Enzyme membrane-bound

Active Transport. Maintenance of the appropriate concentrations of K" and Na" in the intra- and extracellular fluids involves active transport, ie, a process requiring energy (53). Sodium ion in the extracellular fluid (0.136—0.145 AfNa" ) diffuses passively and continuously into the intracellular fluid (<0.01 M Na" ) and must be removed. This sodium ion is pumped from the intracellular to the extracellular fluid, while K" is pumped from the extracellular (ca 0.004 M K" ) to the intracellular fluid (ca 0.14 M K" ) (53—55). The energy for these processes is provided by hydrolysis of adenosine triphosphate (ATP) and requires the enzyme Na" -K" ATPase, a membrane-bound enzyme which is widely distributed in the body. In some cells, eg, brain and kidney, 60—70 wt % of the ATP is used to maintain the required Na" -K" distribution. 

Plasma levels of 3—5 p.g/mL are obtained two hours after adraiinistration of 200 mg ketoconazole. No accumulation in the bloodstream was noted after a 30-wk treatment with this dose. The half-life is approximately eight hours. When ketoconazole is taken with meals, higher plasma levels are obtained. Distribution studies using radioactive ketoconazole in rats show radioactivity mainly in the Hver and the connective tissue. Radioactivity is also present in the subcutaneous tissue and the sebaceous glands. After one dose of 200 mg in humans, ketoconazole is found in urine, saUva, sebum, and cenimen. Like miconazole, the mode of action is based on inhibition of the cytochrome P-450 dependent biosynthesis of ergosterol. This results in disturbed membrane permeabiUty and membrane-bound enzymes (8,10,23,25). 

Ketone body synthesis occurs only in the mitochondrial matrix. The reactions responsible for the formation of ketone bodies are shown in Figure 24.28. The first reaction—the condensation of two molecules of acetyl-CoA to form acetoacetyl-CoA—is catalyzed by thiolase, which is also known as acetoacetyl-CoA thiolase or acetyl-CoA acetyltransferase. This is the same enzyme that carries out the thiolase reaction in /3-oxidation, but here it runs in reverse. The second reaction adds another molecule of acetyl-CoA to give (i-hydroxy-(i-methyl-glutaryl-CoA, commonly abbreviated HMG-CoA. These two mitochondrial matrix reactions are analogous to the first two steps in cholesterol biosynthesis, a cytosolic process, as we shall see in Chapter 25. HMG-CoA is converted to acetoacetate and acetyl-CoA by the action of HMG-CoA lyase in a mixed aldol-Claisen ester cleavage reaction. This reaction is mechanistically similar to the reverse of the citrate synthase reaction in the TCA cycle. A membrane-bound enzyme, /3-hydroxybutyrate dehydrogenase, then can reduce acetoacetate to /3-hydroxybutyrate. 

The rate of catalysis of membrane bound enzymes (Plot B, Figure 1) is more greatly affected than soluble enzymes by lowering the temperature. This is due to the effect of low temperatures on the solidification of the membranes. Thus, an Arrhenius plot of the rate of a membrane-bound enzyme as a function of temperature often shows a discontinuity with a sharp break point (transition temperature) and loss of activity at the temperature where the membrane becomes a gel or more solid phase. 

There has been considerable discussion regarding the mode of action of the sea cucumber and starfish saponins. Both the triterpene and steroidal glycosides inhibit both Na/K ATPase and Ca/Mg ATPase 06) possibly as a result of their aglycone structures. However, their detergent properties cause membrane disruption which will influence the activity of membrane-bound enzymes such as the ATPases. In investigating the actions of saponins on multilamellar liposomes, it was found that cholesterol serves as the binding site for such saponins and that cholesterol-free lip-somes are not lysed by saponins 107). 

Fraser, P.D. and Sandmann, G., In vitro assays of three carotenogenic membrane-bound enzymes from Escherichia coli transformed with different crt genes, Biochem. Biophys. Res. Commun. 185, 9, 1992. 

