Cassava (Manihot esculenta Crantz of cv. KU50) peroxidase and its potential for the detection of some thiol compounds based on the inhibitory effect of 3,3′,5,5′-tetramethylbenzidine oxidation

Abstract

Cassava peroxidase (CSP) isolated from cassava leaves (Manihot esculenta Crantz of cv. KU50) was purified by DEAE and concanavalin-A column chromatography. CSP was a haem-containing cationic glycoprotein with the molecular weight of 38 and 44kDa determined by MALDI-TOF-MS. Its kinetic catalysis in the presence and absence of some thiols were investigated and compared to those of horseradish and soybean peroxidases by using urea hydrogen peroxide (UHP) and 3,3′,5,5′-tetramethylbenzidine (TMB) as substrates. Inhibitory effects of some pesticides containing thiol groups such as thiosemicabazide, thiourea and thiophanate-methyl on TMB oxidation were graphically demonstrated their ability to be either competitive or noncompetitive inhibitors, depending on their properties. The degrees of inhibition were expressed as Ki and IC50 values. The applicable range for the detection of thiosemicabazide in solution was found to be in the range of 10–100μM, whereas those of thiourea and thiophanate-methyl were 40–400μM and 10–460μM, respectively. On the contrary, the inhibitory effects of some phenolic pollutants; 4-nitrophenol, 4-phenylphenol and pentachlorophenol on TMB oxidation were not significantly observed and the phenomenon was discussed.