Human cystatin A is inactivated by engineered truncation. The NH2-terminal region of the cysteine proteinase inhibitor is essential for expression of its inhibitory activity

A comparaison of the inhibitory activity of NH2-terminal human cysteine proteinase fused with porcine adenylate kinase shows that the amino-terminal is necessary for its activity. Removal of six residues from the amino-terminal decreases the inhibitory activity while removal of 15 residues completely inactivates the inhibitor. However the removal of the Met residue from the amino-terminal or substitution of the Pro3 with Leu residue does not affect the inhibitory activity. Substitution of the Gky75 with His residue decreases the loss of activity.