Bile acid synthesis

Bile Acid Synthesis

Q. 1

Secondary bile acids are synthesised by:

 A

Liver

 B

Intestine

 C

Both liver and intestine

 D

Pancreas

Q. 1

Secondary bile acids are synthesised by:

 A

Liver

 B

Intestine

 C

Both liver and intestine

 D

Pancreas

Ans. B

Explanation:

Bile acids are synthesized in the liver.

Bile acids are divided into primary or secondary: Primary bile acids are synthesized in the liver and secondary bile acids are synthesized from primary bile acids in the intestine by colonic bacteria. 
Ref: Harrison, E-18, P-2461.

Q. 2

The four major bile acids found in humans are synthesized from:

 A

Cholesterol

 B

Amino acids

 C

Bilirubin

 D

Protein

Q. 2

The four major bile acids found in humans are synthesized from:

 A

Cholesterol

 B

Amino acids

 C

Bilirubin

 D

Protein

Ans. A

Explanation:

When considering bile as a digestive secretion, it is the bile acids that represent the most important components. They are synthesized from cholesterol and secreted into the bile conjugated to glycine or taurine, a derivative of cysteine. 

Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 25. Overview of Gastrointestinal Function & Regulation. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong’s Review of Medical Physiology, 24e.

Q. 3

Bile acid synthesized in liver (primary bile acids) is:

 A

Lithocholic acid

 B

Cholic acid

 C

Deoxycholic acid

 D

All of the above

Q. 3

Bile acid synthesized in liver (primary bile acids) is:

 A

Lithocholic acid

 B

Cholic acid

 C

Deoxycholic acid

 D

All of the above

Ans. B

Explanation:

 

Bile acids

  • They are steroid acids found predominantly in the bile.
  • Bile acid refers to the protonated (-COOH) form.
  • ile salt refers to the deprotonated or ionized (-COO-) form.
  • Bile salts are bile acids compounded with a cation, usually sodium.
  • The salts of taurocholic acid and glycocholic acid (derivatives of cholic acid) represent approximately eighty percent of all bile salts.
  • The two primary bile acids are cholic acid, and chenodeoxycholic acid.
  • Bile acids, glycine and taurine conjugates, and 7-alpha-dehydroxylated derivatives (deoxycholic acid and lithocholic acid) are all found in intestinal bile.
  • The increase in bile flow is exhibited with an increased secretion of bile acids.
  • e main function of bile acid is to facilitate the formation of micelles, which promotes processing of dietary fat
  • Bile acids are made in the liver by the cytochrome P450-mediated oxidation of cholesterol.
  • They are conjugated with taurine or the amino acid glycine, or with a sulfate or a glucuronide, and are then stored in the gallbladder, which concentrates the salts by removing the water.
  • Rate limiting step is the addition of a hydroxyl group on position 7 of the steroid nucleus by the enzyme cholesterol 7 alpha-hydroxylase.
  • Upon eating a meal, the contents of the gallbladder are secreted into the intestine, where bile acids serve the purpose of emulsifying dietary fats.
  • Bile acids serve other functions, including eliminating cholesterl from the body, drivin the ow of bile to eliminabte catabolites from the liver, emulsifying lipids and fat soluble vitamins in the intestine g to form micelles thatt. can be transported via the lacteal system, and aiding in the reduction of the bacteria flora found in the smal intes ine a biliary tract.

Conjugated bile acids are more efficient at emulsifying fats because at intestinal pH, they are more ionized than unconjugated bile acs.

  • The body produces about 800 mg of cholesterol per day and about half of that is used for bile acid synthesis.
  • In total about 20-30 grams of bile acids are secreted into the intestine daily.

About 90% of excreted bile acids are reabsorbed by active transport in theileum and recycled in what is referred to as the enterohepatic circulation which moves the bile salts from the intestinal system back to the liver and the gallbladder.


Q. 4

Bile acids are reabsorbed from ‑

 A

Duodenum

 B

Proximal jejunum

 C

Distal jejunum

 D

Ileum

Q. 4

Bile acids are reabsorbed from ‑

 A

Duodenum

 B

Proximal jejunum

 C

Distal jejunum

 D

Ileum

Ans. D

Explanation:

Ans. is ‘d’ i.e., Ileum

Quiz In Between


Q. 5

Bile acids are synthesized from ‑

 A

Heme

 B

Cholesterol

 C

Ribulose

 D

Arachidonic acid

Q. 5

Bile acids are synthesized from ‑

 A

Heme

 B

Cholesterol

 C

Ribulose

 D

Arachidonic acid

Ans. B

Explanation:

