Cytochemistry of membrane proteases

Membrane proteases that are detectable by cytochemical means are the classified exopeptidases, aminopeptidases A and M (or N), gamma-glutamyl transpeptidase (which also acts as transferase), dipeptidyl peptidase IV and the endopeptidase, enteropeptidase (also known as enterokinase). Not yet classified are the possible exopeptidase, tripeptidyl peptidase and endopeptidases I (Ala-endopeptidase) and II (Arg-endopeptidase). All these membrane proteases can be investigated with either chromogenic or fluorogenic procedures using synthetic peptide substrates. The most useful substrates are 4-methoxy-2-naphthylamine amino acids and peptides for cytochemical localizations at the light and electron microscope levels, for cytophotometric quantification and the study of membrane protease isoenzymes after analytical isoelectric focusing. Amino acid or peptide derivatives of naphthylamine AS can be recommended for light microscopical localization and cytofluorometric quantification, and 7-amino-4-methylcoumarin and 7-amino-4-trifluoromethylcoumarin amino acids and peptides for the development of enzyme bands after isoelectric focusing. Cytochemistry reveals the heterogeneity in the distribution and species differences of membrane proteases in adult cells, tissues and organs and during development. It also reveals some common localizations, such as in small intestinal enterocytes and proximal tubule cells. The species and organ differences are substantiated and extended considerably by isoelectric focusing in combination with methods for the cytochemical detection of proteases. In addition, continuous cytophotometry or cytofluorometry (section and cultured cell biochemistry) allows the kinetic characteristics, initial reaction rates and maximum activities of all membrane proteases to be determined. The physiological functions of the endopeptidases and exopeptidases are still a matter of debate. However, from cytochemical inhibition studies with natural peptide substrates, e.g. peptide hormones, there is increasing evidence that the proteases detected with synthetic peptides play a decisive role in many physiological circumstances, e.g. in endocrine regulation mechanisms or the regulation of blood pressure. In this respect, capillary endothelium-linked surface membrane proteases may be especially important.