THE ISOLATION AND CHARACTERISATION OF PECTIN METHYLESTERASE FROM PAPAYA

Abstract

The crude enzyme was extracted from ripe papaya fruits by a high ionic strength buffer at pH 7.0. Papaya pectin methylesterase was purified by a protocol consisting of ammonium sulphate precipitation (60-80 %) and cation exchange chromatography (CM-Sepharose CL-6B and Mono S). This method also fractionated papaya PME into two components, labelled as PME 1 and PME 2. Both these components were shown to be homogeneous using polyacrylamide gel electrophoresis and gel filtration. The purification factor obtained for PME 1 and PME 2 are 132.7 and 55.1 respectively. The physical characteristics of PME 1 and PME 2 were studied. Using gel filtration, the molecular weights of PME 1 and PME 2 were estimated to be 28,000 and 27,000 respectively. Ferguson plot analysis, however, gave a molecular weight of 21,000 for the two components. This method of analysis also suggests that PME 1 and PME 2 are isoenzymes as parallel lines were obtained from these plots. The isoelectric point of papaya PME is likely to be greater than pH 9.0, but it could not be determined accurately as it focused too near the cathode. PME 1 and PME 2 showed optimum activities at pH 8.0 and 35 °c. The assay condition used was 0.3 g/100 ml pectin in 0.05 M sodium phosphate buffer, pH 7.0, at 35 °c for 6 minutes. The average KM of PME 1 and PME 2 are 0.0696 and 0.165 g/100 ml pectin respectively while the average VMAX of PME 1 and PME 2 are 737.48 and 796.93 µmoles methanol/min/mg protein. PME 1 and PME 2 are activated by cations. Divalent cations are better activators than monovalent cations. Inhibition studies showed that sucrose was a non-competitive inhibitor of papaya PME with the calculated Kr for PME 1 and PME 2 of 0.56 and 0.60 g/100 ml pectin, while p-chloromercuribenzoic acid had no effect on its activity.