range of 0.75-20 mM in aqueous 15% ethanol, at pH 7.5 (0.1 M Tris buffer) and at 37 degrees C has been undertaken. A plot of k(obsd) against thiol] shows that reactions of mono- and dithiols are first order in thiol concentration. The dependence on pH of these reactions shows that the active species is the thiolate anion. Specific second-order rate constants, k(2) (M(-1) s(-1)) for reaction of the thiolate anions with oltipraz have been determined to be cysteine, 0.040 +/- 0.001; 2-mercaptoethanol, 2.0 +/- 0.02; glutathione, 0.099 +/- 0.001; mercaptoacetic acid anion, 4.0 +/- 0.01; dithiothreitol, 1.33 +/- 0.02; 1,3-propanedithiol, 10 +/- 0.5; 1-mercaptopropane-3-ol, 6.5 +/- 0.1; 1-mercaptopropane-2,3-diol, 1.26 +/- 0.05. A plot of pK(a) against log k(2) for monothiols shows a linear dependence of k(2) on pK(a), beta(nuc) 1.1 +/- 0.07, which accounts for most of the reportedly enhanced reactivity of dithiols over monothiols. The pseudo-first-order rate constant for the solvolysis of oltipraz has been measured as 2.2 (+/-0.2) x 10(-8) s(-1). The kinetics of reaction of three other dithiole-3-thiones with glutathione has also been studied for comparison with oltipraz. The specific second-order rate constants, k(2) (M(-1) s(-1)) are 5-phenyl-1,2-dithiole-3-thione, 4.7 x 10(-)(4); 5-(4-methoxyphenyl)-1,2-dithiole-3-thione, 4.1 x 10(-4); and 1,2-dithiole-3-thione 0.08. Important implications for the mode of biological action of these compounds and the nature of the putative biological targets of the compounds are discussed.