Abstract

Introduction: Soursop (Annona muricataLinn) is an edible lowland tropical fruit-bearing tree that is widely cultivated across regions of the world. It has been extensively researched as a result of its store of acetogenin; a potent anticancer agent. However, there is a dearth of information on the precise mechanism of action of acetogenin. It is therefore imperative to investigate this plant in the hope of discovering a different class of anticancer agent inherent in it. Various studies have demonstrated that cysteine protease inhibitors (CPIs) have considerable therapeutic potential which can be utilized in a variety of disease states including cancer. Aims: Study was designed to isolate, purify and characterize CPI from the fruits and seeds of Soursop. Materials and Methods:Isolation and purification of CPI were achieved by simple methods consisting of ammonium sulphate precipitation, anion exchange chromatography and size exclusion chromatography. Mode of inhibition, optimum pH and temperature, as well as the effect of metals on the enzyme activity was determined using spectrophotometry. Results:The purified CPI from seeds and fruits exhibited competitive and noncompetitive inhibition against papain respectively. However, maximal inhibitory activities for both fruit and seed samples were observed at similar optimal pH and temperature of 8 and 40°C respectively. Although, metal cations such as cobalt (Co2+), copper (Cu2+) and zinc (Zn2+) did not impact a considerable decrease on the inhibitory activity of the CPI; Lead (Pb2+), Magnesium (Mg2+) and manganese (Mn2+) significantly inhibited CPI at a very low concentration (1mM). Conclusion: The antagonistic properties exhibited by the purified CPI certainly indicate its likely suitability for pharmaceutical application in the treatment of some pathological conditions such as cancer, in which uncontrolled proteolytic activities of cysteine proteases are implicated. There is an ample scope for further research on structure elucidation and protein engineering to facilitate its usage in a wide range of application.

Keywords: Cysteine protease inhibitor, Enzyme inhibition, Papain, Purification, Characterization, and Annona muricata