Bile salt precipitation

Efficiency of lipolysis may be compromised by bile salt precipitation. 

Mithani SD. Dissolution and Precipitation of Dipyridamole Effect of pH and Bile Salt Concentration. Thesis The University of Michigan, Michigan, 1998. 

Imbalance In secretion of cholesterol and the bile salts In bile can cause cholesterol to precipitate In the gallbladder, producing cholesterol-based gallstones, which accounts for the most common type of cholelithiasis. 

Human bile sample (500 pi) is placed into a 5-mm nuclear magnetic resonance (NMR) tube. The pH of the solution is adjusted at 6 0.5 using HC1. This range of pH is the optimal value to avoid lowering of the amide integral due to the chemical exchange (alkaline pH) or to precipitation of bile salts (acidic pH). 

Effect of pH. The effect of pH on the deoxycholate series of bile salts in 0.15N NaCl at 20° and 36°C. is summarized in Figure 8. Solutions of free and glycine conjugated bile salts were studied between pH 9.2 and their precipitation pH (NaDC == 6.95, NaGDC = 4.9). Taurine conjugates were studied between pH 10 and 1.6. 

Two complementary experiments show that the orientation and hiding of one or the other face of the steroid ring of cholate can occur when mixtures of lecithin and bile salt are considered. One of these experiments was performed by Etienne (4), who observed the following facts incidentally while extracting lipids from the serum lipoproteins by Delsal s method. This method utilizes a mixture of methanol and methylal (1 to 4) in the cold. The proteins are precipitated, while the lipids are dissolved in the methanol-methylal solvent mixture. If this solution of the lipids is evaporated, the residue is soluble in nonpolar solvents, such as chloroform. However, if sodium cholate is added to the lipoproteins before their extraction, the residue obtained after the methylal-methanol solvent evaporates is insoluble in chloroform. More precisely, while cholesterol and the triglycerides of the lipidic residue are extracted by chloroform, all of the lecithin remains insoluble, associated to the bile salt. The explanation is probably as follows. During evaporation, methylal with its low boiling point (44°C.), evaporates first, and the solvent becomes more and more concentrated with methanol and the residual water from the lipoprotein aqueous solution. Therefore, in the lecithin plus... 

Part of the cholesterol newly synthesized in the liver is excreted into bile in a free non-esterified state (in constant, amount). Cholesteiol in bile is normally complexed with bile salts to form soluble cholic acids, Free cholesterol is not readily soluble and with bile stasis or decreased bile salt concentration may precipitate as gallstones. Most common gallstones are built of alternating layers of cholesterol and calcium bilirubin and consist mainly (80-90%) of cholesterol. Normally. 80% of hepatic cholesterol arising from blood or lymph is metabolized to cholic acids and is eventually excreted into the bile in the form of bile salts. 

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. 

Gallbladder inflammation (cholecystitis) usually presents with acute abdominal pain (colic) with radiation to the right shoulder. The normal composition of bile is about 5% cholesterol, 15% phosphatidylcholine, and 80% bile salt in a micellar liquid form. Increased cholesterol from high-fat diets or genetic conditions can upset the delicate micellar balance, leading to supersaturated cholesterol or cholesterol precipitates that cause gallstone formation. Removal of the gallbladder is a common treatment for this painful condition. 

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]. 

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]. 

Fig. 8. Equilibrium precipitation pH (pHpp,) and apparent p/T (pK ) values of unconjugated and glycine (G)-conjugated CDC and UDC as functions of bile salt concentration (H2O, 37°C). (From ref. 35 with...

The equilibrium pHpp, values and pK values for all 4 bile salts as functions of bile salt concentration (HjO, 37°C) are plotted in Fig. 8 (from ref. 35). For comparable bile salt concentrations, the HA species of UDC and GUDC precipitate at higher pH values than the HA species of CDC and GCDC, despite similar pA values. The physical-chemical reason why the pHpp, of the UDC and GUDC species... 

It is quite probable that enterolith formation in man is a consequence of stasis and bacterial proliferation in the intestine. Microbial enzymes deconjugate bile salts, which raise their pK to a value of about 6, so that much of the free bile acid is un-ionized and precipitates at the / H of intestinal content. When microbial enzymes also convert cholic acid to deoxycholic acid, the latter compound forms choleic acids, which have an even higher pKa and thus precipitate more readily. 

Neomycin is a polybasic, poorly absorbed antibiotic which forms insoluble precipitates with bile salts (99). It lowers serum cholesterol concentrations in man (100-102) and chickens (99) and increases fecal bile acid excretion. It inhibits the hepatotoxic effects of lithocholic acid ingestion in chickens (99) and prevents bacterial conversion of cholate to deoxycho-late (103). Neomycin, 6-12 g/day, induces a malabsorption syndrome, with mucosal changes similar to those of sprue (104). Bile salt metabolism is thus affected in at least three ways by neomycin (1) a binding effect similar to that of cholestyramine, (2) suppression of deconjugation and secondary bile formation caused by antimicrobial properties, and (3) possible impairment of absorption of bile salts by intestinal mucosa. The first probably accounts for most of the increased fecal excretion of bile salts. 

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