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Based on Table II in M. Waite (1996) Phospholipases in Biochemistry of Lipids, Lipoproteins and Membranes (eds. D.E. Vance and J.E. Vance), pp. 211-236 with kind permission of the author and Elsevier Science.

active when ATP and respiratory control drop to low levels. Also, the widespread distribution of phospholipases A2 allows many tissues to perform re-tailoring of the molecular species of membrane lipids by the Lands mechanism. In this process, named after Lands, the American biochemist who first described it, cleavage of the acyl group from the sn-2 position yields a lysophospholipid that can be re-acylated with a new fatty acid from acyl-CoA (Fig. 7.11).

Some phospholipases A2 play key roles in signal transduction. The Group IV cytosolic phospholi-pases A2 translocate to membranes when the intracellular Ca2+ concentration rises. They have a very high specificity for arachidonate at the sn-2 position of phospholipids and a distinct catalytic mechanism compared to the Group I-III enzymes. Ca2+ is not involved in catalysis itself but, by promoting enzyme-membrane interaction, increases activity several-fold. Within its catalytic domain a highly conserved sequence around the active-site serine (ser 228) is identical with that of phospholipase B from Penicillium (below) and another serine (ser 505) can be phosphorylated by mitogen activated protein (MAP) kinase. Phosphorylation increases the activity significantly and this can account for typical agonist stimulation of phospholipase A2 activity. In addition, a number of cellular agonists (such as the inflammatory cytokines interleukin-1 and tumour necrosis factor) increase the cellular content of cytosolic phospho-lipase A2 by gene expression.

Several phospholipases A2 are Ca2+-indepen-dent. One of these, which is found in blood, has particularly good activity against short-chain acyl groups. It is believed to be important for inactivating PAF (Section 7.1.10).

Finally, it is well-recognised that many phospholipases A have high activity against oxidized or oxygen-containing acyl groups. They can, therefore, play a key role in removing such moieties from oxidized lipids in order to preserve normal lipid function.

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