Electrospray Mass Spectrometric Analysis of Lipid Mediators Derived from Arachidonic Containing Membrane Phospholipids

Abstract

Arachidonic acid (5,8,11,14(Z,Z,Z,Z)- eicosatetraenoic acid) is one of the most common polyunsaturated fatty acids present within virtually all mammalian cells. Arachidonic acid plays an important role in biological chemistry because of its unique physical properties such as influencing membrane fluidity as well as serving as a source of a diverse family of oxidized metabolites known to play important physiological and pathophysiological roles within tissues. Even though a very small percentage of the total arachidonic acid within cells is ever involved in the oxidative reactions that lead to the formation of compounds such as prostaglandins [1] and leukotrienes [2], a considerable amount of effort has been placed in designing agents that either inhibit production of these molecules or block their actions. Prostaglandins and leukotrienes exert their biological activity through G-protein linked membrane receptors on the surface of cells, and these specific receptors are linked to numerous signal transduction processes, which lead to a myriad of biological responses mediated by these lipid products. The enzymes that form these lipid mediators utilize arachidonic acid only as a free carboxylic acid, and do not act on arachidonic acid while it is esterified to membrane phospholipids. This is rather curious because the majority of all arachidonate within cells exists in the plasma and organelle membranes in various phospholipid classes esterified to the sn-2 position [3]. The other curious feature of the enzymes responsible for the initial oxidation of arachidonic acid involves the formation of free radical intermediates which can readily react with molecular oxygen. The enzymes cyclooxygenase, 5-lipoxygenase, 15-lipoxygenase, and 12-lipoxygenase utilize iron in various bound forms in the active site to initiate these free radical reactions. Within the active site, the chemical reactivity of these radical species is controlled, resulting in a stereospecific reaction mechanism.