Fluorescence demonstration of dipeptidyl peptidase I (cathepsin C) in skeletal, cardiac, and vascular smooth muscles.

Dipeptidyl peptidase I (cathepsin C) was demonstrated histochemically in soleus, extensor digitorum longus, and cardiac muscles but not in vascular smooth muscle cells of the caudal artery of the rat. The enzyme using Pro-Arg-4-methoxy-beta-naphthylamide as the substrate was found in discrete granules in the striated muscles. The activity was greatest in the soleus muscle, with less activity observed in cardiac tissue, and only a few reactive sites observed in the extensor digitorum longus muscle. Under identical conditions no activity was observed associated with vascular smooth muscle cells. Dipeptidyl peptidase I activity was inhibited completely by 1mM HgCl2 in the incubation solutions and not preserved following conventional chemical fixation techniques.