Histochemical and biochemical studies of dipeptidyl peptidase I (DPP I) in laboratory rodents

Different from other proteases the halide- and thiol-dependent lysosomal dipeptidyl peptidase I (DPP I, cathepsin C, EC 3.4.14.1.) was not thoroughly analysed up to now. Therefore, using complementary methods of catalytic histochemistry and biochemistry, DPP I was studied in many rat and mouse organs and compared with DPP II, another lysosomal protease, whose biochemical and histochemical behaviour is known. Continuous (kinetic) fluorometric measurements revealed Gly-Arg naphthylamine (NA) as the substrate with the highest rate of hydrolysis in which, however, also microsomal alanyl aminopeptidase may participate. A more specific substrate appeared to be Pro-Arg-NA; Gly-Phe-NA, and Ser-Tyr-NA were inferior substrates. The Km values were 0.1 mmol/l and 0.2 mmol/l for Gly-Arg-NA and Pro-Arg-NA. The optimal substrate concentration was between 1 and 2.5 mmol/l and the optimal pH value between 5 less than or equal to pH less than or equal to 5.5. The highest reaction velocity was measured with cacodylate or phosphate buffer. Chloride ions and thiol reagents increased the rate of hydrolysis, but an absolute chloride or thiol dependence was not found. Formaldehyde or glutaraldehyde inhibited DPP I depending on the type and concentration of aldehyde. The DPP I activities in rat and mouse organs varied species-dependently and were highest in the extraorbital gland, liver, jejunum, and kidney. In many of the organs, DPP I activity differed considerably from that of DPP II. For catalytic DPP I histochemistry and localization of the enzyme in lysosomes and secretion granules freeze-dried celloidin-mounted cryosections incubated with Gly-Arg-4-methoxy-2-naphthylamine in the presence of Fast Garnet GBC or Fast Blue B were the method of choice in comparison with other tissue pretreatments, Pro-Arg-MNA as substrate and other coupling agents. Use of thiol reagents and chloride ions did not improve the histochemical results. Using this method DPP I was visualized in many cells of rat and mouse organs where its existence was not yet known before. At many sites, DPP I and DPP II distribution patterns differed considerably. In conclusion, a histochemical chromogenic method is now available, which allows the reliable detection of DPP I as was already possible for DPP II using an MNA substrate.