Function and Inhibition of Intracellular Calcium-independent Phospholipase A2

Abstract

Our previous Minireview (1Dennis E.A. J. Biol. Chem. 1994; 269: 13057-13060Abstract Full Text PDF PubMed Google Scholar) considered the three main kinds of phospholipase A2(PLA2) 1The abbreviations used are: PLA2, phospholipase A2; sPLA2, secretory Ca2+-dependent phospholipase A2; cPLA2, 85-kDa Ca2+-dependent cytosolic phospholipase A2; iPLA2, Ca2+-independent cytosolic phospholipase A2; BEL, bromoenol lactone; PAP-1, Mg2+-dependent phosphatidic acid phosphohydrolase; PAF, platelet-activating factor; AA, arachidonic acid; DAG, 1,2-diacylglycerol; CHO, Chinese hamster ovary. : the well characterized Groups I, II, and III small Ca2+-dependent secretory phospholipase A2s (sPLA2), the 85-kDa Group IV Ca2+-dependent cytosolic phospholipase A2 (cPLA2), and the 80-kDa Ca2+-independent cytosolic phospholipase A2(iPLA2). In the ensuing years, it has become clear that PLA2 represents a growing superfamily of enzymes with many additional sPLA2s (Groups IIC, V, and IX), further definition of the 80-kDa iPLA2 (Group VI), and two Ca2+-independent PLA2s specific for platelet-activating factor (PAF) (Groups VII and VIII) (2Dennis E.A. Trends Biochem. Sci. 1997; 22: 1-2Abstract Full Text PDF PubMed Scopus (758) Google Scholar). All of the well studied sPLA2s appear to use a His-Asp catalytic mechanism and require Ca2+ to be bound tightly in the active site of the enzyme. The well characterized iPLA2appears to require a central Ser for catalysis and of course, no Ca2+. Interestingly, the Group IV cPLA2 does not use Ca2+ for catalysis, but rather the Ca2+dependence seems to relate to a calcium lipid-binding domain (CaLB or C-2 domain) at the N-terminal end responsible for association of the enzyme with the membrane. Thus, the catalytic mechanism and active site Ser do not involve Ca2+ (3Reynolds L.J. Hughes L.L. Louis A.I. Kramer R.M. Dennis E.A. Biochim. Biophys. Acta. 1993; 1167: 272-280Crossref PubMed Scopus (151) Google Scholar, 4Sharp J.D. Pickard R.T. Chiou X.G. Manetta J.V. Kovacevic S. Miller J.R. Varshavsky A.D. Roberts E.F. Strifler B.A. Brems D.N. Kramer R.M. J. Biol. Chem. 1994; 269: 23250-23254Abstract Full Text PDF PubMed Google Scholar, 5Pickard R.T. Chiou X.G. Strifler B.A. DeFelippis M.R. Hyslop P.A. Tebbe A.L. Yee Y.K. Reynolds L.J. Dennis E.A. Kramer R.M. Sharp J.D. J. Biol. Chem. 1996; 271: 19225-19231Abstract Full Text Full Text PDF PubMed Scopus (96) Google Scholar); therefore a mechanistic distinction between the Group IV cPLA2 and the iPLA2s may not be warranted at this time. This is relevant because most of the inhibitors that work on the Group IV cPLA2 also act on the Group VI iPLA2 (6Conde-Frieboes K. Reynolds L.J. Lio Y.C. Hale M.R. Wasserman H.H. Dennis E.A. J. Am. Chem. Soc. 1996; 118: 5519-5525Crossref Scopus (85) Google Scholar, 7Lio Y.C. Reynolds L.J. Balsinde J. Dennis E.A. Biochim. Biophys. Acta. 1996; 1302: 55-60Crossref PubMed Scopus (15) Google Scholar). Inhibitor specificity will be discussed in the next section. We (8Ackermann E.J. Dennis E.A. Biochim. Biophys. Acta. 1995; 1259: 125-136Crossref PubMed Scopus (129) Google Scholar) recently surveyed all of the reported Ca2+-independent PLA2 activities. While there exists a group of lysosomal iPLA2s and a group of characterized ectoenzymes with broad specificity, which may actually be general lipases (8Ackermann E.J. Dennis E.A. Biochim. Biophys. Acta. 1995; 1259: 125-136Crossref PubMed Scopus (129) Google Scholar), sequenced and well characterized intracellular iPLA2s are limited to the 80-kDa Group VI iPLA2and the 29-kDa Group VIII enzyme, which is a PAF acetyl hydrolase (9Hattori M. Adachi H. Tsujimoto M. Arai H. Inoue K. J. Biol. Chem. 1994; 269: 23150-23155Abstract Full Text PDF PubMed Google Scholar). The latter hydrolyzes the acetyl chain present at the sn-2position of PAF and perhaps acts on oxidized phospholipids as well but not on normal phospholipids carrying unoxidized long chain fatty acids at the sn-2 position (9Hattori M. Adachi H. Tsujimoto M. Arai H. Inoue K. J. Biol. Chem. 1994; 269: 23150-23155Abstract Full Text PDF PubMed Google Scholar). This enzyme and a secreted Group VII PAF acetyl hydrolase, both of which are really iPLA2s with a particular substrate specificity, have been considered elsewhere (2Dennis E.A. Trends Biochem. Sci. 1997; 22: 1-2Abstract Full Text PDF PubMed Scopus (758) Google Scholar). The Group VI 80-kDa iPLA2 was first identified in P388D1 macrophages (10Ross M.I. Deems R.A. Jesaitis A.J. Dennis E.A. Ulevitch R.J. Arch. Biochem. Biophys. 1985; 238: 247-258Crossref PubMed Scopus (38) Google Scholar), purified (11Ackermann E.J. Kempner E.S. Dennis E.A. J. Biol. Chem. 1994; 269: 9227-9233Abstract Full Text PDF PubMed Google Scholar), further characterized (12Ackermann E.J. Conde-Frieboes K. Dennis E.A. J. Biol. Chem. 1995; 270: 445-450Abstract Full Text Full Text PDF PubMed Scopus (379) Google Scholar), and then cloned and sequenced by Jones and co-workers (13Tang J. Kriz R.W. Wolfman N. Shaffer M. See