An inactive cathepsin B-like enzyme and cysteine proteinase inhibitors in colon cancer ascites.

The ascitic fluid of a patient with colon cancer was found to contain an inactive cathepsin B-like enzyme. The inactive enzyme with a molecular weight of 40 kDa was converted by pepsin treatment into an active form with a molecular weight of 28 kDa as revealed by Sephadex G-75 gel chromatography. The inactive cathepsin B-like enzyme was considered to represent a precursor form and not an enzyme-inhibitor complex. The activated cathepsin B-like enzyme resembled human liver cathepsin B in its enzymatic characteristics. Cysteine proteinase inhibitor activity was also detected in the same ascitic fluid, and it was separated into two main forms by Sephadex G-75 gel chromatography. The high molecular weight inhibitor fractions reacted with antiserum against alpha-CPI and the low molecular weight fractions reacted with antiserum against cystatin B.