Critical micellar concentration bile salts

Micelles tend to aggregate, and there are many ways to measure their concentration, including surface tension measurements. The midpoint of the concentration range over which micellar aggregation occurs is called the critical micellar concentration (CMC). Below the CMC, added bile-salt molecules dissolve in the form of monomers above the CMC, added bile-salt molecules form micelles, leaving the monomeric concentration essentially constant. The pH at which CMC formation occurs is called the critical micellar pH, (CMpH). Table 1.1 lists values for some of the bile acids mentioned in this review. 

Micellar electrokinetic capillary chromatography (MECC), in contrast to capillary electrophoresis (CE) and capillary zone electrophoresis (CZE), is useful for the separation of neutral and partially charged species [266,267]. In MECC, a surfactant, usually sodium dodecyl sulfate (SDS), is added to the buffer solution above its critical micellar concentration to form micelles. Although SDS is certainly the most popular anionic surfactant in MECC, other surfactants such as bile salts have proved to be very effective in separating nonpolar analytes that could not be resolved using SDS [268]. 

The initial concentration, C0, in square centimeters, was determined directly from the area under the curve of a synthetic boundary trace. Although usual practice is to place solvent in one cell and the solution in the other, the solvent used in this work was a solution of bile salt slightly above its critical micellar concentration, CMC (15), usually 0.5 gram per 100 ml. The solution, which was placed in the other cell, was a more concentrated solution (1.5 to 5.0 grams per 100 ml.). Therefore, since both the solvent and the solution were saturated with monomers, C0 represents an initial concentration difference of bile salts in the micellar phase. The pertinent data for each equilibrium ultracentrifugation experiment are presented in Table I. 

For pancreatic lipase to be active, an additional protein, termed colipase, is required (B26). Pure pancreatic lipase is inhibited by bile salts in concentrations exceeding their critical micellar concentrations (B27). The fiinc-tion of coUpase is to restore lipase activity in the presence of bile salts. Although colipase by itself has no lipolytic activity (B27), defective fot digestion and absorption occur if either lipase or colipase activity is low in the small intestine. Patients with steatorrhea due to either isolated lipase deficiency (F4) or isolated cohpase deficiency (H16) have been reported. A lipase which requires bile acids for activity is human milk lipase (Ol). This enzyme comprises 1% of the protein of human milk, but is inactive i ainst milk fots until its activity is stimulated by bile acids in the small intestine. 

Chemical relaxation methods [52] show evidence of a distribution of relaxation frequencies rather than a single one as found with classic ion detergents [193]. Thus, in agreement with the above conclusions, NaC and NaDC apparently self-associate over a whole range of concentrations and not at some critical micellar concentration. Further, the relaxation frequencies are strongly concentration dependent, suggesting that the distribution of aggregate sizes is wide and shifts upwards as bile salt concentration is increased [52]. 

The isolated brush border vesicles from the plasma membrane of the microvilU is the simplest in vitro system used so far. The interaction of lipid with rabbit intestinal brush border vesicles has been investigated by Proulx et al. [50] who found that PC, phosphatidylethanolamine, cholesterol, diglyceride as well as fatty acids were taken up by vesicles. Barsukov et al. [51] have shown that transfer of PC from PC vesicles to isolated brush border vesicles can occur in the presence of PC-exchange protein. The use of brush border vesicles is an interesting new approach that permits detailed studies of rate of transfer of specific lipids into the plasma membrane of the enterocyte. The model is seriously hmited by the fact that incubation with solutions containing bile salts at a concentration above the critical micellar concentration will result in partial or total solubilization of the membrane vesicles. 

Johns WH and Bates TR., Quantification of the binding tendencies of cholestyramine I effect of structure and added electrolytes on the binding of unconjugated and conjugated bile salt anions, /. Pharm. Sci., 58,179-183 (1969). NB These values were quoted from Ekwall P, Rosendahl T and Lofman N, Bile salt solutions. I. The dissociation constants of Bie cholic and deoxycholic acids, Acta Chem. Scand., 11,590-598 (1957). They were measured at concentrations boBi above and below Bie critical micellar concentration range. 

VIII. MICELLE FORMATION AND CRITICAL MICELLAR CONCENTRATION (CMC) OF BILE SALTS... 

TABLE VIII. Critical Micellar Concentration of Chenodeoxycholate and Other Miscellaneous Bile Salts... 

A number of studies have been carried out to determine the aggregation number (number of molecules per micelle) of bile salt micelles. In common with results of previous studies on the critical micellar concentration (CMC) these studies have given variable results which give aggregation numbers for bile salts from 1 (that is, no micelle formation) to well over 1000 associated molecules (73, 144, 145, 120, 167-169). 

A micellar phase is formed in the intestinal lumen when the bile salt concentration exceeds the critical micellar concentration (approximately 3-4 mM). This concentration of bile salts is usually exceeded during normal digestion. Mixed micelles contain bile salts, fatty acids, monoglycerides, cholesterol, and other lipid-soluble molecules (including fat-soluble vitamins) and are considered to be the major route of delivery of the products of fat digestion to the absorptive mucosal cell. Other nonmicellar phases may coexist in the intestinal lumen with the micellar phase these include an oil phase and a viscous isotropic phase. 

