Hydrolysis phospholipase

The excitatoiy amino acids (EAA), glutamate and aspartate, are the principal excitatory neurotransmitters in the brain. They are released by neurons in several distinct anatomical pathways, such as corticofugal projections, but their distribution is practically ubiquitous in the central nervous system. There are both metabotropic and ionotropic EAA receptors. The metabotropic receptors bind glutamate and are labeled mGluRl to mGluRB. They are coupled via G-proteins to phosphoinositide hydrolysis, phospholipase D, and cAMP production. Ionotropic EAA receptors have been divided into three subtypes /V-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole-proprionic acid (AMPA), and kainate receptors (Nakanishi 1992). 

Although extraction of lipids from membranes can be induced in atomic force apparatus (Leckband et al., 1994) and biomembrane force probe (Evans et al., 1991) experiments, spontaneous dissociation of a lipid from a membrane occurs very rarely because it involves an energy barrier of about 20 kcal/mol (Cevc and Marsh, 1987). However, lipids are known to be extracted from membranes by various enzymes. One such enzyme is phospholipase A2 (PLA2), which complexes with membrane surfaces, destabilizes a phospholipid, extracts it from the membrane, and catalyzes the hydrolysis reaction of the srir2-acyl chain of the lipid, producing lysophospholipids and fatty acids (Slotboom et al., 1982 Dennis, 1983 Jain et al., 1995). SMD simulations were employed to investigate the extraction of a lipid molecule from a DLPE monolayer by human synovial PLA2 (see Eig. 6b), and to compare this process to the extraction of a lipid from a lipid monolayer into the aqueous phase (Stepaniants et al., 1997). 

The venoms of poisonous snakes contain (among other things) a class of enzymes known as phospholipases, enzymes that cause the breakdown of phospholipids. For example, the venoms of the eastern diamondback rattlesnake (Crotalus adamanteus) and the Indian cobra Naja naja) both contain phospholipase Ag, which catalyzes the hydrolysis of fatty acids at the C-2 position of glyc-erophospholipids. 

A subfamily of Rho proteins, the Rnd family of small GTPases, are always GTP-bound and seem to be regulated by expression and localization rather than by nucleotide exchange and hydrolysis. Many Rho GTPase effectors have been identified, including protein and lipid kinases, phospholipase D and numerous adaptor proteins. One of the best characterized effector of RhoA is Rho kinase, which phosphorylates and inactivates myosin phosphatase thereby RhoA causes activation of actomyosin complexes. Rho proteins are preferred targets of bacterial protein toxins ( bacterial toxins). 

Covalent regulation. Following occupation and activation of the M2 acetyl choline receptors, phospholipase C (PLC), is activated and both inositol (l,4,5)-trisphosphate (IP3), and diacylglycerol (DAG), are formed by hydrolysis of phosphatidylinositol (4,5)-bisphosphate (PIP2). 

Two possible pathways for the biosynthesis of 2-AG have been proposed (1) a phospholipase C (PLC) hydrolysis of membrane phospholipids followed by a second hydrolysis of the resulting 1,2-diacylglycerol by diacylglycerol lipase or (2) a phospholipase Ai (PLA,) activity that generates a lysophospholipid, which in turn is hydrolyzed to 2-AG by lysophospholipase C (Fig. 5) (Piomelli, 1998). Alternative pathways may also exist from either triacylglycerols by a neutral lipase activity or lysophosphatidic acid by a dephosphorylase. The fact that PLC and diacylglycerol lipase inhibitors inhibit 2-AG formation in cortical neurons supports the contention that 2-AG is, at least predominantly, biosynthesized by the PLC pathway (Stella, 1997). However, a mixed pathway may also be plausible. 

A. indica L. Indian Aristolochia, also known as Indian birthwort, ishvara (Sanskrit), or adagam (Tamil), is a bitter climber native to India. The medicinal material consists of the rhizome, which is to resolve inflammation (India), counteract insect poison, and as an antipyretic (Philippines and Vietnam). The rhizome contains aristolochic acid, which inhibits in vitro and dose-dependent phospholipid hydrolysis by the human synovial fluid phospholipase A2, snake venom phospholipase A2, porcine pancreatic phospholipase A2, and human platelet phospholipase A2 (2). 

Inositol triphosphate (IP3)-gated channels are also associated with membrane-bound receptors for hormones and neurotransmitters. In this case, binding of a given substance to its receptor causes activation of another membrane-bound protein, phospholipase C. This enzyme promotes hydrolysis of phosphatidylinositol 4,5-diphosphate (PIP2) to IP3. The IP3 then diffuses to the sarcoplasmic reticulum and opens its calcium channels to release Ca++ ions from this intracellular storage site. 

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. 

