Carbon Endoplasmic reticulum

In the endoplasmic reticulum of eukaryotic cells, the oxidation of the terminal carbon of a normal fatty acid—a process termed ch-oxidation—can lead to the synthesis of small amounts of dicarboxylic acids (Figure 24.27). Cytochrome P-450, a monooxygenase enzyme that requires NADPH as a coenzyme and uses O, as a substrate, places a hydroxyl group at the terminal carbon. Subsequent oxidation to a carboxyl group produces a dicarboxylic acid. Either end can form an ester linkage to CoA and be subjected to /3-oxidation, producing a... 

As seen already, palmitate is the primary product of the fatty acid synthase. Cells synthesize many other fatty acids. Shorter chains are easily made if the chain is released before reaching 16 carbons in length. Longer chains are made through special elongation reactions, which occur both in the mitochondria and at the surface of the endoplasmic reticulum. The ER reactions are actually quite similar to those we have just discussed addition of two-carbon units... 

The major mono oxygenases in the endoplasmic reticulum are cytochrome P450s—so named because the enzyme was discovered when it was noted that preparations of microsomes that had been chemically reduced and then exposed to carbon monoxide exhibited a distinct peak at 450 nm. Among reasons that this enzyme is important is the fact that approximately 50% of the drugs humans ingest are metabolized by isoforms of cytochrome P450 these enzymes also act on various carcinogens and pollutants. 

The binding of sulfur and/or an activated intermediate of the phosphorus-containing portion of the parathion molecule to the endoplasmic reticulum leads to a decrease in the amount of cytochrome P-450 detectable as its carbon monoxide complex and to a decrease in the rate of metabolism of substrates such as benz-phetamine ( 19). Neither paraoxon nor any other isolatable metabolite of parathion decreases the amount of cytochrome P-450 or inhibits the ability of microsomes to metabolize substrates such a benzphetamine (19). 

The hepatotoxic effects of carbon tetrachloride have been widely studied in animals. Indeed, carbon tetrachloride is used as a model chemical in many laboratory investigations of the basic mechanism of action of hepatotoxic chemicals. Oral exposure to carbon tetrachloride has been observed to result in a wide spectrum of adverse effects on the liver, the most prominent of which are destruction of the smooth and rough endoplasmic reticulum and its associated enzyme activities (Reynolds and Yee 1968), inhibition of protein synthesis (Lutz and Shires 1978), impaired secretion of triglycerides with resultant fat accumulation (Fischer-Nielsen et al. 1991 Recknagel and Ghoshal 1966 Recknagel and Glende 1973 Waterfield et al. 1991), centrilobular necrosis (Blair et al. 1991 Reynolds and Yee 1968 Waterfield et al. 1991 Waterfield et al. 1991 Weber et al. 1992), and eventually fibrosis and cirrhosis (Allis et al. 1990 Bruckner et al. 1986 Fischer-Nielsen et al. 1991 Weber etal. 1992). 

Dose and duration dependence of free radical generation during carbon tetrachloride metabolism in the endoplasmic reticulum relation between trichloromethyl radical formation and lipid radical format on. 

Figure 7.13 The sequence of events underlying carbon tetrachloride toxicity to the liver cell. Although the process starts in the smooth endoplasmic reticulum at CYP2E1, the destruction spreads throughout the cell.

The first events occurring after a toxic dose of carbon tetrachloride can be observed or detected biochemically around the endoplasmic reticulum. Within one minute of dosing, carbon tetrachloride is covalently bound to microsomal lipid and protein in the ratio of 11 3. Conjugated dienes, indicators of lipid peroxidation, can be detected in lipids within five minutes. 

Whether the toxic effects are mainly due to anemic hypoxia or to the histotoxic effects of carbon monoxide on tissue metabolism is a source of controversy. Carbon monoxide will certainly bind to myoglobin and cytochromes such as cytochrome oxidase in the mitochondria and cytochrome P-450 in the endoplasmic reticulum, and the activity of both of these enzymes is decreased by carbon monoxide exposure. However, the general tissue hypoxia will also decrease the activity of these enzymes. 

FIGURE 17-16 The fatty acids in the endoplasmic reticulum. This alternative to /3 oxidation begins with oxidation of the carbon most distant from the a carbon—the oj (omega) carbon. The substrate is usually a medium-chain fatty acid shown here is lauric acid (laurate). This pathway is generally not the major route for oxidative catabolism of fatty acids... 

Although palmitate, a 16-carbon, fully saturated LCFA (16 0), is to primary end-product of fatty acid synthase activity, it can be further I elongated by the addition of two-carbon units in the endoplasmic] reticulum (ER) and the mitochondria. These organelles use separate enzymic processes. The brain has additional elongation capabilities, allowing it to produce the very-long-chain fatty acids (up to 24 car bons) that are required for synthesis of brain lipids. 

Both catalyze chain cleavage and transfer reactions (Eqs. 17-14 and 17-15) that involve the same group of substrates. These enzymes use the two basic types of C-C bond cleavage, adjacent to a carbonyl group (a) and one carbon removed from a carbonyl group ((3). Both types are needed in the pentose phosphate pathways just as they are in the citric acid cycle. The enzymes of the pentose phosphate pathway are found in the cytoplasm of both animal and plant cells.n7c Mammalian cells appear to have an additional set that is active in the endoplasmic reticulum and plants have another set in the chloroplasts.117c... 

Enzyme complexes occur in the endoplasmic reticulum of animal cells that desaturate at A5 if there is a double bond at the A8 position, or at A6 if there is a double bond at the A9 position. These enzymes are different from each other and from the A9-desaturase discussed in the previous section, but the A5 and A6 desaturases do appear to utilize the same cytochrome b5 reductase and cytochrome b5 mentioned previously. Also present in the endoplasmic reticulum are enzymes that elongate saturated and unsaturated fatty acids by two carbons. As in the biosynthesis of palmitic acid, the fatty acid elongation system uses malonyl-CoA as a donor of the two-carbon unit. A combination of the desaturation and elongation enzymes allows for the biosynthesis of arachidonic acid and docosahexaenoic acid in the mammalian liver. As an example, the pathway by which linoleic acid is converted to arachidonic acid is shown in figure 18.17. Interestingly, cats are unable to synthesize arachidonic acid from linoleic acid. This may be why cats are carnivores and depend on other animals to make arachidonic acid for them. Also note that the elongation system in the endoplasmic reticulum is important for the conversion of palmitoyl-CoA to stearoyl-CoA. 

Formation of squalene from famesyl pyrophosphate. Farnesyl transferase is tightly complexed to the endoplasmic reticulum. Three carbons are labeled . A, ) in different structures for purposes of tracking them as the reaction proceeds. 

The mechanism of action for such peroxidic compounds involves a reductive activation by iron in haem, released as a result of hemoglobin digestion by Plasmodium. This irreversible redox reaction affords carbon-centered free radicals causing the alkylation of haem and of proteins. One such protein (the sarcoplasmic-endoplasmic reticulum ATPase PfATP6) appears to be critical for parasite survival, and there is no indication for resistance by the parasite. However, treatment is expensive and recrudescence of malaria occurs often. Moreover, it was found that at high doses such compounds are neurotoxic. 

In addition to cytochrome P-450 enzymes, another enzyme that mediates phase I oxidations is flavin-containing monooxygenase (FMO), likewise contained in the endoplasmic reticulum. It is especially effective in oxidizing primary, secondary, and tertiary amines. Additionally, it catalyzes oxidation of other nitrogen-containing xenobiotic compounds, as well as those that contain sulfur and phosphorus, but does not bring about hydroxylation of carbon atoms. 

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