Enzymatic Mechanisms and Inhibition of Phospholipase A2
The control of eicosanoid production in inflammatory and other cells depends on the release of free arachidonic acid from the sn-2 position of membrane phospholipids (summarized in Fig. 1). Phospholipase A2 is the simplest and most obvious candidate for the responsible enzyme, which makes its regulation, including both its activation and inhibition, crucial determinants of eicosanoid production (Dennis, 1987). Our laboratory has developed the dual phospholipid model (Roberts et al., 1977) to explain the mechanism of action of an extracellular phospholipase A2; our current understanding of the enzyme is summarized elsewhere (Dennis, 1983; Dennis and Plückthun, 1986). Our studies on the enzymes from snake venom and mammalian pancreas provide a paradigm for the phospholipases responsible for arachidonic acid release. In addition, they provide the best sources of readily available, pure, stable phospholipase A2 for testing potential inhibitors of phospholipase A2. This chapter discusses three inhibitor systems that we have recently characterized. These include amide ether analogues of phospholipids, which comprise an example of a reversible competitive inhibitor (Davidson et al, 1986), manoalide, which is an example of an irreversible inhibitor (Lom-bardo and Dennis, 1985) that is perhaps mechanism based and for which analogues can be studied (Deems et al.,1987), and the lipocortins or calpactins, where calpactin I is an example of inhibition by a substrate depletion model (Davidson et al., 1987). Meaningful interpretation of such studies can only be accomplished within the framework of the enzyme’s kinetics and mechanism of action at the lipid—water interface, which are discussed.
KeywordsLactone Ring Enzymatic Mechanism Phospholipid Vesicle Arachidonic Acid Release Eicosanoid Production
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