Phospholipases, control

In soils, the activity of extracellular enzymes may be affected as a result of their adsorption to clay particles, which, in turn, influences the microbial life in the soil. Pancreatic lipases and phospholipases control the digestion of alimentary fats in the duodenum. These fats are insoluble in water they are present as emulsified globules. Before the dissolved enzymes can exert their action, they have to adsorb at the surface of the globules. Furthermore, intravascular thrombosis is an interfacial process in which adsorption of proteins at the blood vessel wall enhances the adhesion of blood platelets. 

Phorbol esters are promoters that interact with cellular receptors and activate protein kinase C. Usually protein kinase C is activated by Ca++ and diacylglycerol, both of which result from the hydrolysis of phosphoinositides catalyzed by phospholipase C. Phospholipase C is normally activated by several different growth factors. Thus phorbol esters bypass a tightly regulated step in the control of cell growth. Since protein kinase C phosphorylates various proteins, it is not known how this activity participates in establishing a cancerous line of cells. 

Synaptic stimulation, ischemia or seizure activates phospholipase A2 and releases arachidonic and docosahexaenoic acids. Ischemia or seizure triggers accumulation of free AA, DHA and other FFA in the brain( see also Chs 32, 37). This reflects PLA2 activation in excitable membranes [24]. While little is known about the mechanisms that control its activity, the importance of cPLA2 in ischemic brain injury is strongly supported by the recent finding that cPLA2-knockout mice have substantially reduced infarcts and neurologic deficits in a model of stroke [25],... 

The phospholipases (PLC) isozymes cleave the phosphodiester bond in phos-phatidyl-inositol-4,5-bisphosphate (PIP2) releasing two second messenger molecules inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) as shown before. The /1-isozyme are controlled by the Ga or G y subunits of the heterotrimeric G-proteins coupled to surface receptors. The y-isozymes are substrates for tyrosine kinases, such as growth factors. 

Esterases play a role in regulating the platelet-activating factor (PAF), a lipid with hypotensive properties [96], Phospholipase A2 (EC 3.1.1.4) is involved in this pathway by hydrolyzing a precursor to lyso-PAF and a free fatty acid. The activity of PAF, formed by acetylation of lyso-PAF, is controlled by an esterase hydrolyzing the acetate moiety [100]. 

It is well recognized that activity of many mitochondrial enzymes is modulated by the mitochondrial acidic phospholipid cardiolipin. Adenine nucleotide translocase (ANT) of bovine heart mitochondria is known to be tightly associated with six molecules of cardiolipin (Beyer and Klingenberg, 1985), and cardiolipin is absolutely required for the ANT activity (Hoffmann et al, 1994). ADP control of the carrier transition was gradually lost after phospholipase treatment and the ADP conttol could be fully restored through the addition of cardiolipin (Beyer and Nuscher, 1996). Cardiolipin-mediated activation of cytochrome c oxidase (Abramovitch et al, 1990) and... 

In summary, the release of nenrotransmitter from a presynaptic neurone into a synaptic cleft occurs via the process of exocytosis, which is regulated by the increase in Ca " ion concentration in the presynaptic terminal. The increase in Ca " ion concentration is achieved by release of Ca " ions by opening of the Ca ion channel in the endoplasmic reticulum, which is controlled by the concentration of IP3. Failure to release inositol from the inositol phosphates reduces the free inositol concentration, which interferes in the synthesis of PIP2. The phospholipase no longer catalyses a zero order reaction. Consequently, sufficient IP3 to activate the ion channel is released in the presynaptic neurone, so that less nenrotransmitter is released into the synaptic cleft (Figure 12.19). 

The cells in the hypothalamus that control body temperature respond to the cytokines by stimulating the activity of the membrane bound phospholipase, which results in the formation of arachidonic acid, the substrate for the enzyme cyclooxygenase-2 (COX-2) which is the rate-limiting step in the pathway for synthesis of prostaglandins. Prostaglandins influence cells in the hypothalamus that are responsible for temperature regulation. 

Figure 22.13 a-Adrenergic receptor control of contraction of smooth muscle. IP3 represents inositol trisphosphate. Binding of a catecholamine to an a-receptor activates a membrane-bound phospholipase which hydrolyses phosphatidyUnositol bisphosphate within the membrane to produce IP, and diacylglycerol (DAG). IP3 binds a receptor on the sarcoplasmic reticulum in smooth muscle, which activates a Ca ion channel and the cytosolic Ca ion concentration increases, which results in contraction of smooth muscle in arterioles. This results in vasoconstriction and hence decreases blood flow which can leading to an increase in blood pressure. 

The same basic biochemical control mechanism causes contraction of the smooth muscle as well as secretion of aldosterone. The binding of angiotensin to its receptor activates a membrane phospholipase-C. It catalyses the hydrolysis of phosphoinositide phosphatidylinositol bis-phosphate to produce the two intracellular messengers, inositol trisphosphate (IP3) and diacylglycerol (DAG). 

Phosphohpases of type Cy are activated by receptor tyrosine kinases (see Chapter 8), and thus phosphohpase Cy is involved in growth factor controlled signal transduction pathways. The receptor tyrosine kinases (see Chapter 8) phosphorylate the enzyme at specific tyrosine residues and initiate activation of the enzyme. Characteristic for the structure of phospholipase Cy is the occurrence of SH2 and SH3 domains (see Chapter 8). These represent protein modules that serve to attach further partner proteins. 

I agree with Professor McConnell that phospholipid phase transitions may play a role in controlling the activity of a membrane-bound enzyme. However, the case cited is somewhat ambiguous, since porcine phospholipase A2 is a soluble enzyme acting on a phospholipid surface. The major effect of the phase transition in this case is to alter the nature of the substrate rather than the intrinsic catalytic activity of the enzyme. 

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