UNCOVERING A CALCIUM-REGULATED MEMBRANE-BINDING MECHANISM FOR SOYBEANLIPOXYGENASE-1

Lipoxygenases catalyze the biosynthesis of leukotrienes, lipoxins, and other lipid-derived mediators that are involved in a wide variety of pathophysiological processes, including inflammation, allergy, and tum origenesis. Mammalian lipoxygenases are activated by a calcium-mediate d translocation to intracellular membranes upon cell stimulation, and cooperate with cytosolic phospholipase A(2) at the membrane surface to generate eicosanoids. Although it has been documented that plant cell stimulation increases intracellular Ca2+ concentration and activates cytosolic phospholipase A(2), followed by lipoxygenase-catalyzed conve rsion of the liberated linolenic acid to jasmonic acid, no evidence is available for Ca2+-regulated membrane binding and activity of plant l ipoxygenases. Plant lipoxygenases, unlike their mammalian counterparts , are believed to function independently of calcium or membranes. Here we present spectroscopic evidence for a calcium-regulated membrane-bi nding mechanism of soybean lipoxygenase-1 (L-1). Both calcium and memb rane binding affect the structure and the mode of action of L-1. Free L-1 in solution is less accessible to the polar solvent and converts l inoleic acid to conjugated dienes, whereas surface binding increases s olvent accessibility and stimulates conjugated ketodiene production. C alcium exerts a biphasic effect on the structure and activity of L-1. Our results uncover a new regulatory mechanism for plant lipoxygenases and delineate common features in animal and plant cell signaling path ways.