Antibacterial Activity of Fistulin: A Protease Inhibitor Purified from the Leaves of Cassia fistula

Plant protease inhibitors (PPIs) are one of the important components of a plant’s defense machinery. PPIs are active against the insects and microbes which invade the plant. Cassia species possess anti-insecticidal and antimicrobial properties and this study was aimed at investigating the antibacterial efficacy of a PPI present in the leaves of Cassia fistula. A PPI, fistulin, was isolated from the leaves of C. fistula and purified by gel filtration chromatography. The antibacterial activity of the purified fistulin was studied against five bacterial strains, namely, Bacillus subtilis, Staphylococcus aureus, Klebsiella pneumoniae, Pseudomonas aeruginosa and Escherichia coli. The PPI was found to be very active against S. aureus, E. coli, B. subtilis, and K. pneumonia, and its efficacy was comparable to the standard drug, streptomycin sulphate. 1. Introduction Proteases are indispensable to the maintenance and survival of plant and animal systems. The proteolytic events catalysed by these enzymes serve as mediators of signal initiation, transmission, and termination in many of the cellular events such as inflammation, apoptosis, blood clotting, and hormone processing pathways [1]. Despite the fact that these enzymes are indispensable to the cells and organisms that host them, they may be potentially damaging when overexpressed or present in higher concentrations. For this reason the activities of these enzymes need to be strictly regulated and controlled [2]. The synthesis of proteases as inactive preproteins and their substrate specificity are factors which monitor their activities. Nevertheless, these do not fulfill the desired level of regulation and therefore cells and organisms require additional means of control. One important control mechanism involves interaction of the active enzymes with molecules that inhibit their activities. These inhibitors partially or completely inhibit their enzymes and are called protease inhibitors (PIs). PIs comprise a large and diverse group of plant proteins capable of forming reversible protein—protein complexes with enzymes resulting in their inactivation [3]. Protease inhibitors are ubiquitous in nature and are found to be involved in various important biological functions like digestion of proteins, control of blood clotting, apoptosis, and signaling receptors interaction in animal. PIs are of common occurrence in the plant kingdom. Plant PIs (PPIs) are generally small proteins or peptides that occur in storage tissues, such as tubers and seeds and also in the aerial parts of plants [4]. PPIs are usually