Bile acids and salts

As noted above, MeC trimerizes and MeLC does not self-associate in CHCI3. Under these conditions, Foster et al. [202] used vapor pressure osmometry to show that solubilized cholesterol (which dimerizes in CHCI3 [203]) heteroassociated with MeC but not with MeLC. The result was a 1 1 mixed dimer complex of cholesterol and MeC with a molar free energy of formation which was 33% that for the trimerization of MeC in the same solvent [202]. The bonding is presumably via the 3-hydroxyl functions in both steroids this interaction may be of potential importance in the binding of cholesterol to bile acids and salts within membranes and mixed micelles. 

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Bile acids and salts have been found to enhance the absorption of both calcium and vitamin D hence, to increase calcium absorption both directly and indirectly (3,37). However, the ability of some dietary fibers such as lignin and pectin to absorb conjugated and deconjugated bile salts onto their surfaces to be excreted in the feces (a mechanism credited to the hypocholesterolemic effect of some dietary fibers) may result in an overall decrease in calcium absorption from the gastrointestinal tract (7,33,38-40). 

The mechanisms by which various forms of dietary fiber influence calcium bioavailability apparently also differ. In some cases, apparent dietary fiber effects on calcium bioavailability may be secondary to effects on bile acid and salt secretion and reabsorption or to other dietary components. 

Although both LDL and HDL are primarily cholesterol particles, most of the cholesterol measured in the blood is assodated with LDL. The normal role of LDL is to deliver cholesterol to tissues for biosynthesis. When a cell is repairing membrane or dividing, the cholesterol is required for membrane synthesis. Bile acids and salts are made from cholesterol in the liver, and many other tissues require some cholesterol for steroid synthesis. As shown in Figure 1-15-6, about 80% of LDL are picked up by hepatocytes, the remainder by peripheral tissues. ApoB-100 is the only apoprotein on LDL, and endocytosis of LDL is mediated by apoB-100 receptors (LDL receptors) clustered in areas of cell membranes lined with the protdn clathrin. 

Bile salts secreted into the intestine are efficiently reabsorbed (greater than 95 percent) and reused. The mixture of primary and secondary bile acids and bile salts is absorbed primarily in the ileum. They are actively transported from the intestinal mucosal cells into the portal blood, and are efficiently removed by the liver parenchymal cells. [Note Bile acids are hydrophobic and require a carrier in the portal blood. Albumin carries them in a noncovalent complex, just as it transports fatty acids in blood (see p. 179).] The liver converts both primary and secondary bile acids into bile salts by conjugation with glycine or taurine, and secretes them into the bile. The continuous process of secretion of bile salts into the bile, their passage through the duodenum where some are converted to bile acids, and their subsequent return to the liver as a mixture of bile acids and salts is termed the enterohepatic circulation (see Figure 18.11). Between 15 and 30 g of bile salts are secreted from the liver into the duodenum each day, yet only about 0.5 g is lost daily in the feces. Approximately 0.5 g per day is synthesized from cholesterol in the liver to replace the lost bile acids. Bile acid sequestrants, such as cholestyramine,2 bind bile acids in the gut, prevent their reabsorption, and so promote their excretion. They are used in the treatment of hypercholesterolemia because the removal of bile acids relieves the inhibition on bile acid synthesis in the liver, thereby diverting additional cholesterol into that pathway. [Note Dietary fiber also binds bile acids and increases their excretion.]... 

Bile is secreted into the intestine, and more than 95 percent of the bile acids and salts are efficiently reabsorbed. They are actively transported from the intestinal mucosal cells into the portal blood, where they are carried by albumin back to the liver (enterohepatic circulation). In the liver, the primary and secondary bile acids are reconverted to bile salts, and secreted into the bile. 

Cholestyramine or colestipol form an insoluble complex with the bile acids and salts, preventing their reabsorption from the intestine. 

The common bile acids and salts possess a characteristic molecular structure shown schematically for sodium cholate in Fig. 1 [7]. In contrast to the all irons arrangement of the alicyclic ring systems in alio bile salts, a crucial feature of the... 

A simple method for estimating the pH-solubility relationship of bile acids and salts is to carry out aqueous acidometric titration of a bile salt in water with a stronger mineral acid [5,35], Once the molarities of bile salt and mineral acid are known, the titration curves provide a direct measurement of equivalence, equilibrium and metastable pH values, the pH at which precipitation of the HA species occurs (pHpp,), an estimate of the solubilities of the HA species in water (if the system is < CMC) or in water plus micelles (if the system is > CMC) and a calculation of the apparent pK (pATg). The methods, results and interpretation of such titration curves for the common bile salts, titrated with HCl, have been described in detail elsewhere [5,6]. 

Bile An emulsifying agent produced in the Hver and secreted into the duodenum. Its composition includes bile acids and salts, cholesterol, and electrolytes. It aids digestion of fats in the duodenum, [nih]... 

Humans do not oxidize cholesterol for energy. Instead, cholesterol is converted to bile acids such as cholic acid and deoxycholic acid in liver tissue. Bile acids and salts are secreted into bile, which passes into the intestine and emulsifies fats for digestion. Although some bile acids may be reabsorbed in the intestine along with lipids, much cholesterol leaves the body in feces in the form of metabolites such as bile acids and salts. 

Synthesis of bile acids and salts from cholesterol is shown in Figure 19.23. These reactions involve enzymes called microsomal P450 mixed-function oxidases. The first reaction, in which cholesterol is converted to 7-ot-hydroxycholesterol, is the rate limiting step. 

Figure 19.23 Biosynthesis of bile acids and salts from cholesterol. 

Finally, we should not forget to mention nature s own emulsions and foams. Examples of natural emulsions are rubber latex produced by Hevea brasiliensis, fat droplets in milk stabilized by proteins, fats from the diet emulsified in the duodenum and stabilized by bile acids and salts, and blood too may be regarded as an emulsion. Where (micro)organisms living in an aqueous environment produce gaseous metabolites, foams are often formed. This may be desirable, as in beer, or undesirable as in installations for the treatment of wastewater. 

Part of the body s cholesterol is used in the synthesis of bile acids and salts and a smaller proportion gives rise to steroid hormones. Some cholesterol is excreted in the bile and thus is available for reabsorption. 

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