Wk 2 - Molecular Cell Biology: Bile

Question 1

List the two main functions of bile.

Answer 1

1. To aid fat digestion
   
   • Emulsification
   • Absorption
2. For the excretion or homeostatic regulation of important products of metabolism, e.g.
   
   • Bilirubin
   • Cholesterol

Question 2

What is bile?

Answer 2

• A bitter, alkaline, brownish-yellow or greenish-yellow fluid
  
  • Secreted by hepatocytes in the liver
  • Stored** in the **gallbladder
  • Discharged into the duodenum (via the biliary tract)
• A complex fluid containing water, electrolytes and a battery of organic molecules
• These include bile acids, cholesterol, phospholipids, proteins and bilirubin

Question 3

Describe the composition of bile.

Answer 3

• Mainly water (97%)
• Bile acids (0.7%) –> account for 50% of total solutes in bile (of dry mass)
• Bile pigments (0.1%)
• Cholesterol (0.06%)
• and other things make up remaining %

Question 4

What are bile acids formed from?

Answer 4

Bile acids are formed from cholesterol - Hepatic synthesis of bile acids accounts for the majority of cholesterol breakdown in the body.

Question 5

The _____ is the only site of cholesterol excretion.

Answer 5

The liver is the only site of cholesterol excretion.

Question 6

Where does cholesterol come from?

Answer 6

• All cells can make cholesterol, but the liver is the major site of its biosynthesis
• Dietary cholesterol is taken up by the liver as chylomicron remnants

Question 7

Cholesterol is a _____, a type of steroid and is the precursor of many _____.

Answer 7

Cholesterol is a sterol, a type of steroid and is the precursor of many hormones.

Question 8

Describe the metabolism process of cholesterol.

Answer 8

• Cholesterol is metabolised to bile acids in the liver
• This occurs in hepatocytes
• P450 enzymes modify cholesterol in the hepatocyte
• Cholesterol 7 a-hydroxylase (CYP7A1) is the name of the enzyme that adds the hydroxyl group to carbon 7 of cholesterol during bile acid synthesis. This step is the rate limiting step in bile acid production
• CYP7A1 expression is induced by cholesterol and repressed by bile salts

Question 9

What is the rate-limiting step in bile acid production?

Answer 9

Cholesterol 7 a-hydroxylase (CYP7A1) is the name of the enzyme that adds the hydroxyl group to carbon 7 of cholesterol during bile acid synthesis.

This is the rate-limiting step. CYP7A1 is the rate-limiting enzyme.

Question 10

Bile acids have a common structure and differ only in…

Answer 10

Differ only in the number and position of additional hydroxyl groups

Question 11

Prior to secretion, primary bile acids are ____ through the carboxyl group forming ____ linkages to either ____ or ____ – usually the products are called bile ____ but can also be called ‘bile ____’.

Answer 11

Prior to secretion, primary bile acids are conjugated through the carboxyl group forming amide linkages to either glycine or taurine – usually the products are called bile salts but can also be called ‘bile acids’.

Question 12

How are cholesterol and bile acids secreted from hepatocytes into bile?

Answer 12

Bile uses the bile salt export pump (BSEP) - ABCB11.

Liver cells produce bile acids/salts and secrete them into the biliary canaliculi, which drain into the interlobular bile duct (leading to the bile duct).

Question 13

What is the ‘bile salt export pump (BSEP) - ABCB11’?

Answer 13

The bile salt export pump (BSEP) which mainly located at the canalicular membrane of the hepatocyte is an ATP-binding cassette (ABC) transporter encoded by the ABCB11 gene. BSEP mediates the canalicular excretion of numerous bile salt into bile, which is regarded as the rate-controlling step of the vectorial transport of bile acids through hepatocytes.

Question 14

What is the function of bile salt export pump (BSEP)?

Answer 14

BSEP mediates the canalicular excretion of numerous bile salt into bile, which is regarded as the rate-controlling step of the vectorial transport of bile acids through hepatocytes.

Question 15

How are secondary bile acids generated?

