Intestinal Absorption and Metabolism of Xenobiotics

Abstract

There are five possible processes of intestinal absorption of xenobiotics. These are active transport, passive diffusions, pinocytosis, filtration through "pores," and lymphatic absorption. The passive diffusion ismajor process for transport of foreign chemicals across the intestine. Though the lymphatic absorption of drugs is not of any major therapeutic significance, the uptake oftoxic chemicals such as 3-MC, benzpyrene, and DDT through lymphatics may enhance their toxicity, since they are distributed to other organ systems in the body without being metabolized by liver. A number of factors such as diet, motility of intestine, interference with gastrointestinal flora, changes in the rate of gastric emptying, age of the animal, and dibsolution rate of xenobiotic can alter the rate of absorption of chemicals.Liver is the major site of metabolism of xenobiotlcs, but the contribution of intestinal metabolism of xenobiotic can influence the overall bioavailability of chemicals. The xenobiotic metabolizing enzymes oated in endoplasmic reticulum of intestine possess biochemical characteristics similar to that of liver. In general, the rate of metabolism of xenobiotics by intestinal microsomal preparation is lower than that observed with similar hepatic microsomal preparations. The in vitro intestinal metabolism of xenobiotics is affected by several factors including age, sex, diurnal variations, species, and nutritional status of the aninal. The intestinal xenobiotic metabolizing enzymes are stimulated by the pretreatment of animals with foreign chemicals, but this depends on the route of administration of chemicals, drug substrate and the aninal species used. Rabbit intestinal drug metabolizing enzymes seem to be resistant to induction by foreign chemicals.

Structure–activity relationship in the passage of different pyrrolizidine alkaloids through the gastrointestinal barrier: ABCB1 excretes heliotrine and echimidine

“…Hereby the diffusion rate is directly proportional to the gradient but further depends on the molecule's lipid solubility, size, charge, and the area of absorptive surface. Since PA are easily transported through the monolayer, distribution of PA through the Caco‐2 monolayer seems to follow a passive transcellular way, which is the main mechanism of absorption for lipophilic xenobiotics . However, for Hn and Em an active efflux mechanism back to the intestinal lumen mediated by ABCB1 (P‐glycoprotein) was shown in this study.…”

Structure–activity relationship in the passage of different pyrrolizidine alkaloids through the gastrointestinal barrier: ABCB1 excretes heliotrine and echimidine

“…Hereby the diffusion rate is directly proportional to the gradient but further depends on the molecule's lipid solubility, size, charge, and the area of absorptive surface. Since PA are easily transported through the monolayer, distribution of PA through the Caco‐2 monolayer seems to follow a passive transcellular way, which is the main mechanism of absorption for lipophilic xenobiotics . However, for Hn and Em an active efflux mechanism back to the intestinal lumen mediated by ABCB1 (P‐glycoprotein) was shown in this study.…”

Mind the gap—deficits in our knowledge of aspects impacting the bioavailability of phytochemicals and their metabolites—a position paper focusing on carotenoids and polyphenols

“…Transcellular uptake can occur by facilitated, active means or via passive diffusion. The latter is reserved for small and apolar molecules, as these readily pass through the cell membrane . Passive diffusion has been suggested for carotenoids and some apolar polyphenol aglycones .…”

Regional Differences in Human Intestinal Drug Metabolism

“…Phase I reactions are oxidation, reduction, and hydrolysis processes. These chemical reactions change the biologic activity of the parent compound, rendering it less or completely inactive (Chhabra, 1979). In the intestines, the most abundant group of enzymes belongs to the cytochrome P450 subfamily 3A.…”

“…Phase II reactions are conjugation reactions, e.g., glucuronidation and sulfation, that improve the solubility of endo-and xenobiotics (Chhabra, 1979). 7-HC can be used to examine both reactions (van de Kerkhof et al, 2007): glucuronidation of 7-HC is mediated by uridine 59-diphospho-N-acetylgalactosamine glycosyltransferase (UGTs), while sulfotransferase (SULTs) catalyze sulfation reactions (Wang et al, 2003(Wang et al, , 2006.…”

