Microsomal membrane

Plasma membrane, microsomal fraction (fragments of endoplasmic reticulum), and large polyribosomes... 

Phospholipase A2 has been detected in almost any cell in which its presence has been investigated.1 This activity has extensively been studied in liver cells where it is present in different cell organelles such as plasma membranes, microsomes, golgi membranes and lysosomes.1 With the exception of the lysosomal enzyme, all phospholipase A2 activities require an alkaline pH (7.8-9.5) and are stimulated by Ca2+ ions. The lysosomal activity requires acidic pH (U.O-5.0) and is inhibited by Ca2+ ions. 

The overt CPT activities (CPT,) of mitochondria, microsomes and peroxisomes are inhibited by malonyl-CoA, in contrast to the latent CPT isoforms. Of all the hepatic overt CPT activities, only the protein responsible for the activity in the mitochondrial outer membrane (L-CPT I) is well-characterised." Results published in abstract form have indicated that a peroxisomal protein resembling L-CPT 1 may be responsible for the overt CPT activity of these organelles, but the authors suggested that it is immunologi-cally distinct from L-CPT I. A microsomal protein of Mr 47,000 has previously been postulated to represent the microsomal CPT, on the basis that, when microsomes are incubated with [ H]-etomoxir, CoA and ATP, a protein of this molecular size is labelled most prominently. Similar observations were made by who, however, also observed that under these in vitro conditions at least one other protein of Mr 87,000 was also prominently labelled. More recently, it has been confirmed that microsomes express a malonyl-CoA-sensitive overt CPT which has an apparent molecular size of 300 kDa in detergent extracts of the membranes. On the basis of its different kinetic behaviour, ease of solubilisation and stability of catalytic activity when solubilised, it was concluded that the microsomal enzyme is distinct from the mitochondrial outer membrane CPT 1. So, current opinion is that, apart from the common sensitivity of all the overt CPT enzymes to malonyl-CoA inhibition, the proteins responsible for this activity in the mitochondrial outer membrane, microsomes and peroxisomes are distinct molecular entities. 

Therefore, a purpose of the present work was to study the effect of DPhO and BM-DPhO in a wide range of concentration (10 -10 mol/1) on the endoplasmic reticulum membranes (microsomes) isolated from Balb-line mice. Electron paramagnetic resonance (EPR) technique and spin-probe method were used to study the dynamic structure of deep hydrophobic and surface lipid regions of microsomal membranes. We suggested the different effects of DPhO and IM-DPhO on the membrane lipids structure, because iod-methylate derivative is charged. 

Another suggestion is that acid phosphatase of the microsomal membrane represents the origin of the lysosomal enzyme, as has been demonstrated for y5-glucuronidase (Van Lancker and Lentz, 1970 Kato et al., 1970). This requires, however, the assumption that membrane microsomal acid phosphatase undergoes a metabolic transformation in tntx), pos-... 

Mitochondria (guinea pig liver) inner membrane outer membrane Microsomes (bovine liver) rough ER smooth ER B. subtilis E. coli (plasma)... 

Ribosomes and microsomes consisting of endoplasmic reticulum, Golgi, and plasma membrane fragments... 

Squalene epoxidase, like most enzymes responsible for the later steps of sterol biosynthesis [43, 51], is membrane-bound which makes its purification in native form challenging. The purification is additionally complicated by the presence of a large number of cytochrome P450 and other enzymes that have similar hydro-phobicity and size as squalene epoxidase and are hence difficult to remove [52]. Most studies have been carried out with rat liver microsome squalene epoxidase either partially purified or as a homogenate of the cell membrane fraction. In vitro reconstitution of squalene epoxidase activity is absolutely dependent on molecular oxygen, NADPH, FAD, and NADPH-cytochrome c reductase [52, 53]. In this respect, squalene epoxidase resembles the cytochrome P450 enzymes described... 

Fig. 1. Relative composition of root microsomal membranes from 24 land races, varieties and breeding lines of rice which differ in their salt resistance. Campesterol, Stigmasterol and Sitosterol as % of total sterols 16 0, 18 1, 18 2 and 18 3 fatty acids as % of total fatty acids Na transport on a relative scale from (1) lowest to (9) highest. Data of D.R. Lachno, T.J. Flowers A.R. Yeo (unpublished).

Monooxygenases owe their catalytic properties to the hemeprotein cytochrome P450 (Fignre 2.3). Within the membrane of the endoplasmic reticulum (microsomal... 

Endoplasmic reticulum Membranous network of cells that contains many enzymes that metabolize xenobiotics. Hepatic microsomes consist mainly of vesicles derived from the endoplasmic reticulum of liver. 

Cosme J, Johnson EF. Engineering microsomal cytochrome P450 2C5 to be a soluble, monomeric enzyme. Mutations that alter aggregation, phospholipid dependence of catalysis, and membrane binding. /FtoZ Chem 2000 275 2545-53. 

Williams PA, Cosme J, Sridhar V, Johnson EF, McRee DE. Mammalian microsomal cytochrome P450 monooxygenase structural adaptations for membrane binding and functional diversity. Mol Cell 2000 5 121-31. 

The mechanisms involved in the establishment of lipid asymmetry are not well understood. The enzymes involved in the synthesis of phospholipids are located on the cytoplasmic side of microsomal membrane vesicles. Translocases (flippases) exist that transfer certain phospholipids (eg, phosphatidylcholine) from the inner to the outer leaflet. Specific proteins that preferentially bind individual phospholipids also appear to be... 

Stndies of the antoxidation of carotenoids in liposomal suspensions have also been performed since liposomes can mimic the environment of carotenoids in vivo. Kim et al. stndied the antoxidation of lycopene," P-carotene," and phytofluene" " in liposomal snspensions and identified oxidative cleavage compounds. Stabilities to oxidation at room temperature of various carotenoids incorporated in pig liver microsomes have also been studied." The model took into account membrane dynamics. After 3 hr of reactions, P-carotene and lycopene had completely degraded, whereas xanthophylls tested were shown to be more stable. 

Socaciu, C., Jessel, R., and Diehl, H.A., Carotenoid incorporation into microsomes yields, stability and membrane dynamics, Spectrochim. Acta A Mol. Biomol. Sped, 56, 2799, 2000. 

Liebler, D.C. et al.. Antioxidant actions of beta-carotene in liposomal and microsomal membranes role of carotenoid-membrane incorporation and alfa-tocopherol, Arch. Biochem. Biophys., 338, 244, 1997. 

A full understanding of the role of pectin in plant development requires elucidation of the mechanisms that regulate p>ectin biosynthesis (6). Our strategy for studying the biosynthesis of HGA was to 1) establish a PGA-GalAT assay that would allow detection of synthesized HGA, 2) characterize the enzyme in microsomal membranes, 3) characterize the product synthesized by the enzyme in microsomal membranes, and 4) solubilize the enzyme and characterize the solubilized enzyme and its product. 

CHARACTERIZATION OF THE PRODUCTS SYNTHESIZED BY PGA-GALAT IN MICROSOMAL MEMBRANES... 

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