Phospholipase A, enzyme

Phospholipase A Enzyme in the cell membrane that generates arachidonic acid from membrane lipid constituents... 

Phospholipase A enzymes - particularly those from snake venoms or digestive secretions - have been widely studied. Phospholipase Ai is found in microsomal and liposomal fractions (cf. Newkirk and Waite, 1971 Gatt, 1968). It specifically deacylates phosphatidylcholine or phosphatidylethanolamine at the 1-position. Both these substrates are hydrolysed at the same rate by the adrenal medulla lysosomal enzyme, but that from brain prefers phosphatidylcholine as substrate. Detergents will increase phosphatidylethanolamine hydrolysis by the brain enzyme. A phospholipase Ai (which is relatively specific for phosphatidylglycerol) has been reported from the spores of some bacteria (Raybin et aL, 1972), but most bacterial enzymes are unspecific for either the 1- or the 2-positions. 

Just why a different pattern of molecular species is synthesized at low temperature remains uncertain. At the present time attention is directed to the Dunaliella phospholipase A enzymes because of their elevated activity at low temperature (Norman and Thompson, 1986). Since these hydrolases display a specificity towards certain phospholipid molecular species (H. Norman and G. Thompson, unpublished... 

One to three percent of water is mixed thoroughly with the oil at 50-70 °C. The phospholipids hydrate within 1 hour to form a gum with a higher specific density than the oil. The easily hydratable phospholipids are PC, PI and LPC. PE and PA have low hydrating properties and are therefore marked as nonhydratable phospholipids (NHPs). In practice a mix of these various phospholipids is separated in the lecithin gum. After this water degumming process, sometimes enzymatic degumming is applied for hydrolysing the NHP with phospholipase-A enzymes into hydrophilic lysophosphatidylethanolamine... 

Major allergens in all vespid venoms are phospholipase A, a 33.5-kDa enzyme which digests cell membranes (Ves vl for V vulgaris) and antigen 5 (Ves v5), a 23-kDa... 

While most investigations show that sea snake neurotoxins are postsynaptic type, Gawade and Gaitonde (23) stated that Enhydrina schistosa major toxin has dual actions or postsynaptic as well as presynaptic toxicity. E, schistosa venom phospholipase A is both neurotoxic and myotoxic. Neurotoxic action of the enzyme is weak so that there is sufficient time for myonecrotic action to take place (24), Sea snake, L. semifasciata toxin also inhibits transmission in autonomic ganglia, but has no effect on transmission in choroid neurons. 

Figure 11.30 Mechanisms of regulation of phospholipase A2. In all these processes described above, it is phospholipase A that carries out the hydrolysis of membrane phospholipid. Cytokines are local hormones produced by immune cells, T-lymphocytes and macrophages (Chapter 17). Other factors relate to shear stress in endothelial cells and those that stimulate release of granules from mast cells. Eicosanoids are present in the granules and they must be re-synthesised after degranulation in the mast cells. Here the enzymes described above must be present in mast cells.

Enzyme Ei is the phospholipase A, for which there is an excess of substrate in the plasma membrane i.e. a zero order process. (Eor details of this process, see Chapter 11). E, is a phosphatase, which catalyses a first order process. In fact, IP2 can be hydrolysed to produce IPi which is further hydrolysed to produce free inositol. The latter is salvaged by using it to re-form phosphatidylinositol in the phospholipid synthetic pathway and then phosphorylated to prodnce PIP2 (Chapter 11, Eigure 11.21). These reactions are not jnst of biochemical interest bnt are involved in the treatment of bipolar disease a mental disorder. 

A12. Attwood, D., Graham, A. B., and Wood, G. C., The phospholipid-dependence of uridine diphosphate glucuronyltransferase. Reactivation of phospholipase-inactivated enzyme by phospholipids and detergents. Biochem. J. 123, 875-882... 

Drugs, particularly organic bases, may release histamine from mast cells by physically displacing the amine from its storage sites. Morphine, codeine, d-tubocu-rarine, guanethidine, and radiocontrast media can release histamine from mast cells. Basic polypeptides, such as bradykinin, neurotensin, substance P, somatostatin, polymyxin B, and the anaphylatoxins resulting from complement activation, also stimulate histamine release. Venoms often contain basic polypeptides as well as the histamine-releasing enzyme phospholipase A. 

They also influence the inflammatory response by reducing the prostaglandin and leukotriene synthesis that results from activation of enzyme phospholipase A. ... 

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. 

