Heme Catabolism & Bile Salts

Question 1

What two things must be dealt with during heme catabolism?

Answer 1

1. Handling the hydrophobic products of porphyrin ring cleavage
2. Retention, safe mobilization, and re-utilizatoin of iron

Question 2

___% of the total iron is present as heme iron

Answer 2

70%

Question 3

Ferrous iron (Fe²⁺) is an extremely reactive molecule, generating _____ ______ _____

Answer 3

Reactive oxygen species

Question 4

Extravascular hemolysis primarily takes place within the macrophages of what two tissues?

Answer 4

Red pulp macrophages of the spleen and Kupffer cells in the liver

Question 5

What types of macrophages scavenge hemoglobin-haptoglobin complexes in intravascular hemolysis?

Answer 5

CD163(+) macrophages

Question 6

Haptoglobin binds ______ ________

Answer 6

free hemoglobin

Question 7

Where is Haptoglobin produced and secreted?

Answer 7

Haptoglobin is produced mostly in the liver by hepatocytes and secreted into the blood circulation

Question 8

Describe how haptoglobin functions as an antioxidant

Answer 8

Following its release into plasma, hemoglobin dissociates into αß dimers - Oxy-hb dimers are sequestered by haptoglobin preventing both release of free heme and the oxidative damage of heme iron

Question 9

How does the CD163 receptor take up the haptoglobin-hemoglobin complex?

Answer 9

Receptor mediated endocytosis

Question 10

What happens when haptoglobin’s buffering capacity is overwhelmed?

Answer 10

Hb undergoes a rapid conversion to metHb, liberating heme

Question 11

What happens when metHb liberates the free heme?

Answer 11

Free heme then binds to albumin and other plasma components including lipoproteins and is subsequently transferred to hemopexin

Question 12

Describe Hemopexin

• Type of protein:
• Production:
• When do levels increase?

Answer 12

• Hemopexin is an acute phase glycoprotein that binds free heme
• It is produced mostly in the liver by hepatocytes and secreted into blood circulation
• Levels markedly increase during acute phase of inflammation in response to inflammatory cytokines and during heme overload

Question 13

What is the CD91 Scavenger receptor?

Answer 13

binds the hemopexin-heme complex

Question 14

What happens when the heme complex is engulfed by a macrophage?

Answer 14

1. The globin protein is degraded to amino acids in the lysosome
2. Heme is transferred to the cytosol where it is catabolized by heme oxygenase-1 (HO1)

Question 15

What causes Jaundice or Icterus

Answer 15

Jaundice or Icterus results from accumulation of elevated bilirubin in the skin and sclera, imparting a yellow color to these tissues

Question 16

What is bilirubin?

Answer 16

• Bilirubin, an orange pigment derived from the degradation of heme proteins
• potentially toxic waste product that is generally harmless because of binding to serum albumin

Question 17

What is the mechanism of heme ring opening in the macrophage? (2 steps)

Answer 17

1. The Ferroprotoporphyrin IX ring is selectively cleaved at the α-methene bridge, catalyzed by heme oxygenase-1 and requires electrons from NADPH cytochrome P450 oxidoreductase (CYPOR)
2. Nonenzymatic oxidation by molecular oxygen with the elimination of CO^- this leads to the release of iron after addition of electrons and the resulting green pigment is biliverdin

Question 18

What converts bivileridin to bilirubin?

Answer 18

Biliverdin reductase

Question 19

What are some differences between biliverdin and bilirubin?

Answer 19

• Bilirubin is less polar than biliverdin and crosses membranes more readily
• Bilrubin is an antioxidant and appears to be particularly important during the neonatal period

Question 20

How do bilirubin and albumin interact?

Answer 20

• Albumin binding plays a critical role in the desposition of bilirubin in the body
  
  • keeps bilirubin in solution
  • transports it from its primary sites of production to its primary site of excretion

Question 21

What happens to bilirubin once inside the hepatocyte?

Answer 21

1. bilirubin is rapidly removed from the circulation by the liver
   
   • dissociates from albumin before entering the hepatocyte
2. ligandins keep bilirubin in solution
   
   • inhibits the efflux of bilirubin back into the circulation
   • temporary storage
3. conversion to polar conjugates by esterification of the propionic acid carboxyl groups
4. Glucuronic acid is the major conjugating group
   
   • catalyzed uridine diphosphate glucuronosyltransferase (UGT1A1)
5. ​Either one or two glucuronic acid moieties of UDP-glucuronic acid are transferred onto bilirubin yielding the mono- (BMG) or diglucuronide (BDG) species

Question 22

• What is essential for excretion of bilirubin?
• How is this accomplished?

