Biochemistry: Heme catabolism and Bile salts

Question 1

What causes jaundice or scleral icterus?

Answer 1

Accumulation of elevated bilirubin in the skin or sclera, imparting a yellow color to the tissues

Question 2

Inherited disorders of bilirubin metabolism

Answer 2

Hyperbilirubinemia

Question 3

How much of heme catabolism comes from senescent erythrocytes? How much comes from turnover of RBCs?

Answer 3

80% - senescent erythrocytes

20% immature RBCs

Question 4

What is the average lifespan of a RBC and what happens when it reaches this?

Answer 4

120 days – it is taken up by macrophages of the reticuloendothelial system of the liver and spleen

Question 5

Where does heme ring opening occur?

Answer 5

In the macrophage

Question 6

Which enzyme selectively cleaves the ferroprotoporphyrin IX ring at the alpha-methelene bridges? What is required for this step?

Answer 6

HO-1 (heme oxygenase)

Needs electrons from NADPH cytochrome P450 oxidoreductase

Question 7

What is the only known reaction in human tissues that produces CO?

Answer 7

The second step of heme catabolism where there is non-enzymatic oxidation by molecular oxygen

Question 8

What results in the green pigment of biliverdin?

Answer 8

The release of iron after addition of electrons

Question 9

Fe2+/Fe3+ and CO are toxic in their free form. What do they bind to avoid toxicity?

Answer 9

Fe2+/Fe3+ binds ferritin

CO binds hemoglobin

Question 10

What is the enzyme that converts biliverdin to bilirubin? Where does this occur?

Answer 10

Biliverdin reductase
Occurs in the macrophage

*Can use either NADH or NADPH for activity

Question 11

How does bilirubin differ from biliverdin?

Answer 11

Bilirubin is much less polar than biliverdin and it can cross membranes more readily

Question 12

Why is bilirubin thought to be particularly important during the neonatal period?

Answer 12

It is also an antioxidant and other antioxidants are relatively low during the neonatal period

Question 13

What protein does bilirubin bind to travel in the circulation?

Answer 13

Albumin – helps transport bilirubin from primary site of production (red pulp macrophages of the spleen and Kupffer cells of the liver) to its site of excretion (liver)

Question 14

Where does uptake, storage, conjugation and excretion of bilirubin occur?

Answer 14

In the liver

Question 15

How is bilirubin kept in solution once it is inside the hepatocyte?

Answer 15

Through interactions with ligandins – this prevents bilirubin from leaving the liver and going back into the circulation and it is a form of temporary storage for the hepatocyte.

Question 16

What is the enzyme that catalyzes the addition of glucoronic acid to bilirubin? Why does this happen?

Answer 16

UGT1A1 = uridine diphosphate glucuronosyltransferase

This happens to make bilirubin more polar so that it can be excreted across the bile canaliculus

Question 17

Where is UGT1A1 located?

Answer 17

In the ER of the hepatocyte

Question 18

What ATP-dependent transporter is involved in bilirubin excretion?

Answer 18

MOAT = multiorganic anion transporter (this is an energy dependent process)

Question 19

What happens when conjugated bilirubin reaches the intestinal tract?

Answer 19

It is NOT substantially reabsorbed – instead it is degraded by intestinal bacteria into urobilinogen products (which are present in the deconjugated state – not sure when deconjugation happens)

Urobilinogen products = stercobilin (feces) and urobilin (urine)

Question 20

What is the name of the assay that allows for direct quantification of serum bilirubin?

Answer 20

van den Bergh assay

Question 21

How does the van den Bergh assay categorize bilirubin into direct and indirect reacting species?

Answer 21

Only the water soluble, conjugated bilirubin reacts rapidly in this assay – this will give you a value for direct bilirubin

The unconjugated, water insoluble bilirubin reacts slowly and is not detected in short incubations.

When this assay is conducted in methanol, both forms of bilirubin react rapidly and this will give you a value for total bilirubin. Therefore, indirect bilirubin can be calculated.

Total - Direct (conjugated) = Indirect (unconjugated)

Question 22

What are three factors that may contribute to neonatal jaundice in the first 5 days of life?

Answer 22

1. Low activity of UGT1A1
2. Decreased excretory capacity of hepatocytes
3. Increased bilirubin production secondary to accelerated destruction of fetal erythrocytes

Question 23

What is the specific form of brain damage due to hyperbilirubinemia?

Answer 23

Kernicteris – causes athetoid (writhing) cerebral palsy and often hearing loss

Question 24

What is the therapy for neonatal jaundice?

Answer 24

Phototherapy – bilirubin light in the blue-green spectrum changes the configuration of bilirubin that can be excreted in bile without conjugation

Question 25

Absent expression of UGT1A1

Answer 25

Crigler-Najjar syndrome type I

Question 26

Markedly reduced (<20%) expression of UGT1A1

Answer 26

Crigler-Najjar syndrome type II

Question 27

Reduced (~30%) expression of UGT1A1

Answer 27

Gilbert Syndrome

Question 28

Defect in the organic ion transport (MOAT defect) – inability of hepatocytes to secrete conjugated bilirubin into the bile canaliculi after it has been formed –> conjugated bilirubin returns to the blood
Autosomal recessive
Dark pigment to the liver

Answer 28

Dubin-Johnson syndrome

Question 29

What are some other causes of jaundice?

Answer 29

Hemolytic = increased indirect/unconjugated

Obstructive = increased direct/conjugated

Hepatocellular = increased indirect/unconjugated

Question 30

What is the precursor of bile acids?

Answer 30

Cholesterol –bile acids represent a major mechanism by which cholesterol is excreted

Question 31

What is the function of bile acids?

Answer 31

They act as emulsifying agents to prepare dietary triglycerides for hydrolysis by pancreatic lipase and they facilitate absorption of fat-soluble vitamins form the intestine

Question 32

What are the most abundant bile acids?

Answer 32

Derivatives of cholic acid

Question 33

What are the primary bile acids?

Answer 33

Primary bile acids are synthesized in hepatocytes directly from cholesterol = cholic acid & chenodeoxycholic acid

Question 34

What are the secondary bile acids?

Answer 34

Made form primary bile acids by bacteria in the gut via dehydroxylation reactions = deoxycholic acid and lithocholic acid

Question 35

What is conjugation of bile acids?

Answer 35

It forms bile salts by adding a glycine or taurine – this makes them have a lower pKa value and more soluble

Question 36

Why are bile acids more than 95% efficient?

Answer 36

They are reabsorbed through enterohepatic circulation and returned to the liver for recycling (12-32g per day)

Question 37

What is familial hypercholesterolemia?

Answer 37

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 is autosomal dominant

**Defect in the gene encoding the LDL receptor

Question 38

How common is FH?

Answer 38

Heterozygotes number 1 in 500 making it among the most common inborn error in metabolism

Question 39

How does the LDL receptor limit LDL production?

Answer 39

It enhances the removal of the precursor to LDL, IDL, from the circulation

Question 40

What are some treatments for FH?

Answer 40

1. Bile acid binding resins: prevent cholesterol absorption in the ileum and increases fecal excretion of bile acids to increase the conversion of cholesterol to bile acids in the liver –> liver responds by increasing the production of LDL receptors
2. HMG-CoA reductase inhibitors (statins)
3. Diet low in cholesterol and fats