Cholesterol bile salts and

The third class of lipids is steroids. Included in this category of lipids are cholesterol, bile salts, and sex hormones. Steroid structures contain fused rings consisting of three six-carbon rings and a five-carbon ring ... 

Ophthalmic, transmucosal, and transdermal products will be the most sensitive to the strength of binding. These routes of administration experience minimal dilution. However, this may not be a signiLcant concern because the drug typically can also be displaced from the CD cavity at the delivery site by competing lipophiles at the delivery site, such as triglycerides, cholesterol, bile salts, and other hydrophobic compounds, which are often in much higher concentrations (Thompson, 1997). 

H-12) Gallstones. Most gallstones are composed mainly of cholesterol. Bile salts and phospholipids normally prevent the precipitation of cholesterol, but cholesterol stones may form when the cholesterol/bile salt-phospholipid ratio increases excessively. Cheno-deoxycholate may be used as oral therapy for cholesterol gallstones. It not only provides an extra recirculating source of bile acids but inhibits the rate-limiting step in cholesterol biosynthesis. 

This volume of secretion is supplemented in the ductules by ca. 150 ml ductular bile, resulting in a daily production of ca. 600 ml. Bile formation is lower at night than during the day. The most important constituents of the so-called liver bile are the bile acids, phospholipids, proteins, cholesterol and bilirubin. The term bile lipids includes cholesterol, bile salts and phospholipids. The manner in which cholesterol is excreted into the gall bladder is not yet known, nor have any cholesterol-specific transport systems been detected. Cholesterol is primarily broken down into bile acids, (see above) (s. tab. 3.5)... 

XI. Cholesterol, Bile Salt, and Steroid Hormone Metabolism... 

XI. Cholesterol, bile salt, and steroid hormone metabolism 214... 

Fig. 7, Phase diagram showing the physical state of all possible combinations of cholesterol, bile salts, and lecithin (expressed as mole percent) in aqueous solutions. The line AB represents the maximum amount of cholesterol, according to Admirand and Small (A2), which can be dissolved by any mixture of bile salts and lecithin. [From ref. (A2). Reproduced from The Journal of Clinical Investigation, 1968, 47, by copyright permission ofThe American Society for Clinical Investigation.]...

Most of the lipids found in the body fall into the categories of fatty acids and triacylglycerols glycerophospholipids and sphin-golipids eicosanoids cholesterol, bile salts, and steroid hormones and fat-soluble vitamins. These lipids have very diverse chemical structures and functions. However, they are related by a common property their relative insolublity in water. 

In 1956, Hamre (159) reported a detailed study of the liver and biliary tract changes associated with vitamin A deficiency in rats. The animals were fed the diet for approximately 5 weeks. At autopsy, 86% of the rats showed pathological changes. The proximal two-thirds of the common ducts as dilated, and the bile contained masses of epithelial cells. Calculus-like bodies, either hard and chalky white or fragile and dull, were found in the dilated common ducts and consisted predominantly of masses of compacted epithelial cells. Crystals of inorganic salts, cholesterol, bile salts, and others were identified by microscopic examination of crushed stones. All calculi contained both acid-soluble and ether-soluble material. 

Waxes and Triacylglycerols Chemical Properties of Triacylglycerols Phospholipids Steroids Cholesterol, Bile Salts, and Steroid Hormones Cell Membranes... 

Steroids Cholesterol, Bile Salts, and Steroid Hormones... 

Bde salts, cholesterol, phosphoHpids, and other minor components are secreted by the Hver. Bile salts serve three significant physiological functions. The hydrophilic carboxylate group, which is attached via an alkyl chain to the hydrophobic steroid skeleton, allows the bile salts to form water-soluble micelles with cholesterol and phosphoHpids in the bile. These micelles assist in the solvation of cholesterol. By solvating cholesterol, bile salts contribute to the homeostatic regulation of the amount of cholesterol in the whole body. Bile salts are also necessary for the intestinal absorption of dietary fats and fat-soluble vitamins (24—26). 

We turn now to the biosynthesis of lipid structures. We begin with a discussion of the biosynthesis of fatty acids, stressing the basic pathways, additional means of elongation, mechanisms for the introduction of double bonds, and regulation of fatty acid synthesis. Sections then follow on the biosynthesis of glyc-erophospholipids, sphingolipids, eicosanoids, and cholesterol. The transport of lipids through the body in lipoprotein complexes is described, and the chapter closes with discussions of the biosynthesis of bile salts and steroid hormones. 

Irrespective of the physical form of the carotenoid in the plant tissue it needs to be dissolved directly into the bulk lipid phase (emulsion) and then into the mixed micelles formed from the emulsion droplets by the action of lipases and bile. Alternatively it can dissolve directly into the mixed micelles. The micelles then diffuse through the unstirred water layer covering the brush border of the enterocytes and dissociate, and the components are then absorbed. Although lipid absorption at this point is essentially complete, bile salts and sterols (cholesterol) may not be fully absorbed and are not wholly recovered more distally, some being lost into the large intestine. It is not known whether carotenoids incorporated into mixed micelles are fully or only partially absorbed. 

