Characterization of a Thiol Proteinase in Giardia lamblia

Sonicated preparations of Giardia lamblia hydrolyze a variety of protein substrates including human immunoglobulin. Activity is increased by thiol-activating agents and inhibited by thiol proteinase inhibitors. About 55% of activity remains in the soluble fraction after high-speed centrifugation, and pretreatment with a nonionic detergent results in increased soluble activity. This suggests that the enzyme is membrane bound or associated with subcellular particles. Activity elutes as a major peak at 38,000 molecular weight by calibrated sieve chromatography. The favored sites of enzymatic cleavage of IgA1 are between the CH2 and CH3 domain and near the hinge region of the heavy chain. Similar cleavage patterns were identified using sonicated preparations of Entamoeba histolytica and Trichomonas vaginalis.