Phospholipase definition

Serotoninergic System. Figure 1 Graphical representation of the current classification of 5-hydroxytryptamine (5-HT) receptors. Receptor subtypes represented by shaded boxes and lowercase designate receptors that have not been demonstrated to definitively function in native systems. Abbreviations 3-5r cyclic adenosine monophosphate (cAMP) phospholipase C (PLC) negative (-ve) positive (+ve)... 

It appears on the basis of the above studies that phospholipase D does exhibit a stereopreference and also has preference for a phosphoglyceride with a particular substituent on the C-l position. However, further definition of the behavior of phospholipase D requires the use of highly purified enzyme. 

The phospholipases A, (PLAjS) comprise a large group of 1-acyl hydrolases, some of which also degrade neutral lipids (lipases) or remove the acyl group at position 2 in addition to that at position 1 (PLB), and thus must have lysophospholipase activity. Where the enzyme appears to show low selectivity for the sn-l or sn-2 position, the term phospholipase A is used. The term phospholipase B should be restricted to those enzymes where the mechanism involves minimal accumulation of lysophospholipid product. In this section, we consider various enzymes of the PLA type that do not fit a more precise definition in terms of acyl chain selectivity. 

Anti-inflammatory corticosteroids reduce PG production in vivo but not in vitro. They induce the synthesis and/or release of two proteins macrocortin , MW 15,000, first isolated from perfused guinea pig lungs, and lipomodulin , MW 40,000, isolated from rabbit neutrophils. These have been reported to inhibit phospholipase A2 (Blackwell and Flower 1983), but definite proof is still lacking (Bonney and Humes 1984). It is not known whether similar proteins are induced within the CNS. Mepacrine, chlorpromazine, and halothane are all reported to inhibit the enzyme (Blackwell and Flower 1983). 

Early experiments on phospholipase A used an enzyme from snake venom which released one mole of fatty acid from a diacyl phospholipid. The original studies showed quite clearly that unsaturated fatty acids were released from egg or liver phosphatidylcholines and so it was quite natural to assume that the enzyme was specific for unsaturated acids. When fully saturated or fully unsaturated lipids were the substrates, however, one mole of fatty acid was still released from one mole of phospholipid and it was realized that the enzyme must be specific for one or other of the two positions. Early chemical analyses of the products of phospholipase A hydrolysis seemed to indicate that the enzyme released the fatty acid from the 1-position. When it was discovered that the enzyme also liberates fatty acid from plasmalogen (where it is known quite definitely to be located at the 2-position) the problem was reinvestigated more rigorously. 

At present we do not have a definite reason as to why some membrane proteins are attached through phosphatidylinositol. One possibility is that it may provide a means of selectively mobilizing these proteins. Indeed, there is some evidence that endogenous phospholipases may cause the release of VSG and alkaline phosphatase in soluble forms and other proteins such as acetylcholinesterase are also known to be released under certain physiological conditions. 

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