A new stefin type low-M r, cysteine proteinase inhibitor (PLCPI) was isolated from pig polymorphonuclear leukocytes as a contaminant of the cathelin sample. The inhibitor consists of 103 amino acids, and its M r, was calculated to be 11,768. The inhibitor exhibits considerable sequence identity with inhibitors from the stefin family, particularly with human stefin A. The PLCPI is a fast acting inhibitor of papain and cathepsins L and S (k ass ⩾ 1 × 106 M−1 · s−1) and forms very tight complexes with these enzymes (K i, ⩽ 190 pM). The affinity for cathepsins B and H (K i ⩾ 125 nM) was lower. These results also show that the inhibitory activity previously ascribed to cathelin was due to the presence of PLCPI.
