The activity of dipeptidyl(amino)peptidase IV (DAP IV, glycylproline naphthylamidase) was discovered in the endothelial cells of the venous part of capillary bed and of small venules of many organs of the rat, mini-pig, rabbit, cock as well as man. In aortae, large arteries and veins only a portion of vase vasorum displays a positive reaction. Glycyl-proline-4-methoxy-2-naphthylamide (Gly-Pro-MNA) is the substrate of choice both from the viewpoint of enzyme kinetics as well as localization. Phenylalanyl-proline-4-methoxy-2-naphthylamide (Phe-Pro-MNA) is cleaved less easily, however, it enables a good localization. 1- and 2-naphthylamine derivatives of glycylproline display better kinetic properties than Phe-Pro-MNA, however they enable a satisfactory localization under special conditions only. The recommended diazonium salt for the routine is Fast Blue B. The enzyme is quite firmly associated with the structure and chloroform-acetone preextraction of cryostat sections does not influence its activity significantly while improving the localization. Block fixation in aldehydes inhibits the enzyme activity (glutaraldehyde more than formaldehyde). The osmificated azo-dye originated of 4-methoxy-2-naphthylamine and Fast Blue B or hexazonium-p-rosaniline is still partially soluble in solvents used for the usual embedding in epoxyresins for electron microscopical examination. This is a drawback for a reliable demonstration of DAP IV in endothelial cells on the electronmicroscopical level using the epoxy-resin technique. DAP IV of the endothelium is inhibited totally by DFP (10-3M), partially by E 600 (10-3); and slightly by phenanthroline (10-3M). It is unaffected by EDTA (10-3M) and N-ethyl maleimide (10-3M). The combined demonstration of alkaline phosphatase and DAP IV in the same section renders a reliable demonstration of the capillary bed in many organs. The contribution of DAP IV activity of the capillary endothelium to the total DAP IV activity in a particular organ is decisive in the myocardium, striated muscle, aorta and lung; it represents about one half of the total activity in spleen and pancreas and is less expressed in the liver, intestine and particularly in the kidney. In the jejunum of patients sufferring coeliac sprue the activity of capillary endothelium in the propria is decreased or not demonstrable in the acute stage. After a gluten-free diet it is restituted. The activity of DAP IV does not change significantly in aortae of the rabbit and man with atherosclerosis. In plaques of human aortae the capillary endothelium reacts at the most. Vasa vasorum in the adventitia overlying large plaques, which penetrate into the media, display a high DAP IV activity and their number can be increased. In plaques of arteries of cocks there is a positive DAP IV reaction in foam cells. DAP IV does not belong to the enzymes indicating early changes in atherogenesis. The function of DAP IV in the endothelium is not known. It may be a part of the machinery influeneing the protein part of the endothelial coat or may participate in the degradation of some vasoactive peptides. © 1979 Springer-Verlag.
