Cathepsin L is capable of truncating cystatin C of 11 N-terminal amino acids

Cystatin C with the 11 N-terminal amino acids truncated shows a much lower affinity for cysteine proteinases than the intact inhibitor. Such truncatio n of cystatin C is recorded after action of glycyl endopeptidase and cathep sin L. Incubation of cystatin C with papain, cathepsin B or cathepsin H led to no changes in the cystatin C molecule. Isoelectric focusing of the cath epsin L and cystatin C mixture showed the formation of two new bands, One o f them appeared whether E-64 or PMSF was added or not, evidently representi ng a cystatin C/cathepsin I, complex. The other band is the truncated cysta tin C molecule. N-terminal sequencing after separation by HPLC showed that cystatin C is cleaved by cathepsin L at the Gly11-Gly12 bond, The action of cathepsin L on cystatin C may be explained by the cleavage of the scissile bond in an inappropriate complex. (C) 1999 Federation of European Biochemi cal Societies.