Characterization of cysteine proteinase endogenous inhibitors obtained from transformed fibroblasts

: The thermostable endogenous cysteine proteinase inhibitors (CPI) from the primary (REF), immortal (clone IE5) and transformed (clones trF8 and trF8nmcc) fibroblasts were isolated. All the isolated CPI act as reversible competitive inhibitors of cathepsins B and L and of papain. The study of inhibition of cathepsins B and L, purified from the same cell cultures as the CPI, showed that the Ki values for CPI from the cultures of immortal and transformed cells were by one order higher than the Ki values for CPI of primary fibroblasts. The data obtained suggest that immortalization and transformation alter the CPI properties.