The second observation which does not support the unfolded protein model is that when phospholipase A (N. naja venom) was injected into the subphase under the lipid monolayer at equilibrium with globulin, lecithin was readily attacked, as indicated by the rapid fall of surface potential (4, 5, 6). If the penetrated protein were to cover entirely the polar groups of the lipid facing the aqueous subphase (as postulated in the unfolded protein model), the lipid molecules should not be accessible to the lipolytic enzyme. 

Influence of Intermolecular Spacing on Enzymic Hydrolysis of Lecithin Monolayers. When snake venom phospholipase A is injected under a lecithin monolayer, it splits lecithin into lysolecithin and free fatty acid. The change in polar groups of the monolayer results in a change of surface potential. However, if prior to injection of enzyme into the subsolution, a lecithin monolayer is compressed to such a surface pressure that the active site of the enzyme is unable to penetrate the monolayer, hydrolysis does not proceed. For monolayers of dipalmitoyl, egg, soybean, and dioleoyl lecithins the threshold surface pressure values at which hydrolysis does not proceed are 20, 30, 37, and 45 dynes per cm., respectively (40). This is also the same order for area per molecule in their surface pressure-area curves, indicating that enzymic hydrolysis of lecithin monolayers is influenced by the unsaturation of the fatty acyl chains and hence the intermolecular spacing in monolayers (40). 

Dennis, E. A., Phospholipases. In Enzymes XVI. New York Academic Press, 1983. This article provides a thorough review of phospholipases. 

Hepatic microsomes of several animal species possess UDP glucuronyltransferase activity and with p-nitrophenol as a substrate, a 12-fold difference in activity due to species variation is evident. Phospholipase-A activates the enzyme and results of activation experiments indicate that the amount of constraint on the activity of this enzyme is variable in different animal species. 

Kini RM (1997) Venom phospholipase a2 enzymes. John Wiley Sons, Chichester Kini RM, Evans HJ (1989) A model to explain the pharmacological effects of snake venom phospholipases a2. Toxicon 27 613-35... 

The toxicities of insect venoms are low to most people. Despite this, relatively large numbers of fatalities occur each year from insect stings because of allergic reactions in sensitized individuals. These reactions can lead to potentially fatal anaphylactic shock, which affects the nervous system, cardiovascular function, and respiratory function. The agents in bee venom that are responsible for severe allergic reactions are mellitin and two enzymes of high molecular mass — hyaluronidase and phospholipase A-2. 

G-protein results in a decrease of intracellular calcium. Receptors that are able to activate PLC (phospholipase C) enzymes cause release of Ca2+ from intracellular stores and influence Ca2+ entry across the plasma membrane (Werry et al. 2003). 

ER of adrenal, testis, ovary, liver and placenta. It is relatively unstable, being inactivated by freezing, even when pure. A phospholipid environment appears to be an important requirement since, when bovine adrenal microsomal preparations were treated with phospholipase A, 80-85% of phospholipids were hydrolysed with a concomitant loss of 80-90% of enzymic activity [84], Restoration of activity was achieved by adding back to the lipid-depleted membranes aqueous dispersions of microsomal total lipid mixtures [84],... 

Wells, M. A. and Hanahan, D. J. (1969) Studies on phospholipase A. I. Isolation and characterization of two enzymes of Crotalus adamanteus venom, Biochemistry 8, 414-424. 

Several psychrotrophic bacteria produce extracellular phospholipases, the most prevalent in milk being pseudomonads (particularly P. fluorescens), Alcaligenes, Acinetobacter, and Bacillus species (Fox et al., 1976 Owens, 1978a Phillips et al., 1981). Most of these produce phospholipase C, some produce phopholipase Ai and some produce both types (Deeth, 1983). Ser-ratia spp. have been shown to produce only phospholipase A (Deeth, 1983), while P. fragi has been reported not to produce phospholipases (Kwan and Skura, 1985). Phospholipase C from some pseudomonads has been purified and characterised (Doi and Nojima, 1971 Sonoki and Ikezawa, 1975 Stepa-niaketa/., 1987a Ivanov etal., 1996). Like the lipases, many of these enzymes have considerable heat stability and are not destroyed by pasteurization... 

Fig. 2. Ptdlns 4,5-P2-derived second messengers. Ptdlns 4,5-P2 is hydrolysed when a phospholipase C (Ptdlns 4,5-P2 phosphodiesterase) is activated following the binding of specific agonists to their surface receptor proteins. The Ptdlns 4,5-P2 is cleaved to yield diacylglycerol (DG), which is a co-activator of protein kinase C and other enzymes, and Ins( 1,4,5)P, which is capable of releasing Ca2 from intracellular stores. The DG often contains arachidonic acid, which is the source of the prostanoids, which are also capable of controlling diverse cellular functions. The arachidonic acid is cleaved from the parent lipid or from DG by specific phospholipase A2 enzymes.

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