Bile salts, polarity

The structure of cholic acid helps us understand how bile salts such as sodium tauro cholate promote the transport of lipids through a water rich environment The bot tom face of the molecule bears all of the polar groups and the top face is exclusively hydrocarbon like Bile salts emulsify fats by forming micelles m which the fats are on the inside and the bile salts are on the outside The hydrophobic face of the bile salt associates with the fat that is inside the micelle the hydrophilic face is m contact with water on the outside... 

Bile acids, which exist mainly as bile salts, are polar carboxylic acid derivatives of cholesterol that are important in the digestion of food, especially the solubilization of ingested fats. The Na and salts of glycocholic acid and tauro-cholic acid are the principal bile salts (Ligure 25.41). Glycocholate and tauro-cholate are conjugates of cholic acid with glycine and taurine, respectively. 

Because they contain both nonpolar and polar domains, these bile salt conjugates are highly effective as detergents. These substances are made in the liver, stored in the gallbladder, and secreted as needed into the intestines. 

The bile salts have a peculiar molecular structure compared with ordinary detergents, possessing both an ionic polar group and nonionic hydrophilic hydroxyl groups. These molecules might, therefore, have characteristics related to both anionic and nonionic detergents. 

The effect of low concentrations of urea (2M) on the large dihydroxy bile salt micelles is striking, while similar concentrations have no effect on the small trihydroxy or dihydroxy micelles. The effects of urea on micelle formation and aggregate size are undoubtedly complicated (10) and involve changes in solvent structure and thus hydrophobic bonding and hydration of polar groups. For large micelles of dihydroxy bile salt... 

While the detergent molecule has a clear-cut polarity between its hydrophilic and lipophilic parts, the lipophilic part of the bile salt is confined to one side of the steroid nucleus, the other side being spiked with hydrophilic OH groups. In the small primary micelle, bile salt... 

This paper considers the interaction of monoolein, oleic acid, and sodium oleate with bile salt solution in model systems whose compositions have been chosen to simulate those which may occur in small intestinal content. In addition, the behavior of several monoglyceride analogs has been examined to determine the influence of the type of polar head of the lipid on its dispersion by bile salts. Finally, titration experiments were performed to measure the extent of ionization of oleic acid in such systems. 

Figure 4. Phase equilibria for oleyl homologs varying in polar group in bile salt solution...

The kinetic studies indicated that the rate of equilibrium between large monoolein-bile salt aggregates and small monolein-bile salt aggregates was very fast. Information is needed on the rate of exchange of polar lipids between micelles since these aggregates have different structures than soap micelles, where the exchange rates are believed to be rapid (15, 21). 

The other experiment was performed by Isaksson (5) earlier. He extracted desiccated bile, using different solvents in succession. With chloroform, all of the lecithin was extracted but was accompanied by a large part of the bile salts. While bile salts by themselves are insoluble in chloroform, the extract thus obtained contains a proportion by weight of 2 parts of bile salt to 1 of lecithin—about one molecule of lecithin for three molecules of bile salt. Here, it is the lecithin, soluble in chloroform because of its paraffinic chains, which by association solubilizes the bile salt. It is interesting to inquire how these associations are achieved in both cases—i.e., how the molecules of bile salt are arranged and oriented in relation to the molecules of lecithin and to the polar or non-polar solvent. Let us examine first the state of the bile salt molecules in an aqueous phase. 

A nonpolar solubilizate such as hexane penetrates deeply into such a micelle, and is held in the nonpolar interior hydrocarbon environment, while a solubilizate such as an alcohol, which has both polar and nonpolar ends, usually penetrates less, with its polar end at or near the polar surface of the micelle. The vapor pressure of hexane in aqueous solution is diminished by the presence of sodium oleate m a manner analogous to that cited above for systems in nonpolar solvents. A 5% aqueous solution of potassium oleate dissolves more than twice the volume of propylene at a given pressure than does pure water. Dnnethylaminoazobenzene, a water-insoluble dye, is solubilized to the extent of 125 mg per liter by a 0.05 M aqueous solution of potassium myristate. Bile salts solubilize fatty acids, and this fact is considered important physiologically. Cetyl pyridinium chloride, a cationic salt, is also a solubilizing agent, and 100 ml of its A/10 solution solubilizes about 1 g of methyl ethyl-butyl either m aqueous solution. 

