Microsomal microsomes

There are various pathways for free radical-mediated processes in microsomes. Microsomes can stimulate free radical oxidation of various substrates through the formation of superoxide and hydroxyl radicals (the latter in the presence of iron) or by the direct interaction of chain electron carriers with these compounds. One-electron reduction of numerous electron acceptors has been extensively studied in connection with the conversion of quinone drugs and xenobiotics in microsomes into reactive semiquinones, capable of inducing damaging effects in humans. (In 1980s, the microsomal reduction of anticancer anthracycline antibiotics and related compounds were studied in detail due to possible mechanism of their cardiotoxic activity and was discussed by us earlier [37], It has been shown that semiquinones of... 

Reductions Epoxide hydroplase Azo and nitro reduction Carbonyl reductase Disulfide reduction Sulfoxide reduction Quinone reduction Reductive dehalogenation Microsomes, cytosol Gut microflora Cytosol Cytosol Cytosol Cytosol, microsomes Microsomes... 

Figure 2 Determination of irreversible inhibition constants for erythromycin and CYP3A4 microsomes. Microsomes (1 mg/mL microsomal protein) were incubated with erythromycin (0-100 pM) in the presence of NADPH for differing incubation times. The pseudo-first-order rate constant for enzyme inactivation was plotted versus erythromycin concentration to estimate Ki and inact (14.4 pM and 0.045 min-1, respectively). The curve represents the line of best fit. Source From Ref. 32.

Microsomes/microsomal The subcellular fraction containing the fragments of the smooth endoplasmic reticulum (ER) after ultracentrifugation of a cellular homogenate. 

Microsomes were prepared from rats. The Michaelis-Menten constant of highly purified enzyme did not differ from the apparent Km of the enzyme in microsomes. Microsomes also supported formation of norbenzydamine as a result of the presence of cytochrome P450. 

Microsomal Microsomal 17,300 ND Hexachloiobuta-l,3-diene Cumene hydroperoxide... 

When the hepatic tissue is homogenized, the endoplasmic reticulum is broken down into small membrane vesicles called microsomes. Microsomal preparations are obtained by centrifuging the tissue homogenate at 10 000 g for 10 min in order to sediment nuclei, mitochondria and debris, and then by centrifuging again at 100 000 g for 1 h. The microsomes are associated to the pellet and are separated from the supernatant (soluble) fraction (the cytosol). 

Analysis of human CE by Northern blot shows a single band of approximately 2.1 kilobases (kb) (Riddles et al. 1991), and three bands of approximately 2-, 3-, and 4.2-kb occurring with hCE-2 (Schwer et al. 1997). The intensities of the 2.1-kb band were liver 3> heart > stomach > testis > kidney = spleen > colon > other tissues. For hCE-2, the 2-kb band was located in liver > colon > small intestine > heart, the 3-kb band in liver > small intestine > colon > heart, and the 4.2-kb band in brain, testis, and kidney only. Analysis of substrate structure versus efficiency for ester (pyrethroid substrates) revealed that the two CEs recognize different structural features of the substrate (i.e., acid, alcohol, etc.). The catalytic mechanism involves the formation of an acyl-enzyme on an active serine. While earlier studies of pyrethroid metabolism were primarily performed in rodents, knowledge of the substrate structure-activity relationships and the tissue distribution of hCEs are critical for predicting the metabolism and pharmacokinetics of pesticides in humans. Wheelock et al. (2003) used a chiral mixture of the fluorescent substrate cyclopro-panecarboxylic acid, 3-(2,2-dichloroethenyl)-2,2-dimethyl-, cyano(6-methoxy-2-naphthalenyl)methyl ester (CAS No. 395645-12-2) to study the hydrolytic activity of human liver microsomes. Microsomal activity against this substrate was considered to be low (average value of ten samples 2.04 0.68 nmol min mg ), when compared to p-nitrophenyl acetate (CAS No. 830-03-5) at 3,700 2,100 mg ... 

Foster, D. W., and McWhorter, W. P., 1969, Microsomes, microsomal phospholipids and fatty acid synthesis, /. Biol. Chem. 244 260. 

Pyrethroids from Chiysanthemic Acid. The unsaturated side chains of the aHethrolone alcohol moieties of the natural pyrethrins are readily epoxidized by microsomal oxidases and converted to diols, thus detoxifying the insecticides. Esterification of chrysanthemic acid (9), R = CH3, with substituted ben2yl alcohols produces usehil insecticides barthrin [70-43-9J, 2-chloro-3,4-methylenedioxyben2yl (+)-i7j ,/n7 j -chrysanthemate, and dimethrin [70-38-2] 2,4-dimethylben2yl (+)-i7j ,/n7 j -chrysanthemate. These have alimited spectmm of insecticidal activity but are of very low mammalian toxicity, ie, rat oralLD s >20,000 mg/kg. 

Pyrethroids with Modified Chrysanthemate Esters. Newer pyrethroids incorporate optimized chrysanthemic acid components to retard detoxication by microsomal oxidases and these are esterified with a variety of optimized alcohol moieties therefore increasing persistence. 