Answer 22

• Conjugation is essential for bilirubin excretion
• Mechanism:
  
  • Energy-dependent and shared with other organic anions, except bile salts
  • ATP-dependent multiorganic anion transporter (MOAT) has been identified in canalicular membranes and is involved in bilirubin excretion

Question 23

Fate of bilirubin in the gastrointestinal tract

Answer 23

• Bilirubin reaches the intestinal tract mainly conjugated and is not substantially readsorbed
• Rather, bilirubin is degraded by intestinal bacteria into a series of urobilinogen products
• Urobilinogens are present in the deconjugated state (i.e., lost glucuronic acid)
  
  • Urobilinogen has anti-oxidant properties
• Urobilinogen is oxidized to stercobilin
  
  • Major pigment of feces

Question 24

The hepatic processing of bilirubin involves four distinct, but interrelated stages:

Answer 24

1. uptake from the circulation
2. intracellular binding and storage
3. conjugation
4. biliary excretion

Question 25

How can hyperbilirubinemia be broadly divided? (2)

Answer 25

1. Conjugated
2. Unconjugated

Question 26

Quantification of bilirubin:

1. Indirect quantification
2. Direct quantification

Answer 26

1. Indirect quantification
   
   • intensity of yellow discoloration of the skin (total bilirubin)
2. Direct quantification (van den Bergh assay)
   
   • Only the water soluble, conjugated bilirubin reacts rapidly in this assay, which yields a value for direct bilirubin
   • Indirect bilirubin, corresponding to the unconjugated form, is simply calculated by the difference between total and direct

Question 27

• When does neonatal jaundice occur?
• What causes neonatal jaundice?

Answer 27

• About half become clinically jaundiced during the first 5 days of life
• Combination of three factors:
  
  1. Low activity of UGT1A1
  2. Decrease excretory capacity of hepatocytes
  3. Increased bilirubin production secondary to accelerated destruction of fetal erythrocytes

Question 28

Neonatal jaundice

• Serum bilirubin is predominantly _________
• What can happen if neonatal janudice is untreated?
• What is Kernicterus?

Answer 28

• Serum bilirubin is predominantly unconjugated
• If untreated, high bilirubin levels (resulting in intense jaundice) can damage regions of the brain, such as the basal ganglia (yellow discoloration), involved in controlling muscle movement
• Kernicterus is a specific form of brain damage (“bilirubin encephalopathy”) due to hyperbilirubinemia
  
  • causing athetoid (writhing) cerebral palsy
  • often hearing loss

Question 29

How is neonatal jaundice treated?

Answer 29

Phototherapy is the most common treatment modality.

• Exposure of bilirubin to light in the blue-green spectrum
• Changes its configuration to an isomer that can be excreted in bile without conjugation

Question 30

List the inherited uncojugated hyperbilirubinemias (3):

• How is Hepatic bilirubin UGT affected?

Answer 30

• Crigler-Najjar Syndrome Type I
  
  • Hepatic bilirubin UGT ⇒ absent
• Crigler-Najjar Syndrome Type II
  
  • Hepatic bilirubin UGT ⇒ markedly reduced
• Gilbert’s Syndrome
  
  • Hepatic bilirubin UGT ⇒ reduced

Question 31

Conjugated Hyperbilirubinemia:

• Defects in ….
• List the syndromes:
  
  • Appearance of liver
  • Histology of Liver

Answer 31

• Defects of bilirubin secretion
  
  • ​Inability of hepatocytes to secrete conjugated bilirubin into the bile canaliculi after it has been formed.
  • Conjugated bilirubin returns to the blood
• Dubin-Johnson: organic ion transport (MOAT defect)
  
  • Liver appearance: grossly black
  • Histology: dark pigments; predominately centrilobular
• Rotor syndrome
  
  • ​Liver appearance: normal
  • Histology: normal; no increase in pigmentation

Question 32

Other causes of Jaundice:

1. Hemolytic (direct or indirect?)
   
   • Pathogenesis
2. **Obstructive **(direct or indirect?)
   
   • Pathogenesis​
3. Hepatocellular (direct or indirect?)
   
