Bile acids formation

The enzymes in peroxisomes do not attack shorter-chain fatty acids the P-oxidation sequence ends at oc-tanoyl-CoA. Octanoyl and acetyl groups are both further oxidized in mitochondria. Another role of peroxisomal P-oxidation is to shorten the side chain of cholesterol in bile acid formation (Chapter 26). Peroxisomes also take part in the synthesis of ether glycerolipids (Chapter 24), cholesterol, and dolichol (Figure 26-2). 

One role of high density lipoprotein (HDL) is to collect unesterified cholesterol from cells, including endothelial cells of the artery walls, and return it to the liver where it can not only inhibit cholesterol synthesis but also provide the precursor for bile acid formation. The process is known as reverse cholesterol transfer and its overall effect is to lower the amount of cholesterol in cells and in the blood. Even an excessive intracellular level of cholesterol can be lowered by this reverse transfer process (Figure 22.10). Unfortunately, the level of HDL in the subendothelial space of the arteries is very low, so that this safety valve is not available and all the cholesterol in this space is taken up by the macrophage to form cholesteryl ester. This is then locked within the macrophage (i.e. not available to HDL) and causes damage and then death of the cells, as described above. 

The process of bile acid formation in the liver, movement to the gallbladder, and then movement to the duodenum is a pathway that can concentrate toxicants in hepatocytes. From the gallbladder where they are stored, bile acids are released into the gut to aid in the digestion of lipophobic substances. Most of the released bile acids are reabsorbed in the intestines and are returned to the liver by the hepatic portal circulation. 

Bile-acid formation in rats involves hydroxylation to give 7a- and 6j8-hydroxy-derivatives. In many cases, no isotope effect was observed on hydroxylation of the appropriate labelled sterol. These examples involve cytochrome P-450 in the oxidation. However, oxidation of [7a- H,24- C]deoxycholic acid or tauro-deoxycholic acid to the corresponding cholic acid showed an isotope effect of 3.8 on examination of recovered starting material. 

Regulation of bile acid formation from cholesterol occurs at the 7a-hydroxylation step and is mediated by the concentration of bile acids in the enterohepatic circulation. 7a-Hydroxylase is modulated by a phosphorylation-dephosphorylation cascade similar to that of HMG-CoA reductase (Figure 19-11) except that the phosphorylated form of 7a-hydroxylase is more active. 

The conversion of cholesterol to 7a-hydroxycholesterol is the first step and the major regulatory step in bile acid formation (Chapter 9). 

The specificity of the inhibitory effect of bile acids on bile acid formation has been studied by Danielsson [110]. Taurocholic acid, taurochenodeoxycholic acid and taurodeoxycholic acid, fed at the 1% level in the diet for 3-7 days, were found to inhibit cholesterol 7a-hydroxylase activity. Feeding taurohyodeoxycholic acid and... 

Thus, the reduced form of cytochrome P-450 functions as the oxygenactivating biocatalyst of a wide variety of mixed-function oxidations by vertebrate tissues effecting biosynthesis and catabolism of steroid hormones, bile acid formation, and the metabolism of drugs and other xeno-biotics (16). Since reduced pyridine nucleotides do not react directly with hemoproteins, the hydroxylase systems must include components that mediate the electron transport from TPNH to cytochrome P-450. There also must be distinctive diflFerences in composition causing the substrate specificity of the oxygenations. 

Investigation of the 3-hydroxyacyl-CoA dehydrogenase activities in purified rat liver peroxisomes, using the 3-hydroxyacyl-CoAs of straight chain fatty acids, of 2-methyl-branched chain fatty acids and of trihydroxycoprostanic acid as substrates, revealed initially 5 different enzymes (named I to V)." Enzyme IV was a monomeric 78 kD protein, possessed crotonase activity, was induced by clofibrate, and was identified as the inducible multifunctional protein. Interestingly, enzyme III, a monomeric 80 kD protein, also hydrated crotonyl-CoA. In contrast to enzyme IV, enzyme III was not induced by clofibrate." This was the first indication, pubhshed in 1994, that peroxisomes contained a second multifunctional protein. It was named multifunctional protein 2 (MFP-2) (the inducible, firstly isolated protein is referred to as MFP-1). Based on its substrate spectrum, the newly identified multifunctional protein was postulated to be involved in bile acid formation. ... 

