Cathepsin G in human mononuclear phagocytes: comparisons between monocytes and U937 monocyte-like cells

Previous studies demonstrating that a continuous line of human monocyte-like cells (U937) and human monocytes contain elastase probably identical to human neutrophil elastase suggested the possibility that mononuclear phagocytes share other proteinases with neutrophils. The present work establishes that U937 cells contain cathepsin G, an enzyme heretofore found only in neutrophils. U937 cells contain approximately 10 micrograms of cathepsin G-like activity per 10(7) cells, about 25% of the cathepsin G activity in human neutrophils. Normal monocytes have minimal cathepsin G-like activity (approximately 0.1 microgram per 10(7) cells). The cathepsin G-like activity of U937 cells appears to be due to an enzyme that is the same as cathepsin G by several criteria: 1) it is a serine proteinase with activity like cathepsin G against a synthetic chymotrypsin substrate, succinyl-ala-ala-pro-phe-p-nitroanilide; 2) the proteolytic fragments it releases from fibronectin match those released by cathepsin G; 3) like cathepsin G, it can be purified by sequential Trasylol-Sepharose affinity chromatography and carboxymethyl-Sephadex ion exchange chromatography; 4) its amino acid composition and migration on SDS-polyacrylamide gel electrophoresis are indistinguishable from cathepsin G; and 5) it binds with antiserum raised to cathepsin G. The presence of cathepsin G in U937 cells, in much higher concentration than in normal monocytes, indicates either that the content of cathepsin G in monocytes decreases markedly during monocyte differentiation or that U937 cells differ from normal immature monocytes with respect to synthesis of this neutral proteinase.