Like proteases, phospholipases (PLs) are enzymes that catalyze the hydrolysis of a covalent bond, in this particular case of phospholipids. For this reason, a similar approach for the design of FRET... 

Another important group of hydrolytic enzymes are phospho- and cyclophosphodiesterases. They catalyze the hydrolysis of phospho-diester bonds and many of the most relevant biological substrates are nucleic acids. Phospholipase C and D are also important examples. Initial attempts to measure phosphodiesterase activity placed a phosphodiester between a fluorophore and a quencher and the probe was tested in vitro [146], This system was slightly modified by Caturla and used for the identification of catalysts with phosphodiesterase activity [147], More recently, Nagano and co-workers used a coumarin donor and fluorescein as a FRET... 

Parvalbumins Phospholipase A2 High-affinity calcium buffers Hydrolysis of phospholipids with the release of free... 

Belkner et al. [32] demonstrated that 15-LOX oxidized preferably LDL cholesterol esters. Even in the presence of free linoleic acid, cholesteryl linoleate continued to be a major LOX substrate. It was also found that the depletion of LDL from a-tocopherol has not prevented the LDL oxidation. This is of a special interest in connection with the role of a-tocopherol in LDL oxidation. As the majority of cholesteryl esters is normally buried in the core of a lipoprotein particle and cannot be directly oxidized by LOX, it has been suggested that LDL oxidation might be initiated by a-tocopheryl radical formed during the oxidation of a-tocopherol [33,34]. Correspondingly, it was concluded that the oxidation of LDL by soybean and recombinant human 15-LOXs may occur by two pathways (a) LDL-free fatty acids are oxidized enzymatically with the formation of a-tocopheryl radical, and (b) the a-tocopheryl-mediated oxidation of cholesteryl esters occurs via a nonenzymatic way. Pro and con proofs related to the prooxidant role of a-tocopherol were considered in Chapter 25 in connection with the study of nonenzymatic lipid oxidation and in Chapter 29 dedicated to antioxidants. It should be stressed that comparison of the possible effects of a-tocopherol and nitric oxide on LDL oxidation does not support importance of a-tocopherol prooxidant activity. It should be mentioned that the above data describing the activity of cholesteryl esters in LDL oxidation are in contradiction with some earlier results. Thus in 1988, Sparrow et al. [35] suggested that the 15-LOX-catalyzed oxidation of LDL is accelerated in the presence of phospholipase A2, i.e., the hydrolysis of cholesterol esters is an important step in LDL oxidation. 

Muscarinic receptor activation causes inhibition of adenylyl cyclase, stimulation of phospholipase C and regulation of ion channels. Many types of neuron and effector cell respond to muscarinic receptor stimulation. Despite the diversity of responses that ensue, the initial event that follows ligand binding to the muscarinic receptor is, in all cases, the interaction of the receptor with a G protein. Depending on the nature of the G protein and the available effectors, the receptor-G-protein interaction can initiate any of several early biochemical events. Common responses elicited by muscarinic receptor occupation are inhibition of adenylyl cyclase, stimulation of phos-phoinositide hydrolysis and regulation of potassium or other ion channels [47] (Fig. 11-10). The particular receptor subtypes eliciting those responses are discussed below. (See also Chs 20 and 21.)... 

Berstein, G., Blank, J. L., Smrcka, A. V. etal. Reconstitution of agonist stimulated phosphatidylinositol 4-5 bisphosphate hydrolysis using purified mi muscarinic receptor, Gq/11, and phospholipase C-Pl. /. Biol. Chem. 267 8081-8088, 1992. 

The family of heterotrimeric G proteins is involved in transmembrane signaling in the nervous system, with certain exceptions. The exceptions are instances of synaptic transmission mediated via receptors that contain intrinsic enzymatic activity, such as tyrosine kinase or guanylyl cyclase, or via receptors that form ion channels (see Ch. 10). Heterotrimeric G proteins were first identified, named and characterized by Alfred Gilman, Martin Rodbell and others close to 20 years ago. They consist of three distinct subunits, a, (3 and y. These proteins couple the activation of diverse types of plasmalemma receptor to a variety of intracellular processes. In fact, most types of neurotransmitter and peptide hormone receptor, as well as many cytokine and chemokine receptors, fall into a superfamily of structurally related molecules, termed G-protein-coupled receptors. These receptors are named for the role of G proteins in mediating the varied biological effects of the receptors (see Ch. 10). Consequently, numerous effector proteins are influenced by these heterotrimeric G proteins ion channels adenylyl cyclase phosphodiesterase (PDE) phosphoinositide-specific phospholipase C (PI-PLC), which catalyzes the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) and phospholipase A2 (PLA2), which catalyzes the hydrolysis of membrane phospholipids to yield arachidonic acid. In addition, these G proteins have been implicated in... 

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