Structure–activity relationship in the passage of different pyrrolizidine alkaloids through the gastrointestinal barrier: ABCB1 excretes heliotrine and echimidine

“…Hereby the diffusion rate is directly proportional to the gradient but further depends on the molecule's lipid solubility, size, charge, and the area of absorptive surface. Since PA are easily transported through the monolayer, distribution of PA through the Caco‐2 monolayer seems to follow a passive transcellular way, which is the main mechanism of absorption for lipophilic xenobiotics . However, for Hn and Em an active efflux mechanism back to the intestinal lumen mediated by ABCB1 (P‐glycoprotein) was shown in this study.…”

Mind the gap—deficits in our knowledge of aspects impacting the bioavailability of phytochemicals and their metabolites—a position paper focusing on carotenoids and polyphenols

“…Transcellular uptake can occur by facilitated, active means or via passive diffusion. The latter is reserved for small and apolar molecules, as these readily pass through the cell membrane . Passive diffusion has been suggested for carotenoids and some apolar polyphenol aglycones .…”

Regional Differences in Human Intestinal Drug Metabolism

“…Phase I reactions are oxidation, reduction, and hydrolysis processes. These chemical reactions change the biologic activity of the parent compound, rendering it less or completely inactive (Chhabra, 1979). In the intestines, the most abundant group of enzymes belongs to the cytochrome P450 subfamily 3A.…”

“…Phase II reactions are conjugation reactions, e.g., glucuronidation and sulfation, that improve the solubility of endo-and xenobiotics (Chhabra, 1979). 7-HC can be used to examine both reactions (van de Kerkhof et al, 2007): glucuronidation of 7-HC is mediated by uridine 59-diphospho-N-acetylgalactosamine glycosyltransferase (UGTs), while sulfotransferase (SULTs) catalyze sulfation reactions (Wang et al, 2003(Wang et al, , 2006.…”

Structure–activity relationship in the passage of different pyrrolizidine alkaloids through the gastrointestinal barrier: ABCB1 excretes heliotrine and echimidine

“…Hereby the diffusion rate is directly proportional to the gradient but further depends on the molecule's lipid solubility, size, charge, and the area of absorptive surface. Since PA are easily transported through the monolayer, distribution of PA through the Caco‐2 monolayer seems to follow a passive transcellular way, which is the main mechanism of absorption for lipophilic xenobiotics . However, for Hn and Em an active efflux mechanism back to the intestinal lumen mediated by ABCB1 (P‐glycoprotein) was shown in this study.…”

Mind the gap—deficits in our knowledge of aspects impacting the bioavailability of phytochemicals and their metabolites—a position paper focusing on carotenoids and polyphenols

“…Transcellular uptake can occur by facilitated, active means or via passive diffusion. The latter is reserved for small and apolar molecules, as these readily pass through the cell membrane . Passive diffusion has been suggested for carotenoids and some apolar polyphenol aglycones .…”

Regional Differences in Human Intestinal Drug Metabolism

“…Phase I reactions are oxidation, reduction, and hydrolysis processes. These chemical reactions change the biologic activity of the parent compound, rendering it less or completely inactive (Chhabra, 1979). In the intestines, the most abundant group of enzymes belongs to the cytochrome P450 subfamily 3A.…”

“…Phase II reactions are conjugation reactions, e.g., glucuronidation and sulfation, that improve the solubility of endo-and xenobiotics (Chhabra, 1979). 7-HC can be used to examine both reactions (van de Kerkhof et al, 2007): glucuronidation of 7-HC is mediated by uridine 59-diphospho-N-acetylgalactosamine glycosyltransferase (UGTs), while sulfotransferase (SULTs) catalyze sulfation reactions (Wang et al, 2003(Wang et al, , 2006.…”