Microsomes intestinal

Intestinal microsomes Intestinal tissue Phase I metabolism... 

Intestinal absorption of digoxin is less complete compared to digitoxin. In order to improve absorption, acetylated- and methylated-digoxin derivates were developed. Digitoxin is metabolised in hepatic microsomal enzymes and can be cleared independently from renal function. The therapeutical serum level of digoxin is 0.5-2.0 ng/ml and 10-35 ng/ml of digitoxin. Steady state plateau of therapeutic plasma concentrations is reached after 4-5 half-life-times using standard daily doses [5]. 

Like other xenobiotics, cannabinoids also undergo extensive metabolism in the human body to increase their hydrophihc properties for a facihtated ehmination. The metaboHsm of A9-THC has been very well investigated. More than 100 metabolites of A9-THC are known [99] and a good overview of the most important human metaboHtes is given in [100]. MetaboHsm takes place mainly in hepatic microsomes, but also in intestines, brain. 

Sodium/glucose cotransporter (rabbit intestinal brush borders)1641 Stearylcoenzyme A desaturase (rat liver microsomal)[651 Subtilopeptidase amylosacchariticus[661 Succinate dehydrogenase (mitochondrial)1671... 

The coupling of solute transport in the GI lumen with solute lumenal metabolism (homogeneous reaction) and membrane metabolism (heterogeneous reaction) has been discussed by Sinko et al. [54] and is more generally treated in Cussler s text [55], At the cellular level, solute metabolism can occur at the mucosal membrane, in the enterocyte cytosol, and in the endoplasmic reticulum (or microsomal compartment). For peptide drugs, the extent of hydrolysis by lumenal and membrane-bound peptidases reduces drug availability for intestinal absorption [56], Preferential hydrolysis (metabolic specificity) has been targeted for reconversion... 

X Lu, C Li, D Fleisher. Cimetidine sulfoxidation in small intestinal microsomes. Drug Metab Dispos 26 940-942, 1998. 

The P-C isomer selectivity seems to be tissue-specific a preferential uptake of the all-trans isomer was shown in hepatic stellate HSC-T6 cells and in cell-free system from rat liver microsomes, but not in endothelial EAHY cells or U937 monocyte-macrophages (During et al., 2002). When Caco-2 cells were incubated with only 9-cis P-C, all -trans P-C did not increase in cells or in the basolateral medium, indicating that there is no cis-trans isomerization occurring in intestinal cells. Thus, the isomerization of 9-cis P-C observed in vivo (You et al., 1996) could take place in the... 

Fouarge M, Mercier M, Poncelet F (1984) Liver, kidney and small-intestine microsomal-mediated mutagenicity of carcinogenic aromatic amines. Mutat Res 125(1) 23-31... 

Smith, T.S., Graham, M., Munn, E.A., Newton, S.E., Knox, D.P., Coadwell, W.J., McMichael-Phillips, D., Smith, H., Smith, W.D. and Oliver, J.J. (1997) Cloning and characterisation of a microsomal aminopeptidase from the intestine of the nematode Haemonchus contortus. Biochimica et Biophysica Acta 1338, 295—306. 

Gibbs, M. A., Thummel, K. E., Shen, D. D., Kunze, K. L., Inhibition of cytochrome P-450 3A (CYP3A) in human intestinal and liver microsomes comparison of Ki values and impact of CYP3A5 expression, Drug Metab. Dispos. 1999, 27, 180-187. 

Tab. 13.1. Relative rates of glucuronidation in human liver and intestinal microsomes.

UDP-glucuronyltransferases catalyze the addition of glucuronic acid onto phenol, hydroxyl and carboxylic acid functions of molecules. They are expressed in many tissues of the body, including the liver and intestine [2-5], Microsomes from human intestines have been shown to metabolize UDP-glucuronyltransferase substrates including p-nitrophenol [6], 1-naphthol, morphine, and ethinylestradiol [4]. The relative rates of metabolism of these substrates in liver and intestinal microsomes are shown in Table 13.1. 

Overall, the human intestine is capable of metabolizing UDP-glucuronyltransferase substrates, although the rates of metabolism are between 5- and 10-fold lower than those observed in human liver microsomes. However, the presence of a metabolic capacity towards UDP-glucuronyltransferase substrates at the level of the enterocyte can exert a significant gut wall first-pass extraction on oral administration. 

Esterases form a wide family of enzymes that catalyze the hydrolysis of ester bonds. They are ubiquitously expressed in all tissues including the intestine, and are found in both microsomal and cytosolic fractions. Prueksaritonont et al. [6] have studied the metabolism of both p-nitrophenol acetate and acetylsalicylic acid by esterases from human intestinal microsomal and cytosolic systems, and the activity values were 2.76 pmol min-1 mg-1 and 0.96 nmol min-1 mg-1, respectively. Thus, the activity for the hydrolysis of p-nitrophenol acetate in human intestine approaches that in the liver. 

Tab. 13.3. Relative rates metabolism in human intestinal microsomes for a series of CYP3A substrates.

The metabolism of several CYP3A4 substrates in microsomes from the upper small intestine has been compared with liver microsomal metabolism. The results are summarized in Table 13.3. Thus, microsomes from the human upper small intestine can metabolize CYP3A4 substrates at rates approaching those found in human liver microsomes. However, the rate of metabolism in intestinal microsomes can be highly variable (8-fold for sirolimus [17] and 18- to 29-fold for midazolam [19]). 

