Membrane association of active 5-lipoxygenase in resting cells. Evidence for novel regulation of the enzyme in the rat alveolar macrophage.
The enzyme 5-lipoxygenase (5-LO) catalyzes the first two steps in the metabolism of arachidonic acid to leukotrienes, substances which play pivotal roles both in normal host defense and in pathologic states of inflammation. Recent studies in granulocytic cells have shown that activation of 5-LO involves its Ca(2+)-dependent translocation from cytosol to membrane compartments. However, little information exists about the molecular regulation of 5-LO in macrophages, even though these cells comprise the resident effector cell population of most organs. We therefore examined the levels of 5-LO activity and immunoreactive protein in cytosol and membrane fractions of resident rat alveolar (AM) and peritoneal macrophages (PM) and compared them with the well studied human neutrophil (polymorphonuclear leukocyte). In the resting state, PM resembled polymorphonuclear leukocyte in that most of their cell-free 5-LO activity, as well as protein content, were localized to the cytosol fraction. By contrast, resting AM contained most of their activity and almost half of their immunoreactive protein in the crude membrane fraction. The inability of the drug MK-886 to reverse this membrane association suggested that the 5-LO-activating protein was not the site of binding in the resting cell; however, this drug completely inhibited leukotriene B4 synthesis in ionophore A23187-stimulated AM, indicating that an interaction between 5-LO and 5-LO-activating protein was nonetheless required for product synthesis upon stimulation. Translocation of cytosolic 5-LO protein could not be convincingly demonstrated in A23187-stimulated AM, suggesting that the pool of 5-LO enzyme responsible for product formation originated in the membrane rather than the cytosol fraction of the resting cell. The AM therefore represents the first mammalian cell in which 5-LO has been recovered from the membrane fraction (a) of a resting cell and (b) in active form. These novel findings extend our understanding of the molecular regulation of 5-LO and may be of importance in designing strategies to limit inflammation in the lung and other sites.