Bile acids and salts synthesis Flashcards
The function of bile
helps to emulsify and breakdown fats that we ingest
Bile contains …………… and ……………… compounds
organic and inorganic
What makes up the inorganic part of bile and what’s the percentage?
water, 95%
The major organic component of bile is …….
bile salts
The 5 organic compounds in bile salt are
bile acids lecithin bilirubin fatty acids cholesterol
Difference between bile salts and bile acids?
bile salts are conjugated versions of bile acids
Where is bile stored in between meals?
in the gall bladder
What kind of compound is bile acids and what do they contain?
organic and contain 24C hydroxyl group methyl group carbonyl group
In what form do bile acids exist in physiological pH
partially dissociated
Bile acids are amphipathic, this means
they have both hydrophobic and hydrophilic properties
Bile acids are synthesized from
cholesterol
What are the 2 major primary bile acids?
cholic acid
chenodeoxycholic acid
The 3 main processes that convert cholesterol to bile acids
- shortening of hydrocarbon chain by 3C atoms
- Double bonds are reduced
- hydroxyl groups with carbonyl groups are added
Which enzyme catalyzes the rate-limiting step of bile synthesis and where is it found?
cholesterol 7α hydroxylase and it is an ER bound enzyme
The function of cholesterol 7α hydroxylase
adds a hydroxyl group to C7
The function of cholesterol 7α hydroxylase
adds a hydroxyl group to C7
What activates the upregulation of cholesterol 7α hydroxylase?
high cellular levels of cholesterol
What activates the upregulation of cholesterol 7α hydroxylase?
high cellular of cholesterol
What activates the down regulation of cholesterol 7α hydroxylase?
high levels of primary bile acids
By what mechanism does the down regulation of cholesterol 7α hydroxylase activity occur?
negative feedback
cholesterol 7α hydroxylase is only found in
hepatocytes
Liver cells can only release bile acids when
they have been conjugated to bile salts
How can bile acids be transported?
via transport molecules like albumin to travel in the blood
How are bile salts formed?
by the conjugation of bile acids with glycine or taurine at the carboxyl group
What does conjugation of bile acids do to their pKa value and ionization?
lowers the pKa value and allows them to exist in their fully ionized form, making them able to dissolve in aqueous solution
What makes bile salts better emulsifying agents than bile acids?
conjugation
Where does conjugation of bile acids occur?
liver peroxisomes
What are the products of conjugation of primary bile acids?
taurine + cholic/chenodeoxycholic acid = taurocholic/chenodeoxycholic acid
glycine + cholic/chenodeoxycholic acid = glycocholic/chenodeoxycholic acid
The ratio of glycine to taurine in the body is
3:1
What role do bile salts play in cholesterol excretion?
they provide the major method of cholesterol excretion
What can bacteria in the duodenum do to bile salts?
they can convert bile salts back to primary bile acids by removing glycine or taurine
they can also convert primary bile acids to secondary bile acids
By what methods are secondary bile acids formed by bacteria in the duodenum?
deconjugation and 7α dehydroxylation
Name the 2 secondary bile acids and how they are formed?
Deoxycholic acid formed from glycocholic acid
Lithocholic acid formed from tauro-/glyco- chenodeoxycholic acid
What happens to the mixture of bile salts, primary bile acids and secondary bile acids in the ileum?
95% of this mixture is actively reabsorbed into the liver
5% is excreted through faeces
What do mediations like cholestyramine do to bile acids?
they bind and sequester (isolate) them and prevent their reabsorption
What is the importance dietary fibers binding to bile acids?
to promote cholesterol excretion
What is the importance dietary fibers binding to bile acids?
to promote cholesterol excretion
What is the principal sterol in faeces and how is it formed?
coprostanol and it is formed from cholesterol by the bacteria in the lower intestine
What prevents the precipitation of cholesterol in the gall bladder?
bile acids and phospholipids
Bile acids facilitate the intestinal absorption of
fat-soluble vitamins
What is enterohepatic circulation?
when 98-99% of primary and secondary bile acids are returned to the liver via portal circulation
Litholic acid is not absorbed to a large extent because
of its insolubilty
