Chromogenic method of detecting endoproteases

A method for the detection and determination of endoproteases comprising:

• a) incubating a solution suspected of containing an endoprotease with a solution containing a chromogenic peptide substrate for said endoprotease, said chromogenic peptide substrate comprising a compound of the formula I ##STR5## in which X denotes a hydrogen atom, a group which irreversibly masks the terminal amino group, or a conventional protecting group selected from the group consisting of carbobenzoxy, t-butoxycarbonyl and 9-fluorenyloxycarbonyl,
• A and B may be identical or different and denote an alpha-, beta- or gamma-amino acid which comprises 2 to 15 carbon atoms and up to 4 nitrogen atoms, 2 sulfur atoms and 6 oxygen atoms, and B may denote a dipeptide formed from these amino acids,
• C denotes arginine, lysine, tyrosine, phenylalanine or tryptophane, and their homologs,
• R.sub.1 and R.sub.2 may be identical or different and denote a hydrogen atom or an alkyl radical having up to 4 carbon atoms,
• R.sub.3 to R.sub.8 may be identical or different and denote hydrogen, an alkyl radical, an alkoxy radical or a halogen radical,
• Y denotes oxygen, and
• An.sup.- denotes an anion,
• and its water-soluble salts, and
• b) determining the amount of chromophore released by said endoprotease from said chromogenic peptide substrate.