Extraction, purification and characterization of papain cysteine-proteases from the leaves of Carica papaya

Papain is a globular cysteine-protease family consisting of a single polypeptide chain with three disulfide bridges and a sulfhydryl group necessary for proteolytic activity. Its application is extensive to the fields of medicine and food. The aim of this study is to extract, purify and characterize papain enzymes from the leaves of Carica papaya. Crude extracts containing the enzymes were extracted from the leaves of papaya in Mountain Top University, Nigeria, and purified in a 3-step purification using 70% NH4(SO4)2, DEAE-Cellulose column chromatography and Sephadex G-25 column chromatography to achieve a purification fold of 1.6424. The effects of temperature, pH, substrate concentration and Mg2+ concentrations on the activity of the enzymes were determined. This is a premier report on the presence of five cysteine-proteases in the leaves of papaya with different measures of binding affinity and catalytic efficiencies towards the casein-substrate. The purified enzymes from Carica papaya are optimally active from 50 °C - 59 °C and pH 4.5 – 6.6. Their activities are generally enhanced in the presence of Mg2+. The Km and Vmax values for the enzyme in the various pools ranged from 1.47mg/ml - 8.70mg/ml and 0.42µmol/ml/min - 0.4167µmol/ml/min. In this study, papain E displayed optimum activity at pH 5.5 and 50 °C in the presence of Mg2+ with the best catalytic efficiency (Kcat/Km 59.776 - pmL/mg.min) and binding affinity (Km - 0.83 mg/mL) when compared to the isolated enzymes. Values of the parameters assessed from this study could be put to use by the industry in the production of papain and also for household and medical use in storing the product.