Identification and partial characterization of three chitinase forms in Entamoeba invadens with emphasis on their inhibition by allosamidin

Three chitinase forms were identified in Entamoeba invadens cysts following fractionation of a soluble fraction by anionic exchange, size exclusion and hydroxyapatite adsorption chromatographies. The enzymes, named here as A, B and B', showed molecular weights of 64, 33.4 and 33.4 kDa, respectively, as measured by gel filtration. Comparison of their levels of specific activity in partially purified samples revealed chitinase A as the major species. Chitinase B' was a minor component of the chitinolytic complex. Whereas some properties were common to the three forms, analysis of other parameters revealed significant catalytic site-related differences. Accordingly, the three chitinases hydrolyzed the fluorogenic substrate 4-methylumbellifery chitotriose with typical Michaelian kinetics and K-m values of 4.5, 11.8 and 3.8 mu M for A, B and B', respectively. Allosamidin strongly inhibited the three enzyme forms with different kinetics. Dixon plots revealed competitive-type inhibition and Ki values of 10.0, 2.3 and 10.8 nM for A, B and B', respectively. K-m/K-i ratios indicated 450-, 350- and 5130-fold higher affinity for the inhibitor over the substrate for the A, B and B' forms, respectively. Results are discussed in terms of the possibility that the three chitinase species correspond to different enzyme proteins.