INACTIVATION OF HUMAN ALPHA1 PROTEINASE-INHIBITOR BY THIOL PROTEINASES

Human plasma .alpha.1 proteinase inhibitor is the body''s principal modulator of serine proteinases (such as those released from phagocytic cells). Cysteine-active-site proteinases, which are not inhibited, inactivate this important inhibitor by proteolytic cleavage of a scissile peptide bond. Papain carries out this inactivation catalytically, whereas human liver cathepsin B1 acts stoicheiometrically. Thus thiol proteinases could easily disrupt the delicately regulated balance between serine proteinases and .alpha.1 proteinase inhibitor.