This invention provides a method of detecting enzyme activity on a solid
medium. The enzyme substrate has a chromogenic portion comprising a
catechol residue, in which a derivitising moiety is linked to the
aromatic ring of the catechol via a bond, and an enzyme cleavable group
which is attached via an ester or ether linkage to the oxygen atom
derived from a hydroxyl group of the catechol residue. If the enzyme
substrate contacts an enzyme capable of cleaving the enzyme cleavable
groups and the cleaved compound contacts a chelatable metal ion, a
substantially non-diffusable coloured precipitate is formed.