Cysteine‐proteinase‐inhibiting function of T kininogen and of its proteolytic fragments

Previous attempts to liberate T kinin from T kininogen [Moreau et al. (1986) Eur. J. Biochem. 159, 341–346; Gutman et al. (1988) Eur. J. Biochem. 171, 577–582] have shown that complete fragmentation of the precursor molecule into inhibitory peptides was achieved before any vasoactive peptide was released, suggesting a possible physiological significance for this phenomenon. In this study, cysteine‐proteinase‐inhibiting properties of rat T kininogen and of its proteolytic fragments issuing from trypsin and submaxillary gland endopeptidase k hydrolysis, have been investigated using rat lysosomal cathepsins B, H and L, papain and bovine calpains I and II. All three lysosomal cathepsins were inhibited by T kininogen but tighter interactions were observed with cathepsin L and papain. Though higher K1 values were obtained for cathepsins B and H, rate constants for association were found to have high and almost similar values (in the 106 M−1 s−1 range) whathever the enzyme used. Proteolytic fragments also inhibited cathepsin L and papain very strongly and even better than the entire molecule for some of them, but no significant inhibition of cathepsins B and H was observed. Bovine calpains were not inhibited by T kininogen nor by its proteolytic fragments. From the results of this kinetic analysis, which indicates that both the association and the dissociation of lysosomal cysteine proteinases with T kininogen may occur rapidly, an hypothesis has been put forward on the possible in vivo functioning of T kininogen as a proteinase inhibitor.