Antonenkov, V.D., Van Veldhoven, P.P., Waelkens, E. Mannaerts, GP. (1997) J. Biol. Chem. 212, 26023-26031. Substrate specificities of 3-oxoacyl-CoA thiolase A and sterol carrier protein 2/3-oxoacy 1-CoA thiolase purified from normal rat liver peroxisomes. Sterol carrier protein 2/3-oxoacyl-CoA thiolase is involved in the metabolism of 2-methyl-branched fatty acids and bile acid intermediates. Bunya, M., Maebuchi, M., Kamiryo, T., Kurosawa, T, Sato, M., Tohma, M, Jiang, L.L. Hashimoto, T. (1998) J. Biochem. (Tokyo). IS, 347-352. Thiolase involved in bile acid formation. 

The development of present knowledge of the mechanisms of bile acid formation has been possible through a combination of the use of cell-free preparations of liver with the general experimental approach of Bergstrom, Lindstedt, and collaborators. Metabolites formed in the presence of different subcellular fractions of liver homogenates have been isolated and identified. The compounds have been tested as precursors of bile acids in rats with a biliary fistula. The chemical synthesis of a number of unlabeled as well as isotopically labeled steroids has played an important role in these studies. 

The results of Shefer et al. (152) support the conclusion by Lee et al. (146) that taurochenodeoxycholic acid has an effect on bile acid formation in rats with a biliary fistula not entirely related to the physiological regulation of bile acid formation, since the concentration of taurochenodeoxycholic acid needed to repress bile acid biosynthesis is less than half of that of taurocholic acid. 

The homeostatic regulation of bile acid formation could involve action of bile acids on one or both of the rate-determining reactions in the conver-... 

The reason for the low bile acid formation in familial hypercholesterolemia is not known. It could be due to impaired availability of cholesterol to the pool which is utilized for bile acid synthesis, a partial deficiency or inhibition of the enzyme system producing bile acids, deficient hepatic secretion of bile acids, or augmented intestinal reabsorption. An interesting observation is that the bile of hypercholesterolemic patients may contain relatively little or no deoxycholic acid (23,73,157), a finding recorded also in patients... 

In the first step of bile acid formation, cholesterol is hydroxylated in the 7 position by a microsomal hydroxylase that requires NADPH, thus yielding the 5-cholestene-3j5, 7a-diol. Cytochrome c reductase and cytochrome P450 have been suspected to be involved because the reaction is inhibited by carbon monoxide. It is not certain whether the 7a hydroxylation preferentially attacks cholesterol or cholesterol esters. 

Cholestyramine is an ion-exchange resin that traps the bile salts and thereby secures their elimination in the intestine. The bile salts are continuously excreted in the intestine bile acid formation from cholesterol increases, and serum cholesterol levels drop. Existing evidence indicates that cholestyramine administration and ileal bypass are the most effective measures available to reduce serum cholesterol in hypercholesterolemic patients. 

The in vivo work on bile acid formation has been augmented by work with liver homogenates and particulate fractions which has expanded the knowledge concerning the mechanisms involved. Chaikoff et al. (1952) had shown that large amounts of were formed after the administration of cholesterol-26-to... 

Hydroxylation is believed to constitute the initial step in a minor pathway in bile acid formation (95) whereas the importance of 24- and 25-hydroxylation of cholesterol is unknown (96). Steroids related to cholesterol such as sitosterol adn 5P-cholestan-3 -ol have been reported to be bile acid precursors (97,98). Purified cytochrome P-450, prepared from rat liver microsomes, was found to catalyze efficient 25-hydroxylation of 5p-cholestan-3a, 7a, 12a-triol as well as vitamin D3 (99,100). A number of steroids and steroidal triterpenoids have been isolated from plants and microorganisms which contain a single oxygenated functional group at C-25 (90,101-103). 

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