A weakly acidic protease has a powerful proteolytic activity in Entamoeba histolytica.

A thiol dependent protease from homogenates of the parasite Entamoeba histolytica has been identified and partially purified by means of ammonium sulphate fractionation, gel filtration and isoelectric focusing. The protease, having a molecular mass of 21 +/- 2 kDa as judged by gel chromatography, represents a highly potent proteolytic capacity. The protease shows maximal activity against azocasein around the slightly acidic pH of 4.4 at 37 degrees C, but is also active at pH 3.4 and 8.5. At optimal pH, the turnover increases with increasing temperature up to 85 degrees C. The enzyme possesses a thiol group essential for activity, which is inhibited by the thiol blocking reagent p-chloromercuribenzoate. Solutions containing low concentrations of free Hg2+ cause conservation of protease activity. The native protein exhibits an isoelectric point of 4.9. This protease resembles the thiol endopeptidases of mammalian lysosomes, and appears to be a major proteolytic enzyme in Entamoeba histolytica.