Salts of cholic acid and its derivatives are known as bile salts. Bile salts are unlike traditional surfactants in that they are rigid and have multiple polar moieties on one side of the molecule thus exhibiting surface activity. 

Bile salts (bile acids) are the major excretory form of cholesterol. These polar compounds are formed in the liver by converting cholesterol into the activated intermediate cholyl CoA and then combining this compound with either glycine, to form glycocholate, or taurine, to form taurocholate. The detergentlike bile salts are secreted into the intestine where they aid the digestion and uptake of dietary lipids. 

Bile salts (or bile acids) are polar derivatives of cholesterol and constitute the major pathway for the excretion of cholesterol in mammals. In the liver, cholesterol is converted into the activated intermediate cholyl CoA which then reacts either with the amino group of glycine to form glycocholate (Fig. 3a), or with the amino group of taurine (H2N-CH2-CH2-S03", a derivative of cysteine) to form taurocholate (Fig. 3b). After synthesis in the liver, the bile salts glycocholate and taurocholate are stored and concentrated in the gall bladder, before release into the small intestine. Since they contain both polar and nonpolar... 

Bile salts are substances derived from sterols, which make up a substantial part of the solid matter in bile and which play a central role in lipid absorption, by virtue of their surface-active properties. The structure and properties of these salts have been reviewed by Haslewood (305) and Heaton (316). Bile salts essentially have molecules of detergent type hydrocarbon, with a fat-dissolving part and a polar, water-attracting part. The fat-dissolving part consists of the bulk of the steroid nucleus. The hydroxyl groups are so distributed that hydration can readily take place the remainder of the molecule will dissolve the fatty phase. Emulsification of fat/water complexes can thus occur easily. The terms bile acid and bile salt are used somewhat interchangeably in the literature. 

Although bile salts possess a polar head, the hydrocarbon tail is not pure hydrocarbon since there are two or three hydroxyl groups on one side of the molecule. Moreover, like cholesterol (which... 

As a class of tissue, epithelia demarcate body entry points, predisposing a general barrier function with respect to solute entry and translocation. The intestine is lined with enterocytes, which are polarized cells with their apical membrane facing the intestinal lumen that is separated by tight junctions from the basolateral membrane that faces the subepithelial tissues. In addition to their barrier function, the epithelia that line the GI tract serve specialized functions that promote efficient nutrient digestion and absorption and support other organs of the body in water, electrolyte, and bile salt homeostasis. The homeostatic demand on GI tissue that results from this dual function may pose special transport consideration compared with solute translocation across biologically inert barriers. 

Bile salts, which contain polar and nonpolar areas, act as detergents. Th emulsify the fets in the small intestine, breaking them up into tiny soluble droplets (micelles) that provide a broad surfece area for interaction with digestive enzymes (fig. 6.3). 

As polar derivatives of cholesterol, bile salts are highly effective detergents because they contain both polar and nonpolar regions. Bile salts are synthesized in the liver, stored and concentrated in the gall bladder, and then released into the small intestine. Bile salts, the major constituent of bile, solubilize dietary lipids (Section 22.1.1). Solubilization increases in the effective surface area of lipids with two consequences more surface area is exposed to the digestive action of lipases and lipids are more readily absorbed by the intestine. Bile salts are also the major breakdown products of cholesterol. 

Bile salts are required for supporting the activity of pancreatic lipase as well as for maintaining the polar products of fat hydrolysis in solution. While in the lumen of the small intestine, a fraction of the bile salts is modified by the bacteria present If the taurine or glycine rnoictics are removed by microbial enzymes, they are replaced in the liver after reabsorption of the bile salts in the distal ileum. 

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