Microsomes liver

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... 

Ibrahim RB, Wilson JG, Thorsby ME, et al Effect of buprenorphine on CYP3Aactivity in rat and human liver microsomes. Life Sci 66 1293—1298, 2000 Iguchi MY, Handelsman L, Bickel WK, et al Benzodiazepine and sedative use/abuse by methadone maintenance clients. Drug Alcohol Depend 32 257—266, 1993 Isbell H Manifestations and treatment of addiction to narcotic drugs and barbiturates. Med Clin North Am 34 423 38, 1950... 

Figure 5.52 Accurate-mass cone-voltage spectrum of the ethylhydroxy metabolite of Gly-buride, formed in vitro by human liver microsomes. Reprinted with permission from Zhang, H., Henion, J., Yang, Y. and Spooner, N., Anal. Chem., 72, 3342-3348 (2000). Copyright (2000) American Chemical Society.

Figure 5.54 Structures of Praziquantel and its metabolites, cis- and fraw5-4-hydroxy-praziquantel. Reprinted from 7. Chromatogr., B, 708, Lerch, C. and Blaschke, G., Investigation of the stereoselective metabolism of Praziquantel after incubation with rat liver microsomes by capillary electrophoresis and liquid chromatography-mass spectrometry , 267-275, Copyright (1998), with permission from Elsevier Science.

Kitada, M., Igarashi, K., Hirose, S. Kitagawa, H. (1979). Inhibition by polyamines of lipid peroxide formation in rat liver microsomes. Biochemical Biophysical Research Communications, 87, 388-92. 

Evidence suggests that endosulfan can induce microsomal enzyme activity. Increased liver microsomal cytochrome P-450 activity was observed in male and female rats after single and multiple administrations of endosulfan (Siddiqui et al. 1987a Tyagi et al. 1984). Increased enzyme activity was observed in hepatic and extrahepatic tissues. Based on the increase in aminopyrine-A-demethylase and aniline hydroxylase activity, endosulfan has been shown to be a nonspecific inducer of drug metabolism (Agarwal et al. 1978). 

Koshkaryan AO, Aslanyan GT, Mirzoyan MA, et al. 1989. [Impact of isomers and the major metabolite of endosulfan on the state of liver microsomal systems]. Gig Sanit 3 93-94. (Russian)... 

Ritter CL, Malejka-Giganti D. 1982. Mixed function oxidase in the mammary gland and liver microsomes of lactating rats. Biochem Pharmacol 31 239-247. 

Tyagi SR, Singh Y, Sriram K, et al. 1985. Quality and quantity of dietary protein and acute endosulfan metabolic toxicity in rat liver microsomes. Indian J Med Res 81 480-487. 

Types of Carboxylesterase Isolated from Rat Liver Microsomes... 

Under anaerobic conditions, p,p -DDT is converted to p,p -DDD by reductive dechlorination, a biotransfonnation that occurs postmortem in vertebrate tissues such as liver and muscle and in certain anaerobic microorganisms (Walker and Jefferies 1978). Reductive dechlorination is carried out by reduced iron porphyrins. It is carried out by cytochrome P450 of vertebrate liver microsomes when supplied with NADPH in the absence of oxygen (Walker 1969 Walker and Jefferies 1978). Reductive dechlorination by hepatic microsomal cytochrome P450 can account for the relatively rapid conversion of p,p -DDT to p,p -DDD in avian liver immediately after death, and mirrors the reductive dechlorination of other organochlorine substrates (e.g., CCI4 and halothane) under anaerobic conditions. It is uncertain to what extent, if at all, the reductive dechlorination of DDT occurs in vivo in vertebrates (Walker 1974). 

Walker, C.H. (1969). Reductive dechlorination of p,p -DDT by pigeon liver microsomes. Life Science , 111-115. 

The samples of l,6-T2-DBpD and l,6-T2-2,3,7,8-Cl4-DBpD are useful in metabolism and mode of action studies. For example, when incubated with rabbit liver microsomes, l,6-T.>-DBpD is extensively metabolized to polar product(s) but only when these preparations are fortified with reduced nicotinamide-adenine dinucleotide phosphate. Under the same conditions l,6-T2-2,3,7,8-Cl4-DBpD is completely resistant to metabolic attack. In some types of studies, a higher specific activity possibly is desirable i.e., >1 Ci/mmole), and this can be achieved, with the methodology already developed, by using larger amounts of tritium gas or working on a larger synthetic scale so that it is not necessary to add unlabeled materials to assist in crystallization steps where a certain minimum amount of compound is necessary. 

Figure 11-6. Cytochrome P450 hydroxylase cycle in microsomes. The system shown is typical of steroid hydroxylases of the adrenal cortex. Liver microsomal cytochrome P450 hydroxylase does not require the iron-sulfur protein FejSj. Carbon monoxide (CO) inhibits the indicated step.

Miller RE, Guengerich FP. 1982. Oxidation of trichloroethylene by liver microsomal cytochrome P-450 Evidence for chlorine migration in a transition state not involving trichloroethylene oxide. Biochemistry 21 1090-1097. 

Nakajima T, Wang RS, Elovaara E, et al. 1993. Cytochrome P-450-related differences between rats and mice in the metabolism of benzene, toluene and trichloroethylene in liver microsomes. Biochem Pharmacol 45 1079-1085. 

Polyphenols and flavanoids in rat liver microsomal fractions have been demonstrated to inhibit glucuronidation of estrone and estradiol in vitro (Zhu et al, 1998). In addition, flavonoids have also been found to induce phase I and II enzymes in rats including UDP-glucuronosyl transferase (Seiss et al, 1996). However, the effects of phytoestrogens have not been evaluated for either their inhibition or induction of glucuronosyl transferase activity. 

ZHU B T, TANEJA N, LODER D P, BALENTINE D A, CONNEY A H (1998) Effects of tea polypheUols and flavonoids on liver microsomal glucuronidation of oestradiol and oestrone. J Steroid Biochem Mol Biol. 64 207-15. 

Tocotrienols present in rice bran inhibit the liver microsomal enzyme HMGCoA reductase (Qureshi and Qureshi, 1992), the key enzyme involved in the endogenous synthesis of cholesterol, and this helps to lower the circulating cholesterol. Inhibition of another enzyme, ACAT (Acyl coenzyme A acyl transferase), by y-oryzanol results in lowered LDL-C synthesis and enrichment... 

Compound NPYR Species Rat Tissue Preparation Liver microsomes Comments identification of 4-hydroxybutyr-aldehyde (as its 2,4-dinitrophenyl-hydrazone) from a-hydroxylation Reference 9... 

NPYR Rat Liver microsomes HPLC assay for a-hydroxylation 15... 

NPYR Syrian golden hamster Liver microsomes enhanced a-hydroxylation and mutagenicity in ethanol-consuming hamsters 10... 

NPYR Rat Liver microsomes and postmitochondrial supernatant identification of 1,4-butanediol, and 4-hydroxybutyric acid 12... 

NPYR Rat, Hamster Liver microsomes effects of inducers on a-hydroxyla-tion and mutagenicity 19... 

NNN Rat Liver microsomes assay for a-hydroxylation and the effect of a-deuterium substitution 33... 

NNN Rat Liver microsomes identification of products of p-hydroxylation and pyridine-N-oxidation 36... 

NNN Rat, hamster Liver microsomes effects of inducers on rates of a-hydroxy1at i on 35... 

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