Cholesterol and Cholic Acid

Steroid Chemistry at a Glance Daniel Lednicer 2011 John Wiley Sons, Ltd 

More direct evidence for the gross structure of carbon skeleton of steroids came from the isolation of the hydrocarbon 

known as the Diels hydrocarbon, proved to be identical with a sample of the compound synthesized from starting materials of known structure by an unambiguous reaction scheme. 

The strained nature of the cyclobutanone that would result from cychzation as in 4-3 is disfavored over the formation of the anhydride. Reaction thus proceeds to succinic anhydride (4-6). In the absence of instruments, anhydrides can be distinguished from ketones by the fact that the former will lead to a dicarboxylic acid on basic hydrolysis. The neutral ketone can be recovered unchanged under the same conditions. 

The two groups and also other investigators who worked on the problem felt that enough data had been accumulated to propose a stmcture in 1928. Most of the carbon atoms had been accounted for and the results, they deemed, supported 

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Galls tone-solubilizing (gallstone-dissolving) drugs, such as ursodiol (Actigall), suppress die manufacture of cholesterol and cholic acid by die liver. The suppression of die manufacture of cholesterol and cholic add may ultimately result in a decrease in die size of radiolucent gallstones. 

Our study on the distribution of electron transferring proteins in animal sources is still in progress. From present knowledge, adrenodoxin can be found in adrenal cortexes from pig, beef, and rat. Further, a similar protein was isolated from pig testis (see II-A-2), and it was also found in the ovary. However, brain, heart, liver, kidney, and pancreas appear to lack adrenodoxin-like protein. If this is correct, the proteins of the ferredoxin family are located solely in the glands which directly act in the biosynthesis of steroid hormones. It is of interest that adrenodoxin-like protein does not participate in the steroid hydroxylation involved in cholesterol and cholic acid biosyntheses. All of these reactions without the participation of adrenodoxin are similar to enzymes responsible for microsomal non-specific hydroxylation, which consist of the following sequence of electron transfer ... 

The two diastereoisomers of 3a,7a,12a-trihydroxy-5/3-cholestan-26-oic acid (4), isomeric at C-25, have been distinguished by AT-ray crystallographic analy-sis. " It should now be possible to establish the configurations of samples obtained from natural sources. The earlier literature contains apparently conflicting evidence, which may result from the possibility of equilibration at C-25 during hydrolytic steps in the isolation of the natural material, thought to be a key biosynthetic intermediate between cholesterol and cholic acid. 

Staple, E., and S. Gurin The incorporation of radioactive acetate into biliary cholesterol and cholic acid. Biochim. biophys. Acta (Amst.) 15, 372 (1954). 

Scymnol, C27H4JOJ, occurs as a sulphuric ester in shark bile, and is of interest in that it represents a compound intermediate in type between cholesterol and cholic acid, and may indicate their biochemical relationship. Scymnol has the same nucleus as cholic acid, but carries a different side-chain,... 

The liver possesses the power of synthesising both cholesterol and cholic acid, and thus controls the amount of these substances present in the bile. Under normal conditions the amount of cholic acid produced synthetically is probably small, because the bile salts are almost completely reabsorbed from the intestine and return to the liver. 

Sterols are steroids containing one or more hydroxyl groups. Some examples are cholesterol, a component of the cytoplasmic membrane of animal cells, testosterone, a hormone, and cholic acid. 

There was no relationship between rate of synthesis or hepatic content of ubiquinone in the rat and the production and excretion of p-hydroxybenzoate.393 Exposure to low temperatures caused an increase in ubiquinone synthesis, whilst starvation or feeding with cholesterol or cholic acid resulted in a reduction of the conversion of p-hydroxybenzaldehyde into ubiquinone no feedback by the end-product seemed to be operative. 

In the liver, cholesterol has three major fates conversion to bile acids, secretion into the blocKlstream (packaged in lipoproteins), and insertion into the plasma membrane. Conversion of cholesterol to cholic acid, one of the bile acids, requires about 10 enzymes. The rate of bile synthesis is regulated by the first enzyme of the pathway, cholesterol la-hydioxylase, one of the cytochrome P450 enzymes (see the section on Iron in Chapter 10), Cholesterol, mainly in the form of cholesteryl esters, is exported to other organs, after packaging in particles called very-low-density lipoproteins. Synthesis of cholesteryl esters is catalyzed by acyl CoA cho-Jesteroi acy(transferase, a membranc bound enzyme of the ER, Free cholesterol is used in membrane synthesis, where it appears as part of the walls of vesicles in the cytoplasm. These vesicles travel to the plasma membrane, where subsequent fusion results in incorporation of their cholesterol and phospholipids into the plasma membrane. 

Fig. (19). Effects of resveratrol and piceid isolated from P. cuspidatum roots on Uver TC and TG in rats fed com oU-10% cholesterol-1% cholic acid mixture for 1 weeks.

Fig. (20a). Effects of resveratrol and piceid isolated from P. cuspidatum roots on serum LDL-ch and TG in rats fed com oil-10% cholesterol-1% cholic acid mixture for 1 week. Rats were orally administered with lipid emulsion (10 ml/kg body weight) for 1 week. Blood was taken by venous puncture 4 h after administration of the lipid emulsioiL Results are expressed as mean S.E. of 6-7 rats.

