Competitive inhibition of cathepsin C by guanidinium ions and reexamination of substrate inhibition

Cathepsin C, a lysosomal dipeptidyl aminopeptidase, is competitively and re versibly inhibited by guanidinium ions with a K-i approximate to 1.5 mM, Lo ss of activity is not the result of conformational change, subunit dissocia tion or altered mobility of the enzyme, but rather reflects a specific bind ing of guanidinium ions to the active site. The finding that cathepsin C is not inhibited by substrate has allowed the kinetic parameters in the prese nce of guanidinium ion to be determined, Guanidinium significantly decrease s the K-m of substrate hydrolysis, without changing V-max. In a novel appli cation of the transferase reaction, the K-m of the nucleophile substrate ha s been determined (11 mM) and found not to be affected by guanidinium, indi cating its inhibition of substrate binding to the S, but not the S', site, Inhibition is suggested to be the result of shielding a negative charge on the enzyme important for interaction with the substrate. (C) 1999 Academic Press.