Calcium dependent thiol protease caldonopain and its specific endogenous inhibitor in Leishmania donovani

A calcium dependent proteolytic enzyme was detected in the lysed promastigotes of Leishmania donovani, the causative agent of kala-azar. The protease was able to hydrolyse an added substrate, azocasein and showed high affinity for calcium. Rate of azocasein digestion was primarily slow but boosted up after eight hours. It was not inactivated when heated at 55 degrees C for 15 min at pH 7.4. Sulfhydryl reagents significantly reduced the enzymic activity but trypsin-like protease inhibitors hardly had any effect. The enzyme was not sensitive to calmodulin from a heterologous source but registered low activity when treated with chlorpromazine. The caseinolytic activity was stimulated when leishmanial cells were preincubated with ionophore A23187 in presence of 1 mM Ca2+. The enzyme is named caldonopain due to its similarity with a general class of calcium dependent protease calpain present in different tissues and cells. Caldonopain was found to be localized in cytosol along with its specific endogenous inhibitor caldonostatin. The ratio of caldonopain-caldonostatin unit was higher in the infected macrophage compared to the parasitic protozoa and Balb/c macrophage alone. It may be postulated that the amount of both calcium and its protein inhibitor may have a direct impact on the caldonopain-induced biological process to regulate cellular action of this pathogen.