Phospholipase Action

Potential signaling phospholipid derived from phosphatidylcholine by phospholipase action. No receptors known. 

Signaling phospholipid derived from sphingomyelin by phospholipase action. Activates SIP receptors. 

In 1954, Beaufay and de Duve (27) first suggested a relationship between microsomal phospholipid and glucose-6-phosphatase. They observed a loss of enzymic activity from phospholipid-rich microsomal preparations concomitant with extraction with such organic solvents as butanol or treatment with lecithinase. Various studies were carried out to demonstrate that the latter effect was not produced through inhibition of enzymic activity by accumulated products of the hydrolysis of phospholipids. On the basis of their observations that deoxycholate treatment labilized microsomes to phospholipase action, they concluded that . . . the detergent did not exert its primary effect on the dissociation of phospholipids from microsomal protein, but that it probably disrupted... 

Sequential removal of the fatty acids by phospholipase action results in the formation of lysolecithin (glycerophosphorylcholine), then hydrolysis to release choline. Acetylcholine is synthesized in neurons using acetyl CoA. 

This process should be carried out as soon as possible in order to minimize changes in levels of endogenous lipids during storage—for example, phospholipase action may continue at temperatures well below 0°C. Further-... 

Hydrolases. Enzymes catalysing the hydrolytic cleavage ofC —O, C —N and C —C bonds. The systematic name always includes hydrolase but the recommended name is often formed by the addition of ase to the substrate. Examples are esterases, glucosidases, peptidases, proteinases, phospholipases. Other bonds may be cleaved besides those cited, e.g. during the action of sulphatases and phosphatases. 

Eicosanoids, so named because they are all derived from 20-carbon fatty acids, are ubiquitous breakdown products of phospholipids. In response to appropriate stimuli, cells activate the breakdown of selected phospholipids (Figure 25.27). Phospholipase Ag (Chapter 8) selectively cleaves fatty acids from the C-2 position of phospholipids. Often these are unsaturated fatty acids, among which is arachidonic acid. Arachidonic acid may also be released from phospholipids by the combined actions of phospholipase C (which yields diacyl-glycerols) and diacylglycerol lipase (which releases fatty acids). 

The second type of material includes spores, which may or may not produce disease symptoms but which can germinate in the insect gut and give rise to vegetative bacterial cells which in turn may produce, and exoenzymes such as phospholipases (lecithinases) or hyaluronidase. The phospholipases may produce direct toxic symptoms owing to their action on nervous or other phospholipid-containing tissue. Hyaluronidase breaks down hyaluronic acid and produces effects on animal tissue which are morphologically similar to the breakdown of insect gut wall in the presence of microbial insecticide preparations. 

Diacylglycerol is glycerol esterified to two fatty acids at the sn-1 and sn-2 positions. It is a membrane-embedded product of phospholipase C action and an activator of protein kinase C. It is also an intermediate in the biosynthesis of triacylglycerol, phosphatidyletha-nolamine and phosphatidylcholine. 

Phospholipases. Figure 1 Generic phospholipid with sites of action of phospholipases shown. 

A second group of myotoxic toxins, found almost exclusively in the venoms of cobras, are the cytotoxins (often called cobratoxins, cytolysins, cardiotoxins, or direct lytic factors). These, rather than phospholipases, are almost certainly the primary cause of muscle damage following bites by cobras. Their mechanism of action is not properly known, but it is certainly the case that their action is potentiated by the presence of phospholipases in the venom, even if the phospholipases concerned are not, themselves, myotoxic. The cytotoxins of cobra venom possess no hydrolytic activity of any kind. 

TBT and TFT are membrane-active molecules, and their mechanism of action appears to be strongly dependent on organotin(IV) lipophilicity. They function as ionophores and produce hemolysis, release Ca(II) from sarcoplasmic reticulum, alter phosphatodylseiine-induced histamine release, alter mitochondrial membrane permeability and perturb membrane enzymes. Organotin(IV) compounds have been shown to affect cell signaling they activate protein kinase and increase free arachidonic acid through the activation of phospholipase... 

There are three groups of eicosanoids that are synthesized from C20 eicosanoic acids derived from the essential fatty acids linoleate and a-linolenate, or directly from dietary arachidonate and eicosapentaenoate (Figure 23-5). Arachidonate, usually derived from the 2 position of phospholipids in the plasma membrane by the action of phospholipase Aj (Figure 24-6)—but also from the diet—is the substrate for the synthesis of the PG2, 1X2 series (prostanoids) by the cyclooxygenase pathway, or the LT4 and LX4 series by the lipoxygenase pathway, with the two pathways competing for the arachidonate substrate (Figure 23-5). 

The a subunits and the Py complex have actions independent of those on adenylyl cyclase (see Figure 43-4 and Table 43-3). Some forms of tt stimulate channels and inhibit Ca channels, and some ttj molecules have the opposite effects. Members of the G, family activate the phospholipase C group of enzymes. The py complexes have been associated with channel stimulation and phospholipase C activation. G proteins are involved in many important biologic processes in addition to hormone action. Notable examples include olfaction (oColf) <1 vision (aj. Some examples are listed in Table 43-3. GPCRs are implicated in a number of diseases and are major targets for pharmaceutical agents. 

Figure 45-6. Interaction and synergism between antioxidant systems operating in the lipid phase (membranes) of the cell and the aqueous phase (cytosol). (R-,free radical PUFA-00-, peroxyl free radical of polyunsaturated fatty acid in membrane phospholipid PUFA-OOH, hydroperoxy polyunsaturated fatty acid in membrane phospholipid released as hydroperoxy free fatty acid into cytosol by the action of phospholipase Aj PUFA-OH, hydroxy polyunsaturated fatty acid TocOH, vitamin E (a-tocopherol) TocO, free radical of a-tocopherol Se, selenium GSH, reduced glutathione GS-SG, oxidized glutathione, which is returned to the reduced state after reaction with NADPH catalyzed by glutathione reductase PUFA-H, polyunsaturated fatty acid.)...

Immunologic abnormahties (eg, transfusion reactions, the presence in plasma of warm and cold antibodies that lyse red blood cells, and unusual sensitivity to complement) also fall in this class, as do toxins released by various infectious agents, such as certain bacteria (eg, Clostridium). Some snakes release venoms that act to lyse the red cell membrane (eg, via the action of phospholipases or proteinases). 

Phospholipase A activity was subsequently demonstrated to be present in venom, and it too required Ca (25). DEAE-cellulose fractionation yielded four proteins, two of which were phospholipase A and hemolytic, and two of which had neither phospholipase A nor hemolytic activities. Either of the latter two proteins enhanced to various degrees the hemolytic activity of either of the two phospholipases. The findings suggest considerable analogy with synergistic mechanisms underlying the hemolytic action of the venoms of a number of snakes. 

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. 

The ion channel receptors are relatively simple in functional terms because the primary response to receptor activation is generated by the ion channel which is an integral part of the protein. Therefore, no accessory proteins are needed to observe the response to nicotinic AChR activation and the full functioning of the receptor can be observed by isolating and purifying the protein biochemically and reconstituting the protein in an artificial lipid membrane. In contrast, the G-protein-coupled receptors require both G-proteins and those elements such as phospholipase-C illustrated in Fig. 3.1, in order to observe the response to receptor activation (in this case a rise in intracellular calcium concentration resulting from the action of IP3 on intracellular calcium stores). 

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