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Carboxylesterases (EC 3.1.1.1) of chicken brain were investigated by applying kinetic analysis of organophosphorus inhibition. By iterative elimination of exponential inhibition curves and by sequential inhibition experiments using a combination of two organophosphorus inhibitors, 11 different carboxylesterases of chicken brain were characterized with respect to their phenyl valerate-hydrolyzing activity (milliunits per gram of brain) and their inhibition by O,O-diethyl O-4-nitrophenyl phosphate (Paraoxon), O,O-diisopropylphosphorofluoridate, and N,N'-diisopropylphosphorodiamidic fluoride (Mipafox). The bimolecular inhibition rate constants (liters . mole-1 . min-1) were calculated for the 11 enzymes and 3 organophosphorus compounds. The corresponding data for acetylcholinesterase (EC 3.1.1.7) in chicken brain were determined. The importance of inhibition rate constants for the development of acute cholinergic symptoms, delayed neurotoxicity, and atypical organophosphate effects is shown.
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PMID:Inhibition of brain carboxylesterases by neurotoxic and nonneurotoxic organophosphorus compounds. 686 14

The reaction rates (ks) of vitamin E (alpha-, beta-, gamma-, delta-tocopherols, TocH), ubiquinol-10, and related antioxidants (tocol, ubiquinol-0, and hydroquinone) with aroxyl (ArO(.-)) radical have been measured in micellar solution by stopped-flow spectrophotometer. The ks values increased in the order of hydroquinone < tocol < delta-TocH < ubiquinol-0 < gamma-TocH approximately beta-TocH < ubiquinol-10 < alpha-TocH at pH 4 approximately 8. The antioxidants which have lower oxidation potentials showed higher reactivities. The ks values of alpha-, beta-, gamma-, delta-tocopherol, and tocol remained constant between pH 4 and 10, and decreased rapidly at pH 11 approximately 12 by increasing pH value. From the pH dependence of ks values, the pKa values (= 13.1 approximately 12.6) have been determined for these tocopherols. The ks values of ubiquinol-10 also remained constant between pH 4 and 9, and increased rapidly at pH 9.5. Ubiquinol-10 is dibasic acid and can exist in three different molecular forms, depending on the pH value. By comparing the ks values with the mole fraction of each molecular form of ubiquinol-10, the reaction rate ks1 (= 1.21 x 10(5) M(-1)s(-1)) for the undissociated form, ks2 (= 1.04 x 10(6) M(-1)s(-1)) for monoanion and ks3 (= 0 M(-1)s(-1)) for dianion, and the pKa1 and pKa2 values (= 11.4 and 12.7) were determined. The ks2 value is 8.6 times as large as the ks1 value. Similar analyses were performed for ubiquinol-0 and hydroquinone. It was found that the relative ratio of ks values (100:21:20:2.9) of alpha-, beta-, gamma-, delta-tocopherols in micellar dispersion has good correlation with the relative biopotency ratios for rat fetal resorption, rat hemeolysis, and chicken muscle dystrophy. The relative antioxidant activities of alpha-tocopherol and ubiquinol-10 have been discussed based on the ks values obtained and their concentrations in serum and several tissues (heart, muscle, liver, kidney, and brain).
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PMID:Structure-activity relationship of the free-radical-scavenging reaction by vitamin E (alpha-, beta-, gamma-, delta-Tocopherols) and ubiquinol-10: pH dependence of the reaction rates. 1722 24

Detailed kinetic studies have been performed for the reaction of aroxyl (ArO.) radical with vitamin E (alpha-, beta-, gamma-, delta-tocopherol, TocH), ubiquinol-10, and related antioxidants in micellar solution, using a stopped-flow spectrophotometer. The second-order reaction rates (ks) obtained increased in the order of hydroquinone < tocol<delta-TocH<ubiquinol-0<gamma-TocH-beta-TocH<ubiquinol-10<alpha-TocH at pH 4-8. The antioxidants which have lower oxidation potentials (Ep) showed higher reactivities. The reaction rates obtained in micellar solution were pH dependent because of the dissociation of OH groups in the antioxidants. For instance, by comparing the ks values with the mole fraction of each molecular form of ubiquinol-10, the reaction rate ks1 (1.21 x 10(5) M(-1)s(-1)) for undissociated form, ks2 (1.04 x 10(6) M(-1)s(-1)) for monoanion and ks3 (0 M(-1)s(-1)) for dianion, and the pKa1 and pKa2 values (11.4 and 12.7) were determined. It was found that the relative ratio of ks values (100:21:20:2.9) of alpha-, beta-, gamma-, delta-tocopherols in micellar dispersion has good correlation with the relative biological activities for rat fetal resorption, rat haemolysis, and chicken muscle dystrophy. The relative antioxidant activities of alpha-tocopherol and ubiquinol-10 have been discussed on the basis of the products of the ks values and their concentrations in serum and several tissues (heart, muscle, liver, kidney, and brain).
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PMID:Comparison between the free-radical-scavenging activities with vitamin E and ubiquinol in biological systems based on their reaction rates: a research account. 1909