Evidence for distinct isoenzymic forms of a cell surface protease on normal colonic cells and on carcinoma cells from...

Both normal and carcinoma cells of the colon possess the cell surface protease guanidinobenzoatase (GB). GB is similar to plasminogen activator. In fixed sections, the insolubilised GB can be located by the fluorescent probe 9-amino acridine (9-AA) which binds to the active centre of GB, causing cells possessing active GB to fluoresce yellow. Both cell types possess cytoplasmic proteins which can be extracted in isotonic saline and were shown to inhibit their respective cell surface GB. The formation of an enzyme-inhibitor complex was demonstrated by the failure of 9-AA to react with the cell surface GB inhibitor complex. The data presented indicate that the colonic carcinoma cells possess an isoenzymic form of GB not recognised by the inhibitors of GB extracted from normal colonic cells.