   • Pathogenesis

Answer 32

1. Hemolytic (increased indirect/unconjugated)
   
   • Excessive erythrocyte destruction
   • Formation of bilirubin in amounts exceeding the conjugating ability of the liver and hence its excretion into the bile
   • Free bilirubin increases in plasma as a result
2. Obstructive (increased direct/conjugated)
   
   • Caused by partial or complete blocking of the bile ducts
   • Conjugated bilirubin is prevented from being excreted into the intestine
   • Increased amounts in the plasma
3. Hepatocellular (increased indirect/unconjugated)
   
   • Damage to the liver by toxins, poisons, cardiac failure, or acute or chronic disease
   • Impairs the liver’s capacity to conjugate circulating bilirubin and hence excrete it

Question 33

What two enzymatic processes are involved in cholesterol synthesis?

Answer 33

1. Farnesylation
2. Glycosylation

Question 34

Cholesterol can be derived from the ___ or _________ de novo in virtually all cells

Answer 34

Cholesterol can be derived from the diet or synthesized de novo in virtually all cells

Question 35

Bile acids

• Synthesis:
• Secretion:
• Storage:
• Function:

Answer 35

• Synthesis: synthesized from cholesterol in the liver
• Secretion: secreted into bile canaliculi
• **Storage: **
  
  • carried to the gallbladder for storage
  • excreted from the small intestine
• Function:
  
  • emulsifying agents to prepare dietary triglycerides for hydrolysis by pancreatic lipase
  • facilitate absorption of fat-soluble vitamins from the intestine

Question 36

• Most abundant bile acids are derivatives of _____ ____
• How are bile acids a major mechanism by which cholesterol is excreted?

Answer 36

• Most abundant bile acids are derivatives of cholic acid
• Carbon skeleton of cholesterol is not degraded (oxidized to CO₂ and H₂O) in humans but is excreted in bile as free cholesterol and as bile acids

Question 37

• List the primary bile acids:
  
  • Synthesis:
• List the primary bile salts:
  
  • Synthesis:

Answer 37

• Primary bile acids: **cholic acid & chenodeoxycholic acid **
  
  • synthesized in hepatocytes directly from cholesterol
• Primary bile acids: **deoxycholic acid & lithocholic acid **
  
  • converted to secondary bile acids by bacteria in the gut via dehydroxylation reactions

Question 38

• Where are primary and secondary bile acids absorbed?
• Why is conjugation important for bile acids?

Answer 38

• Primary and secondary bile acids are reabsorbed by the intestine (lower ileum) into portal blood, and taken up by hepatocytes where they are conjugated to glycine or taurine, forming bile salts.
• Conjugation is important as it converts the bile acids into molecules (i.e., bile salts) with a lower pKa value
  
  • renders them more soluble in the small intestine

Question 39

Why is enterohepatic circulation important for bile acid regulation?

Answer 39

The capacity of the liver to produce bile acids is insufficient to meet physiological demands, so the body relies on an efficient enterohepatic circulation that carries bile acids from the intestine back to the liver

Question 40

**Familial Hypercholesterolemia (FH): **

• Clinical Characteristics
• Genetic Defect

Answer 40

• Clinical Characteristics
  
  1. elevated concentration of LDL in the plasma
  2. deposition of LDL-derived cholesterol in tendons and skin (xanthomas) and in arteries (atheromas)
  3. inheritance as an autosomal dominant trait with a gene dosage effect (homozygotes are more severely affected than heterozygotes)
• Genetic Defect:
  
  • ​mutation in the gene encoding the LDL receptor

Question 41

• What is the incidence of heterozygotes with FH?
• What is the treatment for FH?

Answer 41

• Heterozygotes number 1 in 500 persons
• Treatment is directed at lowering the plasma level of LDL

Question 42

• What type of endocytosis is exhibted by the LDL receptor?
• What is the function of the LDL receptor?

Answer 42

• receptor-mediated endocytosis
• help maintain a constant level of cholesterol within the cell in the face of fluctuations in the supply of lipoproteins

Question 43

What is the dual role of the LDL receptor?

Answer 43

1. Limits LDL production by enhancing the removal of the precursor, IDL (B-100, apo E ⇒ higher affinity for LDL receptor), from the circulation.
2. LDL receptor enhances LDL degradation by mediating cellular uptake of LDL (apo B-100)

Question 44

What are the treatment options for FH heterozygotes?

Answer 44

1. Bile acid binding resins
   
   • bind biles acids in the intestinal lumen, preventing their absorption from the ileum.
   • Liver responds to cholesterol deficiency by increasing production of LDL receptors
   • alone, can decrease LDL cholesterol levels by 10-20%
2. HMG-CoA reductase inhibitors (statins)
   
   • combined with bile acid binding resins, can decrease LDL cholesterol by >60%
   • drug of choice
3. Diet low in cholesterol and fats
   
   • alone, can decrease LDL cholesterol levels by 10-20%