CLINICAL CASES
305
Bile salts: The ionized form of bile acids, which is the state that bile acids
exist at physiologic conditions.
Cholestyramine: A synthetic, strongly basic anion exchange resin that will
strongly bind bile salts when taken orally and prevent their reabsorption
in the intestinal tract.
Conjugated bile salts: Bile salts whose carboxylate groups have been
enzymatically condensed with the amino groups of either of the amino
acids glycine or taurine to form the glyco- or tauroconjugates. This con-
jugation extends the pH range over which the bile salts are ionized and
therefore effective emulsifying agents.
Ursodeoxycholic acid: A naturally occurring bile acid (originally isolated
from the bile of bears) that is present in minor concentrations in the bile
(approximately 1 to 10 percent). When administered orally, it helps to
dissolve cholesterol gallstones.
D ISC U SSIO N
Bile salt molecules secreted by the gallbladder are essential for the emul-
sification and absorption of fats. They are the salt forms of bile acids, which
are the major product of cholesterol catabolism in the liver. Bile salts form
micelles as their hydrophobic face contacts the fat (triacylglycerol), and their
polar face maintains contact with the aqueous environment. This micelle for-
mation allows water-soluble digestive enzymes to digest the entrapped triacyl-
glycerol molecule, releasing fatty acids that are readily absorbed by the
digestive system.
There are two primary bile acids formed in the liver from cholesterol:
cholic acid and chenodeoxycholic acid. The formation of bile acids prevents
cholesterol accumulation in organs; the body cannot break down the steroid
ring of cholesterol. At physiologic pH, bile acids will always be in the form of
bile salts. Bile salts are conjugated with glycine or taurine in the liver prior to
excretion, forming glyco- or tauroconjugates. Bacterial enzymes present in the
intestine produce the secondary bile salts, deoxycholate and lithocholate, by
reducing the primary bile salts.
Bile salts perform an important function and are recycled by the body. The
body produces 400 mg of bile salts per day from cholesterol; this represents
the fate of half of the cholesterol used daily in metabolism (800 mg). However,
20 to 30 g of bile acids is maintained in the enterohepatic circulation. Less
than 0.5 g per day is lost to excretion. Bile salts are produced in the liver,
stored in the gallbladder, and secreted through the bile duct into the duodenum
where they act on triacylglycerol molecules in the intestines. In the gut, the
glycine or taurine moiety is removed from the bile salt. It is reabsorbed in the
small intestine and returned to the liver for reuse via the portal vein. Bile salts
are absorbed by passive diffusion along the entire small intestine, and a spe-
cialized Na+-bile salt cotransporter is present in the lower ileum.
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