Lipophilic xenobiotics

In phase 1, the pollutant is converted into a more water-soluble metabolites, by oxidation, hydrolysis, hydration, or reduction. Usually, phase 1 metabolism introduces one or more hydroxyl groups. In phase 2, a water-soluble endogenous species (usually an anion) is attached to the metabolite— very commonly through a hydroxyl group introduced during phase 1. Although this scheme describes the course of most biotransformations of lipophilic xenobiotics, there can be departures from it. 

Many of the phase 1 enzymes are located in hydrophobic membrane environments. In vertebrates, they are particularly associated with the endoplasmic reticulum of the liver, in keeping with their role in detoxication. Lipophilic xenobiotics are moved to the liver after absorption from the gut, notably in the hepatic portal system of mammals. Once absorbed into hepatocytes, they will diffuse, or be transported, to the hydrophobic endoplasmic reticulum. Within the endoplasmic reticulum, enzymes convert them to more polar metabolites, which tend to diffuse out of the membrane and into the cytosol. Either in the membrane, or more extensively in the cytosol, conjugases convert them into water-soluble conjugates that are ready for excretion. Phase 1 enzymes are located mainly in the endoplasmic reticulum, and phase 2 enzymes mainly in the cytosol. 

Vertebrate liver is a very rich source of enzymes that metabolize lipophilic xenobiotics, and subcellular fractions are prepared to study metabolism. Sometimes, other tissues such as brain, kidney, testis, and ovary are also treated in this way. A typical subcellular fractionation of liver might be as follows ... 

Looking at the foregoing results overall, the rates of loss in vivo are related to the rates of metabolism in vitro, measured or estimated. As with the OC insecticides, problems of persistence are associated with compounds that are not readily metabolized, for example, 2,2, 4,4, 5,5 -HCB in the foregoing examples. For further discussion of the dependence of elimination of lipophilic xenobiotics on metabolism, see Walker (1981). 

Ligandin A form of glutathione-5-transferase with a marked capacity for binding certain lipophilic xenobiotics. 

Lipoproteins Macromolecules that are associations of lipids with proteins. Involved in the transport of both lipids and lipophilic xenobiotics in the blood. 

Monooxygenases (MOs) Enzyme systems of the endoplasmic reticulum of many cell types, which can catalyze the oxidation of a great diversity of lipophilic xenobiotics, are particularly well developed in hepatocytes. Forms of cytochrome P450 constitute the catalytic centers of monooxygenases. 

Virtually all organisms possess biotransformation or detoxification enzymes that convert lipophilic xenobiotics to water-soluble and excretable metabolites (Yu et al. 1995). In the metabolic process, PAHs are altered by Phase I metabolism into various products such as epoxides, phenols,... 

An alternative process that can lead to the termination or alteration of biologic activity is metabolism. In general, lipophilic xenobiotics are transformed to more polar and hence more readily excreted products. The role that metabolism plays in the inactivation of lipid-soluble drugs can be quite dramatic. For example, lipophilic barbiturates such as thiopental and pentobarbital would have extremely long half-lives if it were not for their metabolic conversion to more water-soluble compounds. 

Squalene is suggested to enhance elimination of lipophilic xenobiotics by several experimental evidences. Its nonpolar structure promotes a promising affinity for unionized drugs. Richter and Schafer (1982) studied squalene for elimination of [14C]hexachlorobenzene (HCB) as an alternative method to paraffin treatment. Animal models have been fed by squalene and paraffin as 8% of the diet. Results indicated that squalene supplementation was as effective as paraffin on fecal excretion of HCB. 

The cytochrome P450 system is the principal enzyme system for the metabolism of lipophilic xenobiotics. It is a heme-containing, membrane-bound, multi-enzyme system which is present in many tissues in vivo but is present at the highest level in liver. A coenzyme, cytochrome P450 NADPH oxidoreductase (OR), is essential for P450 catalytic function and cytochrome bs may stimulate catalytic activities of some enzymes. In human liver, it is estimated that there are 15-20 different xenobiotic-metabolizing cytochrome P450 forms. A standard nomenclature, based on relatedness of the amino acid sequences, has been developed (Nelson et al., 1993). The most recent... 

