Evidence for membrane association of plasminogen activator activity in mouse macrophages

Production and release of high levels of plasminogen activator, a serine protease often referred to as a secretory product, has been considered as a biochemical index for mouse macrophage "activation". Although the mechanism for plasminogen activator release is not known, several characteristics of the release process suggest that the enzyme may be shed from the cell surface of activated macrophages rather than secreted. In this paper, we show that plasminogen activator activity in thioglycollate elicited macrophages is predominantly associated with a subcellular fraction consisting mainly of membranes and granules which are pelletable at 100,000 x g. Furthermore, plasminogen activator activity can be solubilized only by detergents and not by treatments which are known to release granule-bound contents as well as loosely associated peripheral membrane proteins. Thus, these results suggest that macrophage plasminogen activator is firmly found to cellular membranes.