Answer 15

• Bacterial enzymes within the intestine act upon a proportion of primary bile acids/salts
• …which hydrolyses the amide link to glycine or taurine and then removes the C7 hydroxyl group
• Secondary bile acids can enter the bloodstream, return to the liver, and undergo conjugation to form other bile acids

Question 16

Explain how primary and secondary bile acids/salts are metabolised in the liver and intestines.

Answer 16

• The liver secretes primary and secondary bile salts into the duodenum.
• Along the proximal and distal ileum, primary bile salts are reabsorbed into the portal circulation.
• After formation by deconjugation, the secondary bile acids are reabsorbed predominantly in the colon.
• From portal circulation, bile salts or acids are reabsorbed into the liver from the blood by an active transport mechanism.
• The bile acids are conjugated before re-secretion.

Bile 15-30g of salts/day; portal circulation 15-30g of salts/day

Faecal excretion of primary and secondary salts is 0.5g/day

Question 17

What enteric hormones regulate bile secretion?

Answer 17

1. CCK (cholecystokinin)
2. Secretin

Question 18

What stimulates CCK secretion?

Answer 18

The most potent stimulus for release of cholecystokinin is the presence of fat in the duodenum.

Question 19

What secretes CCK?

Answer 19

It is secreted by cells of the mucosa of the duodenum and jejunum

Question 20

What is the action of CCK?

Answer 20

Once released, it stimulates contractions of the gallbladder and common bile duct, resulting in delivery of bile into the gut.

Question 21

What stimulates secretin release?

Answer 21

Secretin is released in response to low duodenal pH caused by the presence of hydrochloric acid from the stomach

Question 22

What is the effect of secretin on the gut?

Answer 22

Its effect on the biliary system is to stimulate biliary duct cells to secrete bicarbonate and water, which expands the volume of bile and increases its flow out into the intestine.

Question 23

What is the function of the gallbladder?

Answer 23

Stores & concentrates bile during the fasting state when the flow of bile is lowest

Question 24

What is the maximum volume the gallbladder can hold?

Answer 24

The maximum volume it can hold is 30- 60 mL.

But as bile is concentrated 5-fold in the gallbladder by absorption of water (active transport of Na+) and small electrolytes (all of the organic molecules are retained), but has the capacity to concentrate 20-fold. In this way, the gall bladder can store 12 hours worth, or ~400mL of secreted bile.

Question 25

Why do gallstones form?

Answer 25

Due to cholesterol excess

• Too much cholesterol relative to bile acids and phospholipids, means that the cholesterol becomes insoluble
• Excess cholesterol crystallises
• These effects are compounded in the gallbladder where the bile concentration is highest

Question 26

What is the main cause of gallstones?

Answer 26

The main cause of gall stones is a decrease of bile acids in the bile, which may be caused by:

• Malabsorption of bile acids from the intestine (ileal disease?)
• Obstruction of biliary tract
• Hepatic dysfunction
• Genetic susceptibility
• Excessive cholesterol (?)

Question 27

List the signs and symptoms of gallstones.

Answer 27

Gallstones may cause different signs and symptoms depending on location.

nb cystic relatively benign but in bile duct and duodenal papilla more problematic!

Question 28

What are the main two functions of bile?

Answer 28

• Bile contains bile acids which are critical for digestion and absorption of fats and fat-soluble vitamins in the small intestine
• Many waste products, including bilirubin, are eliminated from the body by secretion into bile and elimination in faeces

Question 29

Bile acids are ____, with detergent properties.

Answer 29

Bile acids are _amphipathic*_, with detergent properties.

*Amphipathic = a molecule containing both polar (water-soluble/hydophilic) and nonpolar (water-soluble/hydrophobic) regions

Question 30

What are micelles?

Answer 30

Micelles are essentially small aggregates (4-8 nm in diameter) of mixed lipids and bile acids suspended within the ingesta.

Question 31

How are micelles formed?

Answer 31

Monoglycerides and fatty acids are liberated through the action of lipase, they retain their association with bile acids and complex with other lipids to form structures called micelles.

Question 32

What are bile pigments?

Answer 32

Bile pigments are any of the colouring materials found in bile and are derived from porphoryins (hemoglobin, cytochromes, myoglobin)

Question 33

What destroys aged RBCs?

Answer 33

Aged red blood cells are naturally destroyed by macrophages which phagocytose (engulf and digest) the cellular debris

Question 34

Where does the phagocytosis of aged RBCs occur?

