There is also a considerable diversity of specificity of phospholipase enzymes responsible for phospholipid hydrolysis, in terms of both positional and molecular species selectivity. Phospholipase A activity in rat liver will act selectively to remove sn-1 16:0 from PC species containing sn-2 18:2, whereas cytosolic phospholipase A2 (PLA2) is specific for species containing sn-2 20:4n-6. In contrast, secretory PLA2 must be bound to negatively charged phospholipids for activation, but it is not acyl specific. Mammalian phospholipase Cs (PLCs) act preferentially on phos-phatidylinositol-4,5-bisphosphate rather than on PI or PC, whereas agonist-stimulated phospholipase Ds (PLDs) are selective for PC species. However, although the distribution of phospholipases is tissue specific, the contribution of their activities to the regulation of phospholipid compositions in most tissues has not been well defined.
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