Retinol formed by retinyl ester hydrolysis (or originating as such in the diet) and dietary -carotene are solubilized in mixed micelles as discussed above, thus enabling these molecules to reach the microvillus membrane. In studies with everted rat gut sacs in vitro, El-Gorab et al. (1975) reported that micellar solutions significantly enhance uptake of both retinol and p-carotene over emulsions. Maximal uptake occurred at the critical micellar concentration of the bile salt mixture. At higher detergent concentrations, 3-carotene uptake declined whereas retinol absorption remained high. 

Lucangioli, S., Carducci, C., Tripodi, V., Kenndler, E. and 2001, Retention of bile salts in micellar electrokinetic chromatography Relation of capacity factor to octanol-water partition coefficient and critical micellar concentration. J. Chromatogr. B 765, 113. 

Bile salts have most of the properties of an anionic detergent, forming micellar solutions above their critical micellar concentrations. They possess a low capacity for the solubilisation of non-polar substances but a high one for polar ones. The triglycerides of the dietary fats would thus be expected to be emulsified to a limited extent in the duodenum by bile salts, aided by the free fatty acids formed in the stomach and the other bile constituents. 

The physical chemistry of micellar structure and formation has been reviewed extensively elsewhere[40,45-47], and is only briefly summarized. The concentration at which micellar aggregation of bile salts molecules occurs (critical micellar concentration, CMC) is affected by bile salt structure, pH, temperature and a variety of other factors. Conjugated bile salts have a higher CMC than the unconjugates, and the CMC for trihydroxycholanates (cholic acid) is higher than for the dihydroxy derivatives. Among the latter, deoxy-cholate forms micelles at a lower CMC than does chenodeoxycholate. 

Bile Salts Most widely used bile salts in lipidic nano-carriers are sodium deoxycholate sodium taurocholate [63, 88, 91]. Conjugated bile salts sit at the lipid/water interface and at the "Critical micellar concentration" and form micelles. Kumar and Kaur [97] have reported that bile salt was found to stabilize nanoparticles by forming a monomolecular layer on the surface of nanoparticles. 

In response to a meal, cholecystokinin is released from the intestine and causes relaxation of the sphincter of Oddi and contraction of the gallbladder (see Chapter 48). This allows a concentrated solution of micelles (consisting of bile salts, lecithin, and cholesterol) to enter the intestine. In the intestinal lumen, dietary cholesterol and the products of triglyceride digestion (predominantly free fatty acids and monoglycerides) are incorporated into mixed micelles. Micelles deliver lipolytic products to the mucosal surface. To carry out these functions, a critical micellar bile acid concentration of 2ramoI/L is necessary. 

Based on in vitro studies, it was reported that bile salts above their critical micellar concentrahon (CMC) are required for orlislal to be able to effectively inhibit HPL [29] and lipoprotein lipase [111]. The fact that the inhibited HPL could be reactivated by reducing the bile salt concentration below its CMC suggested that bile salts (above their CMC) may stabilize the acyl-hpase complex [29]. 

On the basis of surface and bulk interaction with water. Small [85] classified bile acids as insoluble amphiphiles and bile salts as soluble amphiphiles. On account of the undissociated carboxylic acid group, the aqueous solubility of bile acids is limited [35] in contrast, many bile salts have high aqueous solubilities as monomers [33] and, in addition, their aqueous solubilities are greatly enhanced by the formation of micelles [5,6]. Because many bile salts are weak electrolytes, their ionization and solubility properties are more complicated than those of simple inorganic or organic electrolytes [5,35]. For example, the p/Tj, values of bile acids in water vary markedly as functions of bile salt concentration and, because micelles formed by the A (anionic) species can solubilize the HA (acid) species [5,35], the equilibrium precipitation pH values of bile acids also vary as functions of bile salt concentration. Finally, certain bile salts are characterized by insolubility at ambient temperatures [2,5,6,86,87], only becoming soluble as micelles at elevated temperatures (the critical micellar temperature) [6]. 

Fig. 12. Partial phase diagrams for the dilute region of aqueous solutions of the disodium salts of sulfated monohydroxy bile salts glycolithocholate sulfate (GLCS at pH 10.0) and taurolithocholate sulfate (TLCS at pH 7.0, inset). The solid solubility curves and the interrupted CMC curves demarcate areas where crystals (and monomers), micelles (and monomers), and monomers alone are found. The critical micellar temperature (CMT) represents an equilibrium between micelles and hydrated crystals connected via the monomer concentration at the CMC. The Krafft point is a triple point and only represents the CMT at the CMC. (After ref. 6 with permission.)...

Fig. 10. Phase equilibria of the bile acid (as sodium salt)-fatty acid soap-water phase diagram at constant water concentration in relation to temperature. Mixtures with varying molar ratios of bile acid/sodium soap (total concentration 40 mM) were incubated, and the temperature at which the system became clear was plotted solutions were buffered top i 12. The curves indicate the critical micellar temperature of the system and have also been termed mixed Krafft points (46). The CMT of the bile acids is extremely low.

Fig. 36. Critical micellar temperature (CMT) of alkaline metal salts of lithocholic acid as a function of the atomic volume of the alkaline metal. CMT vertical axis, atomic volume horizontal axis. Percent solids refers to the total amount of bile salt in g/100 ml water. Note that as the atomic volume of the alkaline metal decreases, the Krafft point for any given concentration of bile salt increases. There is a striking rise with lithium, which has the smallest atomic volume.

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