Investigation of proteases with chromogenic substrates after isoelectric focusing (IEF)

A technique is described for the detection of protease isoenzymes which is more sensitive than disc electrophoresis. Supernatants of crude rat and human organ homogenates are subjected to analytical isoelectric focusing (IEF) and the gel strips are finally incubated in histochemical media containing 4-methoxy-2-naphthylamine amino acids or peptides and diazonium salts for simultaneous or post-coupling. The incubation media are identical with those used for section histochemistry of proteases. This combination of IEF and proteases histochemistry yields excellent and reproducible data which cannot be obtained by protease histochemistry alone. Post-coupling delivers less and more diffuse bands than simultaneous coupling. For simultaneous coupling, Fast Blue B and Fast Black K are the most suitable diazonium salts. More bands are found in agarose gels compared with polyacrylamide. Sex-differences exist for endopeptidases in the submandibular gland, but are absent in other rat organs. Despite their uniform membrane localization in tissue sections, aminopeptidase (AP) A and M and dipeptidylpeptidase (DPP) IV and gamma-glutamyltranspeptidase (GGT) show striking heterogeneous band patterns depending on the investigated organ. The similar band patterns of APA and APM can be specified by the use of activators or inhibitors. In rat kidney, up to 26 bands are obtained with DPP II and IV substrates, 3 for APA and APM and up to 12 for GGT. DPP IV of human liver is different from that in rat liver.