Different light microscopical procedures for
the histochemical demonstration of catalase were tested
in cryostat sections of mussel digestive gland tissue by
using both benzidine and diaminobenzidine (DAB) as
hydrogen donors. The selected procedure, which was
also applied to mouse liver for comparative purposes,
consisted of incubation in media containing 0.2% DAB
and 0.3% H202 at pH 10.4 for 35 min at 42 "C. Addition
of 0.01 M imidazole to the incubation medium increased
the staining intensity of the histochemical procedure.
The positive reaction product was localized in epithelial
cells lining the digestive tubules and the ducts. The
histochemical reaction was inhibited partially by
aminotriazole or sodium azide and disappeared
completely by omission of H202 from the incubation
medium. On the other hand, heat resistant nonenzymatic
reactions were observed in sites known to
contain lipofuscins such as epithelial cells of the
gastrointestinal tract and connective tissue brown cells