It is possible that there is present in your extract another protease
which attacks the protease you are assayng and which is inhibited by
the 3,4-dichloroisocoumarin.
Cheers,
--
Craig Marshall craigm at sanger.otago.ac.nz
Biochemistry Department Phone +64 3 479 7570
University of Otago Fax +64 3 479 7866
P.O. Box 56, Dunedin, New Zealand