Typical reactions catalyzed by UGT enzymes require the cofactor uridine diphosphate-glucuronic acid, UDPGA. However, the C-terminus of all UGT enzymes contains a membrane-spanning domain that anchors the enzyme in the endoplasmic reticulum, and the enzyme faces the lumen of the endoplasmic reticulum, where it is ideally placed to conjugate lipophilic xenobiotics and their metabolites generated by oxidation, reduction, or hydrolysis. The lumenal... 

Conjugation of lipophilic xenobiotics to polar cellular constituents renders the xenobiotic more water-soluble. While the lipophilic parent xenobiotics could readily diffuse into the cells, the increase in polarity associated with conjugation greatly reduces the ability of the compound to diffuse across the lipid bilayer of the cell membrane thus trapping the compound within the cell. The polar conjugates must therefore rely upon active transport processes to facilitate efflux from the cell. Hepatocytes, as well as other cells involved in chemical detoxification, are rich with members of the ATP-binding cassette superfamily of active transport proteins (ABC transporters). Cellular efflux of xenobiotics by these transporters is often referred to as Phase III elimination because Phase I or II detoxification processes often precede and are a requirement of Phase III elimination. A detailed description and discussion of elimination and transporters is presented in Chapter 15. 

The following discussion will center on mixed-function oxidases involving the hemoprotein cytochrome P-450, the active center of which consists of protoporphyrin IX. Mixed-function oxidases based on cytochrome P-450 are perhaps best known for their role in the primary metabolism of lipophilic xenobiotics in mammals, birds, fish and many invertebrates including insects. While this function is often critical in determining the survival of an organism... 

Lipophilic xenobiotics are superior in number (about 70-80%). Because of their high solubility in fats - thus also in the phospholipid double layer of the cellular and subcellular membranes - these substances rapidly (and without hindrance) reach the liver cells and organelles according to the laws of nonionic diffusion. 

Such lipid-soluble, non-excretable substances — if subject to glomerular filtration — are almost completely reabsorbed from the primary urine by renal tubules, with the result that they are hardly ever excreted via the kidney. This might involve the danger of accumulations of lipophilic xenobiotics in the body, especially in fatty tissue. Therefore hydrophilization of these substances (with... 

Microsomes isolated from hepatic tissue appear to retain all of the mixed-function oxidase capabilities of intact hepa-tocytes because of this, microsomal preparations (with the necessary cofactors, e.g.. NADPH. Mg- ) are u.scd frequently for in vitro drug metabolism studies. Because of its membrane-bound nature, the cytochrome P-450 monooxy-gena.se system appears to be housed in a lipoidal environment. This may explain, in part, why lipophilic xenobiotics arc generally good sub.straics for the monooxygenase system. ... 

The major purpose of biotransformation is to chemically modify (metabolize) poorly excretable lipophilic compounds to more hydrophilic chemicals that are readily excreted in urine and/or bile. Without metabolism, lipophilic xenobiotics accumulate in biota, increasing the potential for toxicity. Examples of such compounds are highly halogenated polychlorinated biphenyls (PCBs) and polychlorinated dibenzofu-rans (TCDD and dioxins) that occur as tissue residues in humans. On the contrary, biotransformation is normally not required for xenobiotics with high water solubility because of rapid excretion in urine. 

Oxidation is the most common metabolic reaction for lipophilic xenobiotic and endobiotic compounds, in part because most mammalian tissues are well oxygenated. 

Although mercapturic acids are normally the major thioether products of lipophilic xenobiotics found in urine of mammals, small amounts of the corresponding S-cysteine conjugates are also frequently excreted. All four thioether products formed during mercapturic acid biosynthesis are routinely excreted in bile. 

A number of enzyme systems have evolved in animals and plants which effectively convert lipophilic xenobiotics to more polar compounds that are efficiently excreted. Phase I enzymes, responsible for oxidation, reduction, and/or hydrolysis, are integrated with phase II or conjugation enzymes for reactions of both types and are normally required for the formation of products polar enough to be readily excreted. The intracellular level of these enzymes, and thus, the capacity for biotransformation, increases in a coordinate fashion in response to exposure to xenobiotic compounds. This response is... 

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