MODULATION OF HUMAN TYPE-II SECRETORY PHOSPHOLIPASE A(2) BY SPHINGOMYELIN AND ANNEXIN-VI

Conjectural results have been reported on the capacity of inflammatory secreted phospholipase A(2) (sPLA(2)) to hydrolyse mammalian membrane phospholipids. Development of an assay based on the release of non-es terified fatty acids by the enzyme acting on the organized phospholipi d mixture constituting the membrane matrix has led to the identificati on of two prominent effecters, sphingomyelin (SPH) and annexin. Recomb inant hu mall type II sPLA(2) hydrolyses red-cell membrane phospholipi ds with a marked preference for the inner leaflet. This preference is apparently related to the high content of SPH in the outer leaflet, wh ich inhibits sPLA(2). This inhibition by SPH is specific for sPLA(2). Cholesterol counteracts the inhibition of sPLA(2) by SPH, suggesting t hat the SPH-to-cholesterol ratio accounts in vivo for the variable sus ceptibility of cell membranes to sPLA(2). Different effects were obser ved of the presence of the non-hydrolysable D-alpha-dipalmitoyl phosph atidylcholine (D-DPPC), which renders the membranes rigid but does not inhibit sPLA(2). Annexin VI was shown, along with other annexins, to inhibit sPLA(2) activity by sequestering the phospholipid substrate. T he present study has provided the first evidence that annexin VI, in c oncentrations that inhibit hydrolysis of purified phospholipid substra tes, stimulated the hydrolysis of membrane phospholipids by sPLA(2). T he activation requires the presence of membrane proteins. The effect i s specific for type II sPLA(2) and is not reproducible with type I PLA (2). The activation by annexin VI of sPLA(2) acting on red cell membra nes results in the preferential release of polyunsaturated fatty acids . It suggests that type II sPLA(2) in conjunction with annexin VI, mig ht be involved in the final step of endocytosis and/or exocytosis prov iding the free polyunsaturated fatty acids acting synergistically to c ause membrane fusion.