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 dithiolethi ones. Amalysis of the kinetics of reaction of 4-methyl-5-(pyrazin-2-yl)-1,2 -dithiole-3-thione (oltipraz) I with monothiols and dithiols in the range o f 0.75-20 mM in aqueous 15% ethanol, at pH 7.5 (0.1 M Tris buffer) and at 3 7 degreesC has been undertaken. A plot of k(obsd) against [thiol] shows tha t reactions of mono- and dithiols are first order in thiol concentration. T he 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 c ysteine, 0.040 +/- 0.001; 2-mercaptoethanol, 2.0 +/- 0.02; glutathione, 0.0 99 +/- 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-mereaptopropane-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 reactivit y 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). T he kinetics of reaction of three other dithiole-3-thiones with glutathione has also been studied for comparison with oltipraz. The specific second-ord er rate constants, k2 (M-1 s-1) are 5-phenyl-1,2-dithiole-3-thione, 4.7 x 1 0(-4); 5-(4-methoxyphenyl)-1,2-dithiole-3-thione, 4.1 x 10(-4); and 1,2-dit hiole-3-thione 0.08. Important implications for the mode of biological acti on of these compounds and the nature of the putative biological targets of the compounds are discussed.