Ridley H, CA Watts, DJ Richardson, CS Butler (2006) Resolution of distinct membrane-bound enzymes from Enterobacter cloacae SKLDla-1 that are responsible for selective reduction of nitrate and selenate anions. Appl Environ Microbiol 12 5173-5180. 

Superoxide is produced by the NADPH oxidoreduc-tase (oxidase), which is a membrane-bound enzyme complex containing a flavoprotein that catalyses the transfer of single electrons from NADPH in the cytosol to extracellular oxygen. NADPH is mainly supplied by the hexose monophosphate shunt. In resting cells, the oxidase complex is inactive and disassembled, but is rapidly reconstituted and activated by chemotactic mechanisms or phagocytosis (Baggiolini and Thelen, 1991). 

Two forms of this enzyme are known, a membrane-bound form mainly found in microsomes of all cells and a soluble form present in red blood cells. Structurally, the soluble form with 275 amino acid residues lacks a hydrophobic segment at the NH2 terminus which is present in the membrane-bound enzyme with 300 amino acid residues. Both isoforms are produced by a single gene on chromosome 22 (T17,Y4). 

Inside the cytoplasm of the presynaptic neuron the monoamines are exposed to the mitochondrial outer membrane-bound enzyme monoamine oxidase (MAO). MAO breaks the monoamines down into inactive metabolites before they are taken up into the vesicles. However, if MAO is inhibited, then the monoamines enter the vesicles and are available for release. MAO inhibitors, such as moclobemide, have been used in the treatment of depression, since they increase the availability of noradrenaline and serotonin. Selegiline is used for Parkinson s disease, since it raises dopamine levels. 

Hawi S.R., Rochanakij S., Adar F., Campbell W.B., Nithipatikom K., Detection of membrane-bound enzymes in cells using immunoassay and Raman microspectroscopy, Anal. Biochem. 1998 259 212-217. 

The second popular hypothesis for the therapeutic mechanism of action of Li+ is its interference with another receptor-coupled, second messenger system, that of the plasma-membrane bound enzyme, adenylate... 

LUDWIG-MULLER, J., HILGENBERG, W., A plasma membrane-bound enzyme oxidases L-tryptophan to indole-3-acetaldoxime, Physiol. Plant., 1988, 74, 240-250. 

Membrane-bound Enzyme of Vascular Smooth Muscle Cells... 

Seth, P., Fung, H-L., Biochemical characterization of a membrane-bound enzyme responsible for generating nitric oxide from nitroglycerin in vascular smooth muscle cells. Biochem. Pharmacol. 46... 

Despite intense study of the chemical reactivity of the inorganic NO donor SNP with a number of electrophiles and nucleophiles (in particular thiols), the mechanism of NO release from this drug also remains incompletely understood. In biological systems, both enzymatic and non-enzymatic pathways appear to be involved [28]. Nitric oxide release is thought to be preceded by a one-electron reduction step followed by release of cyanide, and an inner-sphere charge transfer reaction between the ni-trosonium ion (NO+) and the ferrous iron (Fe2+). Upon addition of SNP to tissues, formation of iron nitrosyl complexes, which are in equilibrium with S-nitrosothiols, has been observed. A membrane-bound enzyme may be involved in the generation of NO from SNP in vascular tissue [35], but the exact nature of this reducing activity is unknown. 

Membrane-based separation, lactic acid production and, 14 120 Membrane biocompatibility, in hemodialysis, 26 823—824 Membrane bioreactors, 16 26 Membrane-bound enzymes, 10 338 Membrane cell process, 9 620 Membrane cells... 

Fig. 4.1). Since the P450s are membrane-bound enzymes in vivo, adding lipid to reconstitute a functional enzyme system in vitro more closely mirrors the in vivo environment and presumably serves as a matrix to allow the two enzymes, P450 and P450 reductase, to interact properly. 

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