Answer 34

This occurs primarily in the liver and the spleen (‘reticular endothelial system’)

Question 35

What happens to haemoglobin during metabolism?

Answer 35

• Broken down to heme and globin
• Heme is catabolised

Question 36

Describe the catabolism of heme in the liver.

Answer 36

• About 75% of bilirubin is derived from this heme catabolism (250-350 mg / day)
• The ring structure of the heme is firstly oxidatively cleaved to biliverdin by a Cytochome P450 enzyme (heme oxygenase)
• Biliverdin is reduced to bilirubin by biliverdin reductase

Question 37

What is the byproduct of heme oxidation?

Answer 37

Biliverdin

Question 38

What is the byproduct of biliverdin reduction?

Answer 38

Bilirubin

Question 39

Describe the metabolism of bilirubin.

Answer 39

1. Bilirubin is not water soluble and must be complexed with albumin for transportation to the liver
2. The liver takes up bilirubin and conjuates it with glucuronic acid which makes it more water-soluble
3. Conjugated bilirubin is secreted by hepatocytes into biliary canaliculi
4. Conjugated bilirubin is catabolised by bacteria in the gut first to form urobilinogen
5. This then is converted to stercobilin (which colours feces brown). (without this modification stools may be yellow or green)
6. Urobilinogen and some stercobilin is reabsorbed from the gut and recycled through the liver or kidneys to be excreted as urine

Question 40

Conjugated bilirubin is catabolised by ____ in the gut first to form ____.

Answer 40

Conjugated bilirubin is catabolised by bacteria in the gut first to form urobilinogen.

Question 41

In order for urobilinogen and stercobilin to be excreted, ____ must have been catabolised in the _____ already.

Answer 41

In order for urobilinogen and stercobilin to be excreted, bilirubin must have been catabolised in the gut (liver) already.

ie liver is functioning ok –> conjugation is occuring!

Question 42

Aside from excess cholesterol, what else causes gallstones?

Answer 42

Aside from excess cholesterol. Gall stones may also be caused by excessive bilirubin secretion –> known as ‘pigment gallstones’ (account for 20%)

Question 43

In what cases are pigment gallstones most likely to occur?

Answer 43

Pigment gall stones may occur particularly in haemolytic anaemias, eg sickle cell, and biliary tract infections

Question 44

What are the two types of gallstones?

Answer 44

1. Cholesterol containing stones (80% of gallstones)
2. Pigment stones of calcium bilirubinate ie calcified bilirubin (20% of gallstones)

Question 45

What is normal plasma conc of bilirubin?

Answer 45

The normal plasma concentration of bilirubin is less than 17 micromol/L

Question 46

At what concentration of plasma bilirubin would someone have jaundice?

Answer 46

Concentrations > 50 micromol/L are readily recognised as bilirubin imparts a yellow colour to the skin – jaundice.

Question 47

What are the different types of jaundice? And what causes them?

Answer 47

• Hemolytic jaundice (Pre-hepatic jaundice) - too much bilirubin is produced (caused by excessive lysis of rbc e.g. various anaemias, including sickle cell or G6PD deficiency)
• Hepatocellular jaundice (intrahepatic jaundice) - disease induced cell necrosis reduces the liver’s ability to metabolise (conjugate) and/or excrete bilirubin into bile (caused by cirrhosis, hepatitis, liver tumours, less often by mutations in metabolic enzymes)
• Obstructive jaundice (post hepatic jaundice) - blockage in the bile duct (gall stone, cyst, tumour)

Question 48

Conjugated bilirubin is increased in ____ (prehpetaic/intrahepatic/posthepatic) and ____ jaundice (prehpetaic/intrahepatic/posthepatic) but not ____ (prehpetaic/intrahepatic/posthepatic) jaundice.

Answer 48

Conjugated bilirubin is increased in intrahepatic and posthepatic jaundice but not prehepatic jaundice.

Question 49

Urine urobilinogen is present in ____ jaundice and ____ jaundice but not ____ jaundice.

Answer 49

Urine urobilinogen is present in prehepatic jaundice and intrahepatic jaundice but not posthepatic jaundice.