The overall metabolism of vitamin A in the body is regulated by esterases. Dietary retinyl esters are hydrolyzed enzymatically in the intestinal lumen, and free retinol enters the enterocyte, where it is re-esterified. The resulting esters are then packed into chylomicrons delivered via the lymphatic system to the liver, where they are again hydrolyzed and re-esterified for storage. Prior to mobilization from the liver, the retinyl esters are hydrolyzed, and free retinol is complexed with the retinol-binding protein for secretion from the liver [101]. Different esterases are involved in this sequence. Hydrolysis of dietary retinyl esters in the lumen is catalyzed by pancreatic sterol esterase (steryl-ester acylhydrolase, cholesterol esterase, EC 3.1.1.13) [102], A bile salt independent retinyl-palmitate esterase (EC 3.1.1.21) located in the liver cell plasma hydrolyzes retinyl esters delivered to the liver by chylomicrons. Another neutral retinyl ester hydrolase has been found in the nuclear and cytosolic fractions of liver homogenates. This enzyme is stimulated by bile salts and has properties nearly identical to those observed for... 

The intestinal absorption of dietary cholesterol esters occurs only after hydrolysis by sterol esterase steryl-ester acylhydrolase (cholesterol esterase, EC 3.1.1.13) in the presence of taurocholate [113][114], This enzyme is synthesized and secreted by the pancreas. The free cholesterol so produced then diffuses through the lumen to the plasma membrane of the intestinal epithelial cells, where it is re-esterified. The resulting cholesterol esters are then transported into the intestinal lymph [115]. The mechanism of cholesterol reesterification remained unclear until it was shown that cholesterol esterase EC 3.1.1.13 has both bile-salt-independent and bile-salt-dependent cholesterol ester synthetic activities, and that it may catalyze the net synthesis of cholesterol esters under physiological conditions [116-118], It seems that cholesterol esterase can switch between hydrolytic and synthetic activities, controlled by the bile salt and/or proton concentration in the enzyme s microenvironment. Cholesterol esterase is also found in other tissues, e.g., in the liver and testis [119][120], The enzyme is able to catalyze the hydrolysis of acylglycerols and phospholipids at the micellar interface, but also to act as a cholesterol transfer protein in phospholipid vesicles independently of esterase activity [121],... 

Bile salts In contrast to cholesterol, bile salts are absorbed mainly in the jejunum. They are returned to the liver through the hepatic portal vein (in association with proteins) and can thence be re-secreted into the bile. The transport of bile salts between liver and intestine is known... 

Bile. The liver forms a thin secretion (bile) that is stored in the gallbladder after water and salts have been extracted from it. From the gallbladder, it is released into the duodenum. The most important constituents of bile are water and inorganic salts, bile acids and bile salts (see p. 314), phospholipids, bile pigments, and cholesterol. Bile salts, together with phospholipids, emulsify insoluble food lipids and activate the lipases. Without bile, fats would be inadequately cleaved, if at all, resulting in fatty stool (steatorrhea). Resorption of fat-soluble vitamins would also be affected. 

They are useful only in hyperlipoproteinemias involving elevated levels of LDL i.e. type Ila, lib and V. They are basic ion exchange resins. They are neither digested nor absorbed in the gut. They bind bile acids in intestine and interrupt their entero-hepatic circulation, leading to increased faecal excretion of bile salts and cholesterol. There is increased hepatic conversion of choles-terol to bile acids. More LDL receptors are expressed on liver cells leading to increased clearance of IDL, LDL and indirectly of VLDL. 

If more cholesterol enters the bile than can be solubilized by the available bile salts and phosphatidylcholine, cholesterol gallstone disease (cholelithiasis) can occur. This is generally caused by gross malabsorption of bile acids from the intestine, obstruction of the biliary tract, or severe hepatic dysfunction, leading to abnormalities in bile or bile salt production. 

Dietary cholesterol, together with triacylglycerols, is absorbed from the intestinal tract and enters the large lipoprotein chylomicrons (see Fig. 21-1). Absorption of cholesterol is incomplete, usually amounting to less than 40% of that in the diet. Absorption requires bile salts and is influenced by other factors.186 As it is needed cholesterol is taken from the plasma lipoproteins into cells by endocytosis. Much of the newly absorbed cholesterol is taken up by the liver. The liver also secretes cholesterol, in the form of esters with fatty acids, into the bloodstream. 

Deters M, Kirchner G, Koal T, Resch K, Kaever V. Everolimus/cyclosporine interactions on bile flow and biliary excretion of bile salts and cholesterol in rats. Dig Dis Sci 2004 49(l) 30-7. 

Does Citrus Pectin Bind Bile Salts A possible mechanism by which dietary pectin may cause lowering of cholesterol levels in rats has been reported (1 9). In these in vitro studies, pectin was found to inhibit the transport of taurocholic acid from everted sacs of rat intestine. The absorption of labelled cholesterol was depressed by the addition of 5% pectin to the diet as evidenced by increased excretion of labelled cholesterol and diminished cholesterol deposition in the liver. It was concluded from these studies that pectin lowers cholesterol levels in cholesterol-fed rats primarily by binding bile salts and, consequently, by impairing cholesterol absorption. Results similar to those obtained with dietary pectin and described have also been reported for other non-nutritive substances such as guar gum, psyllium seed colloid and seruglucan (20). 

Cholesterol is the precursor of bile salts, and their secretion into the intestine is stimulated after eating food and during the digestion and absorption of triglyceride. 

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