Characterization of peroxidase in post-harvested cassava Manihot esculenta Crantz Tubers

Abstract

Peroxidase (EC 1.11.1.7) is a ubiquitous plant enzyme that catalyzes the oxidation of cellular components by H2O2. A peroxidase was purified from the parenchyma of cassava roots stored for 7 days by 40-80% ammonium sulfate precipitation follow ed by Concanavalin a Sepharose 4B and Sephadex G-200 columns respectively. The enzyme was purified 16 fold with a specific activity of 560 U/mg. The native molecular weight of the enzyme was found to be 105 kD by gel filtration and the subunit molecular weight was estimated to be 54 kD by SDSPAGE, indicating a homodimer form of the enzyme. The purified enzyme was separated into 5 forms on isoelectric focusing gel with pI ’s 5.1, 5.2, 5.4, 5.8 and 6. The enzyme was a glycoprotein with high content of carbohydrate and showed soret band at 398 nm. The enzyme was stable in broad pH range of 4-11 with optimum pH at 5 and optimum temperature at 60℃. The enzyme retained about 80% of its activity during incubation at temperature upto 50℃ for 4 hr. Partial deglycosylation of the enzyme with 3% TFMS reduced about 50% carbohydrate w/w The partial deglycosylated enzyme showed decrease in pH and temperature stability, reduce intensity of pI’s 5.1, 5.2, 5.8, 6 bands and appearance o f a new band at pI 5.3. The cassava parenchyma peroxidase catalyzed the oxidation of the following substrates: H2O2, DAB, guaiacol, o-dianisidine, pyrogallol and syringaldazine. Partial deglycosylated enzyme showed a decrease in the Km of all the substrates except syringaldazine.