Drugs may also undergo hydrolysis by intestinal esterases (hydrolases), more specifically carboxylesterases (EC 3.1.1.1) in the intestinal lumen and at the brush border membrane [58, 59]. It has been shown that intestinal hydrolase activity in humans was closer to that of the rat than the dog or Caco-2 cells [60]. In these studies, six propranolol ester prodrugs and p-nitrophenylacetate were used as substrates, and the hydrolase activity found was ranked in the order human > rat Caco-2 cells > dog for intestinal microsomes. The rank order in hydrolase activity for the intestinal cytosolic fraction was rat > Caco-2 cells = human > dog. The hydrolase activity towards p-nitrophenylacetate and tenofovir disoproxil has also been reported in various intestinal segments from rats, pigs and humans. The enzyme activity in intestinal homogenates was found to be both site-specific (duodenum > jejunum > ileum > colon) and species-dependent (rat > man > Pig)-... 

The enzyme involved in the degradation of the dyes has been shown to be azoreductase. The enzymes were first isolated from the intestinal microflora and was later found to be produced by the cytosolic and microsomal fractions of the liver [47]. The enzyme was sensitive to oxygen and was inactivated by oxygen. In experiments involving intestinal anaerobic bacteria, Rafii et al. found the requirement of... 

Sediments and biota collected from the Hersey River, Michigan, in 1978, were heavily contaminated with phenanthrene, benz[a]anthracene, and benzo[a]pyrene when compared to a control site. Elevated PAH concentrations were recorded in sediments, whole insect larvae, crayfish muscle, and flesh of lampreys (family Petromyzontidae), brown trout (Salmo trutta), and white suckers (Catostomus commersoni), in that general order (Black et al. 1981). The polluted collection locale was the former site of a creosote wood preservation facility between 1902 and 1949, and, at the time of the study, received Reed City wastewater treatment plant effluent, described as an oily material with a naphthalene-like odor (Black et al. 1981). In San Francisco Bay, elevated PAH concentrations in fish livers reflected elevated sediment PAH concentrations (Stehr et al. 1997). In Chesapeake Bay, spot (Leiostomus xanthurus) collected from a PAH-contaminated tributary (up to 96 mg PAHs/kg DW sediment) had elevated cytochrome P-450 and EROD activity in liver and intestine microsomes (Van Veld et al. 1990). Intestinal P-450 activity was 80 to 100 times higher in fish from highly contaminated sites than in conspecifics from reference sites intestinal EROD activity had a similar trend. Liver P-450 and EROD activity was about 8 times higher in spot from the contaminated sites when compared to the reference sites. Liver P-450 activity correlated positively with sediment PAH, but intestinal P-450 activity seemed to reflect dietary exposure (Van Veld et al. 1990). The poor correlation between hepatic concentrations of PAHs and P-4501A is attributed to the rapid metabolism of these compounds (van der Weiden et al. 1994). 

Gallbladder had 85% of radioactivity. Tb 1/2 values were 0.8 days for liver, 3.3 days for intestine, 3.5 days for gallbladder, and 8.2 days for liver Diets rich in fish oils produced marked increases in activities of P-450 cytochromes, 7-EROD, and other enzyme activities bass fed diets devoid of fish oil had lower microsomal enzyme activities... 

Miranda, C.L. and R.S. Chhabra. 1980. Species differences in stimulation of intestinal and hepatic microsomal nixed-function oxidase enzymes. Biochem. Pharmacol. 29 1161-1165. 

An important DMPK property of a NCE is oral bioavailability (F) of the compound in various pre-clinical species.3 The oral bioavailability of a compound is dependent on several factors including intestinal permeability (estimated by the Caco-2 assay) and hepatic clearance (estimated with an in vitro metabolic stability assay).3 30 The metabolic stability assay is typically performed by incubating test compounds in liver microsomes or hepatocytes. The results can provide estimates of in vivo stability in terms of metabolic liabilities.3 8 59 62 Several authors described this assay as an important tool for the rapid assessment of the DMPK properties of NCEs.3 6 8111819 26 44 59 62-65... 

It was reported that the distribution and activities of esterases that catalyze pyrethroid metabolism using several human and rat tissues, including small intestine, liver, and serum, were examined [30]. The major esterase in human intestine was hCE2. //c/n.v-Permethrin was effectively hydrolyzed by pooled human intestinal microsomes (five individuals), while deltamethrin and bioresmethrin were not. This result correlated well with the substrate specificity of recombinant hCE2. In contrast, pooled rat intestinal microsomes (five animals) hydrolyzed trans-permethrin 4.5 times slower than the human intestinal microsomes. Furthermore, pooled samples of cytosol from human or rat liver were ca. half as hydrolytically active as the corresponding microsome fraction toward pyrethroids however, the cytosolic fractions had significant amounts (ca. 40%) of the total hydrolytic activity. Moreover, a sixfold interindividual variation in hCEl protein expression in human hepatic cytosols was observed. 

Wynalda MA, Hutzler JM, Koets MD, Podoll T, and Wienkers LC (2003) In vitro metabolism of clindamycin in human liver and intestinal microsomes. Drug Metab. Disp. 31 878-887. 

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