Bile Acids and Bile Salts

Bile Acids

Bile salts are synthesized in the liver from cholesterol. The first step is formation of bile acids:

 

 

1. An α-hydroxyl group is added to carbon 7 of cholesterol by 7α-hydroxylase (rate-limiting step).
2. The double bond of cholesterol is reduced, and further hydroxylations occur, resulting in two compounds. One has two α-hydroxyl groups at positions 3 and 7 (diol); and the other has three α-hydroxyl groups at positions 3,7, and 12 (triol).
3. The side chain is oxidized and converted to a branched, 5-carbon chain, containing a carboxylic acid at the end.
4. The bile acid with two hydroxyl groups at positions 3 and 7 is chenocholic acid.
5. The bile with three hydroxyl groups at positions 3, 7, and 12 is cholic acid.
6. The bile acids have pK of approximately 6.
7. Above pH 6, the molecules are salts (they ionize and carry a negative charge).
8. At pH 6 (the pH of the intestinal lumen), half of the molecules are ionized.
9. Below pH 6, the bile acids are protonated, and their charge decreases as the pH is lowered.

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Bile Salts

 

 

 

• The bile salts are activated by ATP and coenzyme A forming their CoA derivatives, which can form conjugates with either glycine or taurine.
• Glycine forms an amide with the carboxyl group of a bile salt, forming glycocholic acid or glycochenocholic acid.
• The bile salts that are conjugates of glycine have pK of approximately 4.
• These bile salts are more completely ionized at pH 6 in the gut lumen and served as better detergents
• Taurine, which is derived from cysteine, forms an amide with the carboxyl group of a bile acid.
• The bile salts of taurine have a pH of approximately 2. They ionize very readily in the gut and are the best detergents among the bile salts.

 

 

 

 

 

 

 

 

 

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