Ans. is ‘b’ i.e., Cholesterol

  • Primary bile acids are cholic acid and chenodeoxycholic acid, which are synthesized from cholesterol in liver.
  • In the intestine some of the primary bile acids are converted into secondary bile acids, i.e., deoxycholic acid (formed from cholic acid) and lithocholic acid (derived from chenodexoxycholic acid).
  • Glycine and taurine conjugates of these bile acids are called as bile salts.
  • For example, cholic acid is a bile acid, and its glycine conjugate (glycocholic acid) is a bile salt.
  • Bile salts help in digestion and absorption of fat by emulsification and micelles formation.
  • Bile salts act as detergents, i.e., they have surface tension lowering action.
  • Detergent action is due to amphipathic nature of bile salts (Note : Amphipathic molecules are molecules that contain both hydrophobic non-polar as well as hydrophilic-polar ends).



Q. 6

Rate limiting enzyme in bile acid synthesis ‑

 A

Desmolase

 B

21 α-hydroxylase

 C

7α-hydroxylase

 D

12α-hydroxylase

Q. 6

Rate limiting enzyme in bile acid synthesis ‑

 A

Desmolase

 B

21 α-hydroxylase

 C

7α-hydroxylase

 D

12α-hydroxylase

Ans. C

Explanation:

Ans. is ‘c’ i.e., 7α-hydroxylase

About half of the cholesterol in the body is ultimately metabolized to bile acids.

The primary bile acids are synthesized from cholesterol in liver. These are cholic acid and chenodeoxycholic acid.

Rate limiting enzyme in primary bile acids synthesis is 7α – hydroxylase (cholesterol 7α – hydroxylase).

This enzyme is inhibited by bile acids and induced by cholesterol.

Thyroid hormones induce transcription of 7a-hydroxylase, thus in patients with hypothyroidism plasma cholesterol tends to rise (because of inhibition of 7α-hydroxylase which in turn inhibits conversion of cholesterol to bile acids).


Q. 7

Which of the following is/are bile acids?

 A

Cholic acid

 B

Lithocholic acid

 C

Deoxycholic acid

 D

All of the above

Q. 7

Which of the following is/are bile acids?

 A

Cholic acid

 B

Lithocholic acid

 C

Deoxycholic acid

 D

All of the above

Ans. D

Explanation:

 

Primary bile acids are cholic acid and chenodeoxycholic acid, which are synthesized from cholesterol in liver. In the intestine some of the primary bile acids are converted into secondary bile acids, i.e., deoxycholic acid (formed from cholic acid) and lithocholic acid (derived from chenodexoxycholic acid).

  • Glycine and taurine conjugates of these bile acids are called as bile salts. For example, cholic acid is a bile acid, and its glycine conjugate (glycocholic acid) is a bile salt.
  • Bile salts = Sodium or potassium + Amino acid (glycine or taurine) + Bile acids (Cholic acid or chenodeoxycholic acid)
  • So, Bile salts are : –
  • Sodium + glycine + cholic acid = Sodium-glyco-cholic acid (sodium-glyco-cholate)
  • Sodium + taurine + cholic acid = Sodium-tauro-cholic acid (Sodium-tauro-cholate)
  • Sodium + glycine + chenodeoxycholic aicd = Sodium-glyco-chenodeoxycholate
  • Sodium + taurine + chenodeoxycholic acid = Sodium-tauro-chenodeoxycholate o Similarly potassiun bile salts are potassium-glycocholate, potassium-taurocholate, potassium-glyco­chenodexoxycholate, and potassium-tauro-chenodexoxycholate.

Q. 8

Primary bile acid is‑

 A

Deoxycholic acid

 B

Lithocholic acid

 C

Chenodeoxycholic acid

 D

None

Q. 8

Primary bile acid is‑

 A

Deoxycholic acid

 B

Lithocholic acid

 C

Chenodeoxycholic acid

 D

None

Ans. C

Explanation:

Ans. is ‘c’ i.e., Chenodeoxycholic acid 

  • Primary bile acids are cholic acid and chenodeoxycholic acid, which are synthesized from cholesterol in liver. In the intestine some of the primary bile acids are converted into secondary bile acids, i.e., deoxycholic acid (formed from cholic acid) and lithocholic acid (derived from chenodexoxycholic acid).
  • Glycine and taurine conjugates of these bile acids are called as bile salts. For example, cholic acid is a bile acid, and its glycine conjugate (glycocholic acid) is a bile salt.

Quiz In Between



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