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-esterified fatty acids by the enzyme acting on the organized phospholipid mixture constituting the membrane matrix has led to the identification of two prominent effecters, sphingomyelin (SPH) and annexin. Recombinant hu mall type II sPLA(2) hydrolyses red-cell membrane phospholipids with a marked preference for the inner leaflet. This preference is apparently related to the high content of SPH in the outer leaflet, which inhibits sPLA(2). This inhibition by SPH is specific for sPLA(2). Cholesterol counteracts the inhibition of sPLA(2) by SPH, suggesting that the SPH-to-cholesterol ratio accounts in vivo for the variable susceptibility of cell membranes to sPLA(2). Different effects were observed of the presence of the non-hydrolysable D-alpha-dipalmitoyl phosphatidylcholine (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. The present study has provided the first evidence that annexin VI, in concentrations that inhibit hydrolysis of purified phospholipid substrates, stimulated the hydrolysis of membrane phospholipids by sPLA(2). The activation requires the presence of membrane proteins. The effect is 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 membranes results in the preferential release of polyunsaturated fatty acids. It suggests that type II sPLA(2) in conjunction with annexin VI, might be involved in the final step of endocytosis and/or exocytosis providing the free polyunsaturated fatty acids acting synergistically to cause membrane fusion.