The insertion of hydroxyl groups into the 23- or 24-position of 5P-cholestane-3a,7a,12a,25-tetrol was found to be stereospecific. Although all these compounds were potential precursors of bile acid, studies in vivo and in vitro experiments using [3P- H] and (24- C) 5P-cholestane-3a,7a,12a,25-tetrol (46) (Figs.6, 7), (24- C) 5p-cholestane-3a,7a,12a,24R,25-pentol and (24- C) 5P-cholestane-3a,7a,12a,24S,25-pentol demonstrated the existence of a new 25-hydroxylation pathway for the transformation of cholesterol to cholic acid in these patients (2,10). The reaction sequence involved the stereospecific formation of a 24S-hydroxy pentol, 5P-cholestane-3a,7a,12a,24S,25-pentol, 3a7a,12a,25-tetrahydroxy-5P-cholestan-24-one and did not involve SP-cholestanoic acids as intermediates (Fig. 8). The two bile pentols, SP-cholestane-3a,7a,12a,24R, 25-pentol and 5P-cholestane-3a,7a,12a,23R,25-... 

Combination of Chinese herb active components. The combination of Chinese herb active components (baicalin, jasminoidin, and cholic acid) (CBJC) has shown positive effects in rats treated with ibotenic acid. The expression levels of 19 genes in the forebrain were significantly influenced by CBJC approximately 60 % of these genes were related to neuroprotection and neurogenesis, whereas others were related to antioxidation, protein degradation, cholesterol metabolism, stress response, angiogenesis, and apoptosis [298]. 

Primary bile acids Synthesized from cholesterol as cholic acid and chen-odeoxycholic acid. They are secreted as taurine and glycine conjugates. 

S2). More recent studies have shown that patients with cirrhosis are able to efficiently convert 7a-hydroxy-cholesterol into cholic acid (G8, P8), suggesting that 12a-hydroxylase activity is near normal. Other evidence from in vivo studies in man with labeled preciusors suggests that 12a-hydroxylase activity is not important in the regulation of the ratio between cholic acid and chenodeoxycholic add in human bile (B21). The possibility that different pools of cholesterol are utilized for the biosynthesis of cholic acid and chenodeoxycholic acid is now being investigated. 

C5. Carey,. B., Conversion of cholesterol to trihydroxycoprostanic acid and cholic acid in man. J. Clin. Invest. 43, 1443-1448 (1964). 

The above investigations led to the formulation of the sequence of reactions shown in Fig. 3 for the nuclear changes in the conversion of cholesterol into cholic acid and chenodeoxycholic acid. In this scheme, the 12a-hydroxyl group is introduced at the stage of 7a-hydroxy-4-cholesten-3-one. It was shown that also 7a-hy-droxycholesterol could be 12a-hydroxylated by the microsomal fraction of a liver homogenate [31]. Since that hydroxylation occurred at a much lower rate, it was believed to represent a minor pathway [32]. The conversions shown in Fig. 3 were later also demonstrated in human Uver [33]. 

A study of the reduction of [24- C]3-oxo-5j8-cholanic acid in bile fistula rats given [l- Hjjethanol showed that all metabolites had a 3a-hydroxy group and all radioactive products (lithocholate, 3a,6/8-dihydroxy-5 -cholanate, chenodeoxycho-late and y8-muricholate) contained about 13 atom% excess deuterium in the 3/9 position. Thus, the 3)8-hydroxy-5/9-steroid dehydrogenase isoenzyme of alcohol dehydrogenase [172] has no function in the reductive metabolism of bile acids. Cholic acid was not radioactive but contained deuterium at the 3)8, 5)8 and other positions, probably because of the transfer of deuterium from ethanol via NADH to NADPH, which it utilized in the biosynthesis of cholesterol and bile acids and in oxido reduction of the 3-hydroxyl group of the latter [173]. 

Huijghebaert et al. [23] isolated a bile salt sulfatase-producing strain designated, Clostridium S, from rat feces. This bacterium hydrolyzed the 3-sulfates of lithocholic acid, chenodeoxycholic acid, deoxycholic acid and cholic acid but not the 7-or 12-monosulfates. Sulfatase activity required the 3-sulfate group to be in the equatorial position. A free C-24 or C-26 carboxyl group was also required for sulfatase activity in whole cells of this bacterium. The 3-sulfate of cholesterol, Cj,-and Cji-steroids were not hydrolyzed by Clostridium S, [24]. Nevertheless, C,9- and C2]-steroid sulfates are hydrolyzed in the gut by microbial activity suggesting that the intestinal microflora may contain bacteria with steroid sulfatases possessing different substrate specificities. However, it should be noted that enzyme substrate specificity studies carried out in whole cells may reflect both cell wall permeability and enzyme specificity. 

What structural features account for the differences in the solubility of cholesterol, estradiol and cholic acid in the body (see Fig. 5.23)... 

A. Conversion of Cholesterol to Cholic Acid and Chenocholic Acid... 

Biosynthesis The primary B. are synthesized in the liver from cholesterol by a complicated, multi-step reaction sequence. The 7o-hydroxy group is introduced first while the 12-hydroxy group is added later to a further intermediate with subsequent formation of both chenodeoxycholic acid and cholic acid. Deoxycholic acid is not synthesized in the liver but rather in the intestines by 7a-dehydroxylation of cholic acid by intestinal bacteria. 

Early work in vitro on the sequence of reactions in the conversion of cholesterol into bile acids was carried out with mitochondrial preparations from rat and mouse liver (11). These preparations were found to catalyze predominantly reactions involving the oxidation of the side chain of C27-steroids. No evidence for the formation of 12a-hydroxylated metabolites was obtained. In 1963, Mendelsohn and Staple (12) reported the conversion of cholesterol into 5/ -cholestane-3a,7a-12a-triol in the presence of a 20,000 supernatant fluid of rat liver homogenate. This finding provided the first experimental evidence for the long surmised role of 5/ -cholestane-3a,7a,12a-triol as an intermediate in the conversion of cholesterol into cholic acid. Subsequent work with this enzyme preparation and subfractions of it has led to the elucidation of the sequences of reactions in the conversion of cholesterol into 5/ -cholestane-3a,7a,12a-triol (Fig. 1). 

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