Phospholipases Degrade Phospholipids

Enzymes that degrade phospholipids are called phospholipases. They are classified according to the bond cleaved in a phospholipid (fig. 19.11). Phospholipases Al and A2 selec- tively remove fatty acids from the sn-1 and sn-2 positions, i respectively. Phospholipase C cleaves between glycerol and the phosphate moieties phospholipase D hydrolyzes the [ head-group moiety X from the phospholipid. Lysophospho- lipids, which lack a fatty acid at the sn-1 or sn-2 position, j are degraded by lysophospholipases. f Phospholipases are found in all types of cells and in 

I various subcellular locations within eukaryotic cells. Some of these enzymes are specific for particular polar head-groups others are nonspecific. Phospholipase A2 is a major component of snake venom (cobra and rattlesnake) and is partially involved in the deadly effects of these venoms. Because of the high concentration of phospholipase A2 in these venoms, this enzyme has been studied intensively. The pancreas is also rich in phospholipase A2, which is secreted into the intestine for digestion of dietary phospholipids. 

In some cases the functions of phospholipases in cells are purely degradative and result in the release of the phospholipid components (fatty acids, glycerol, phosphate, and head-groups). But in many cases phospholipases have important roles in synthesis and regulation. For example, we have seen how phospholipase A2 catalyzes the first step in the remodeling of phosphatidylcholine to the surfactant 

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Based mainly on cell-free assays, 10 enzymatic activities that degrade phospholipids, intermediates in the phospholipid biosynthetic pathway, or triacylglycerol have been reported (Table 2). The detergent-resistant phospholipase A, (encoded by pldA) of the outer membrane, characterized by Nojima and colleagues (Y. Nakagawa, 1991), is the most studied of these enzymes. This enzyme is unusually resistant to inactivation by heat and ionic detergents... 

The intracellular signaltransduction of ofi-adrenoceptors is effectuated by a G-protein-dependent activation of the phospholipase C. This enzyme cleaves phosphatidylinositol, a phospholipid present in cell membranes, into inositol-1,4-5-triphosphate (IP3) and diacylglycerol (DAG). IP3 is a strong inductor of intracellular calcium release which leads to an increase of smooth muscle tone or the liberation of hormones stored in vesicles. Noradrenaline which is released by exocytosis, spreads by diffusion only. Only a small fraction of the total amount of the transmitter released will actually reach the postsynaptic membrane and bind to its specific receptors. Another fraction escapes the synapic cleft by diffusion and is finally enzymatically degraded in the interstitial fluid. Another fraction is taken up postsynaptically and metabolized enzymatically by the target cells (uptake 2). By far most of the transmitter (90%) is actively taken up by the releasing neuron itself (uptake 1 or neuronal re-uptake). In the... 

Certain classes of lipids are susceptible to degradation under specific conditions. For example, all ester-linked fatty acids in triacylglycerols, phospholipids, and sterol esters are released by mild acid or alkaline treatment, and somewhat harsher hydrolysis conditions release amide-bound fatty acids from sphingolipids. Enzymes that specifically hydrolyze certain lipids are also useful in the determination of lipid structure. Phospholipases A, C, and D (Fig. 10-15) each split particular bonds in phospholipids and yield products with characteristic solubilities and chromatographic behaviors. Phospholipase C, for example, releases a water-soluble phosphoryl alcohol (such as phosphocholine from phosphatidylcholine) and a chloroform-soluble diacylglycerol, each of which can be characterized separately to determine the structure of the intact phospholipid. The combination of specific hydrolysis with characterization of the products by thin-layer, gas-liquid, or high-performance liquid chromatography often allows determination of a lipid structure. 

In the duodenum, dietary lipids are degraded by pancreatic enzymes triacylglycerol by pancreatic lipase, phospholipids by phospholipase A2 and lysophospholipase, and cholesteryl esters by cholesterol esterase. Enzyme release from the pancreas is controlled by cholecystokinin, produced by cells in the intestinal mucosa. 

Lysophospholipids have been found in butter serum by Cho et al. (1977). They characterized the sn-1 and -2 lysophosphatidylcholines and phosphatidylethanolamines. It is not known if these compounds are products of degradation or remnants of biosynthesis. Cho et al. (1977) searched for, but did not find, another possible product of enzymatic degradation of milk, phosphatidic acid. Phosphatidic acid can be formed by the action of phospholipase D on phosphatidylcholine, for example, but this enzymatic activity was not detected. The compound is also an important intermediate in the biosynthesis of lipids, but the concentration in tissue is always very low. The amount is also low in milk. Cho et al. (1977) found 1.2 and 0.9 (percent of total lipid P) of the lyso compounds above. The quantities of the other phospholipids were phosphatidylethanolamine, 27.3 -choline, 29.1 -serine, 13.4 -inositol, 2.5 and sphingomyelin, 25.6. 

There are numerous reactions by which the acyl groups or polar head-groups of phospholipids might be modified or exchanged. Phospholipids are degraded by specific phospholipases. 

Andresen T. L. and Jorgensen K. (2005). Synthesis and membrane behavior of a new class of unnatural phospholipid analogs useful as phospholipase A2 degradable liposomal drug carriers. Biochim. Biophys. Acta Biomembr. 1669 1-7. 

Phospholipases are potentially important in milk and milk products because of their ability to degrade the phospholipids of the milk fat globule membrane, thereby increasing the susceptibility of the milk fat to lipolytic attack (Fox et al., 1976 Griffiths 1983). 

Deeth, H.C. 1983. Phospholipid degradation by phospholipases of some psychrotrophic bacteria. In Fats for the Future. Proc. Int. Conf. Oils Fats Waxes, 132-135, Duromark Publications, Auckland. 

Many of the phospholipases, on the other hand, are very selective enzymes. The pancreatic phospholipase 2 (or phospholipase A2) hydrolyzes only esters in position 2 of sn-3 phospholipids i.e., the enzyme is stereospecific (Table II). Phospholipases I (or Ai) attack only the I-position. Lysophospholipases remove the remaining fatty acid, either in position I or 2, from the lysophospholipid. Phospholipases play a role in the post-mortem degradation of meat and fish. 

Phospholipases are found in most cell compartments. Mitochondria have a heat stable and Ca -dependent phospholipase 2 in their outer membranes (23). Phospholipases 1 and 2 occur in microsomal and lysosomal cell fractions. Very little is known about these enzymes. The lysosomal enzymes may be responsible for the post-mortem degradation of phospholipids in meat and fish. 

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