The reactivity of the individual O—P insecticides is determined by the magnitude of the electrophilic character of the phosphoms atom, the strength of the bond P—X, and the steric effects of the substituents. The electrophilic nature of the central P atom is determined by the relative positions of the shared electron pairs, between atoms bonded to phosphoms, and is a function of the relative electronegativities of the two atoms in each bond (P, 2.1 O, 3.5 S, 2.5 N, 3.0 and C, 2.5). Therefore, it is clear that in phosphate esters (P=0) the phosphoms is much more electrophilic and these are more reactive than phosphorothioate esters (P=S). The latter generally are so stable as to be relatively unreactive with AChE. They owe their biological activity to m vivo oxidation by a microsomal oxidase, a reaction that takes place in insect gut and fat body tissues and in the mammalian Hver. A typical example is the oxidation of parathion (61) to paraoxon [311-45-5] (110). 

Mutagenicity. The AJ-nitrosamines, in general, induce mutations in standard bacterial-tester strains (117). As with carcinogenicity, enzymatic activation, typically with Hver microsomal preparations, is required. Certain substituted A/-nitrosamine derivatives (12) induce mutations without microsomal activation (31,33,34). Because the a-acetoxy derivatives can hydroly2e to the corresponding a-hydroxy compounds, this is consistent with the hypothesis that enzymatic oxidation leads to the formation of such unstable a-hydroxy intermediates (13) (118). However, for simple /V-nitrosamines, no systematic relationship has been found between carcinogenicity and mutagenicity (117,119—123). 

The hver microsomal dmg-metabolizing system is of particular importance. This oxidative pathway is mediated by isozymes of the cytochrome family (Fig. 4). At least ten enzyme families exist to accommodate the abiUty of humans to handle many foreign molecules (21). 

The development of easy-to-use assays for determining theophylline blood levels afforded a handle on maintenance of effective but nontoxic levels. The relatively good availabihty of such assays in the United States probably contributed to the historical preference for theophylline treatment by U.S. physicians. Careful titration of the dose must be done on a patient-by-patient basis because individual rates of metaboHsm vary widely. Most ( 85%) of an oral dose of theophylline is metabolized by Hver microsomal enzymes. As a result many dmgs, eg, cimetidine [51481-61-9], anticonvulsants, or conditions, eg, fever, cigarette smoking, Hver disease, which affect Hver function alter theophylline blood levels. 

Celanese Chemical Co., Inc., Mutagenicity Evaluation of n-PropylAlcohol in the Ames Salmonella Microsome Plate Test, Litton Bionetics, Inc., Kensington,... 

Pyrazolone-type dmgs, such as phenylbutazone and sulfinpyrazone, ate metabolized in the Hver by microsomal enzymes, forming glucutonide metabohtes that ate easily excreted because of enhanced water solubility. 

Chinese Herbal Medicines. Many traditional Chinese medicines have been screened for radioprotective activity in experimental animals. In one study of more than a thousand Chinese herbs, a number of agents increased the survival rate of dogs exposed to a lethal dose of y-rays by 30—40%, and some symptoms of radiation injury were ameHorated. These effects are potentially related to stimulation of the hemopoietic and immune systems (130). Extracts of five Chinese dmg plants, as weU as aspirin, effectively protected mice exposed to 7.5—8.0 Gy (750—800 rad) of y-radiation, and increased survival rates by 8—50% (131). Several Chinese traditional medicines, adininistered ip before or after irradiation, protected against Hpid peroxidation in a variety of mouse tissues, including BM, Hver, and spleen, as weU as in mouse Hver microsomal suspensions irradiated in vitro (132). 

Literature reports iadicate that sodium sorbate causes weak genotoxic effects such as chromosomal aberrations and mutations ia mammalian cells (172,173). This effect is thought to be caused by oxidative products of sodium sorbate ia stored solutions (173—175). The main oxidation product of sodium sorbate, 4,5-oxohexenoate, is mutagenic ia a Salmonella mammahan-microsome test (176). Sorbic acid and potassium sorbate were not genotoxic under the same test procedures (167,172,174—177). 

Vane and co-workers isolated a new prostaglandin (initially called PGX) from microsomal fraction of stomach. The structure was established by chemical synthesis from PGP2a (Ref. 1). 

S u/woue//j/mammalian microsome assay (Ames test)... 

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

E. M. Benson, A. J. Tomlinson and S. Nayloi, Time course analysis of a microsomal incubation of a therapeutic dmg using preconcenti ation capillary electrophoresis (Pc-CE) , 7. High Resolut. Chromatogr. 17 671-673 (1994). 

Medicinal chemistry has frequently drawn inspiration and important new leads from the examination of natural products, and this was proven to be the case once more. In 1992, researchers at Merck and Glaxo announced, almost simultaneously, the independent discovery of the same new class of natural products from two different fungi. As a consequence, the same family of natural products has two names - the zaragozic acids (Merck)4 or the squalestatins (Glaxo).5 A typical member of the family, zaragozic acid A (squa-lestatin SI) (1) was shown to have a tremendous affinity for squalene synthase (K, = 79 pM for rat microsomal squalene synthase) and could even lower serum cholesterol levels in vivo in a population of marmosets.6... 

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... 

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