5-Lipoxygeiiase (5-LO) is a Ca2+ stimulated enzyme that initializes the formation of proinflammatory leukotrienes from arachidonic acid (AA). In this report, we demonstrate that a soluble protein of the monocytic cell line Mono Mae 6 confers 5-LO activity Ca2+-dependent in vitro. Thus. in broken cell preparations of human polymorphonuclear leukocytes (PMNL) and rat basophilic leukemia (RBL)-1 cells, 5-LO converted AA (>20 muM) in the absence of Ca2+ whereas Ca2+ was absolutely required for 5-LO activity in broken cell preparations of MM6 cells. 5-LO partially purified from MM6 cells was substantially active in the absence of Ca2+. Recombination experiments revealed that the cytosolic fraction of MM6 cells contains a factor that suppresses the activity of partially purified 5-LO from PMNL. RBL-1, and MW cells in the absence but not in the presence of Ca2+. Further characterization showed that this factor is a 80-100 kDa heat-sensitive protein. (C) 2002 Elsevier Science (USA). All rights reserved.