Bilirubin Metabolism & Bile Salts - Dahms

Question 1

How much hemoglobin is synthesized each day to replace that lost by RBC turnover?

Answer 1

6-7g

Question 2

How much of total body iron is present as RBC heme?

Answer 2

70%

Question 3

1. What plasma protein conjugates free Hb in the blood?
2. What is it important for?
3. Under what circumstances could this protein become depleted?

Answer 3

1. Haptoglobin
2. Delivering free Hb to macrophages for catabolism
3. Intravascular hemolysis

Question 4

What cell do the first steps of heme catabolism occur in?

Answer 4

Macrophages, which ingest senescent RBCs and/or Hb+Haptoglobin complexes

Question 5

What steps of heme catabolism occur in macrophages?

Answer 5

1. Heme ring opening
2. Conversion of linear heme to biliverdin
3. Conversion to biliverdin to bilirubin

Question 6

What carries bilirubin from macrophages to the liver?

Answer 6

Albumin

Question 7

Once bilirubin reaches the liver, what catabolic process occurs there?

Answer 7

Bilirubin is conjugated with glucuronic acid & excreted in the bile

Question 8

What processes of heme catabolism occur in the GI tract?

From the GI tract, what two paths can the heme catabolic products take?

Answer 8

Bacteria deconjugate and further convert the conjugated bilirubin into urobilinogen.

Urobilinogen can be:

1. Oxidized by intenstinal bacteria to stercobilin and excreted in the feces
2. Reabsorbed into the blood stream from the intestines

Question 9

How are urobilinogen products excreted from the bloodstream?

Answer 9

The kidney converts urobilinogen products to urobilin, which are excreted in the urine.

Question 10

What is the classic clinical finding of hyperbilirubinemia?

Answer 10

Jaundice (icterus)

Yellow discoloration of skin and sclera due to accumulation of elevated bilirubin

Question 11

Again, what is a normal RBC lifespan?

Senescent RBCs are taken up by what for catabolism? Be specific.

Answer 11

120 days

Macrophages of the reticuloendothelial system of the liver & spleen

Question 12

What percentage of heme catabolism occurs from senescent eryhtrocytes?

What sources make up the remaining percentage?

Answer 12

80% from senescent RBCS

Remaining 20% from immature RBC destruction, cytochrome enzyme turnover, etc.

Question 13

The first step of heme catabolism involves opening of the heme ring.

1. What enzyme catalyzes this opening?
2. What specific bond in heme is cleaved?
3. What is a necessary cofactor for this reaction?

Answer 13

1. HO-1 (Heme oxygenase-1)
2. The alpha-methene bridge
3. electrons from NADPH Cytochrome P450 Oxidoreductase (CYPOR)

Question 14

Once the heme ring is opened, it must be converted to biliverdin.

1. What enzyme catalyzes this reaction?
2. What two by-products are liberated by this reaction?
3. What happens to these by-products?

Answer 14

1. Trick question (sorry) - reaction is believed to be nonenzymatic, catalyzed instead by molecular O₂​
2. CO and Fe^2+/3+
   
   • N.B. Only known rxn in human body to produce CO
3. Both are toxic compounds in their free forms.
   
   • CO is bound by Hb
   • Fe^2+/3+ is bound by ferritin

Question 15

During heme catabolism, biliverdin must be converted to bilirubin.

1. What enzyme catalyzes this reaction?
2. What cofactor is required?

Answer 15

1. Biliverdin reductase
2. NADH or NADPH

Question 16

Compare bilirubin with biliverdin.

Answer 16

Bilirubin is less polar than biliverdin, and so can cross membranes more easily.

Bilirubin also appears to be an important antioxidant during the neonatal period.

Question 17

What is the role of albumin in heme catabolism?

Answer 17

Albumin is important for keeping bilirubin soluble and delivering it from its primary site of production (macrophages) to its primary site of excretion (liver).

Question 18

Once bilirubin is delivered into hepatocytes, what cytosolic proteins temporarily bind bilirubin until it is further processed (conjugated)?

Answer 18

Ligandins

Question 19

Once in the liver, bilirubin is conjugated.

1. What is the major conjugating group used for bilirubin?
2. What enzyme is the conjugation catalyzed by?
3. How can the products of this reaction differ?

Answer 19

1. Glucuronic acid (glucuronidation reaction)
2. UGT1A1 (uridine diphosphate glucuronosyltransferase 1A1)
3. Either one or two gluruonic acid moieties can be conjugated, yielding either:
   
   • mono- (BMG) or
   • diglucuronide (BDG) species

Question 20

How is bilirubin excreted from hepatocytes?

What cofactor is needed for this process?

Answer 20

MOAT (Multiorganic anion transporter), located along the canalicular membranes

ATP-dependent process

Question 21

Again, what are the major heme breakdown products in:

1. Urine?
2. Feces?

What single compound are both of these products derived from?

Answer 21

1. Urobilin (gives urine its yellow color)
2. Stercobilin (major pigment of feces)

Both are derived from urobilinogen

Question 22

What quantitative assay is there for measuring bilirubin values?

What type of bilirubin does this test measure when run in water? Why?

What type of bilirubin is detected when the test in run in methanol? Why?

Answer 22

1. van den Bergh assay
2. In water, measures direct (conjugated) bilirubin. Unconjugate bilirubin is insoluble in H₂O and is not detected.
3. In methanol, both conjugated and unconguated bilirubin are soluble, so running the test in MeOH yields total bilirubin

N.B. You can calculate indirect (unconjugated) bilirubin from these two tests with total - direct = indirect

Question 23

1. What is neonatal jaundice?
2. How common is it?
3. When does it occur?
4. How serious is it?

Answer 23

1. Jaundice due to the neonate’s low liver function at birth.
2. Every newborn has hyperbiliubinemia by adult standards. About half of newborns become clinically jaundiced.
3. First five days of life
4. **Most **cases are innocuous, but severe cases can result in encephalopathy.

Question 24

Is direct or indirect bilirubin responsible for neonatal jaundice?

How does this bilirubin accumulate around birth in the first place?

Answer 24

Indirect (unconjugated) bilirubin in the serum is reponsible.

Bilirubin accumulates due to:

• Low UGT1A1 (conjugating enzyme) activity at birth
• Low excretory capacity of hepatocytes
• Increased bilirubin production
  
  • Due to accerlerated RBC destruction
    
    • (Not sure what causes this)

Question 25

What is **kernicterus?**

Answer 25

"**Bilirubin** **encephalopathy**" * The type of encephalopathy seen in neonatal jaundice * Causes **athetoid** (writhing) **cerebral** **palsy** * Often causes **hearing** **loss**

Question 26

Why might neonatal jaundice be an evolved, physiological function?

Answer 26

* Hyperbilirubinemia might be an important antioxidant defense for the newborn * Bilirubin can inhibit lipid peroxidation, as it can donate H⁺

Question 27

If needed, how is neonatal jaundice treated? Describe the mechanism of action.

Answer 27

**Phototherapy** Exposure to **blue-green light (430-490nm)** changes bilirubin's **configuration** to an isomer that can be excreted in the bile **without** **conjugation**.

Question 28

Name three syndromes that include an **inherited _unconjugated_ bilirubinemia**. What serum bilirubin level is typically found in each? Which is most severe?

Answer 28

1. Crigler-Najjar Syndrome, Type I * 20-50mg/dL 2. Crigler-Najjar Syndrome, Type II 1. \<20mg/dL 3. Gilbert Syndrome * Usually \<3mg/dL Worst: CN Syndrome, Type I. **Higher bilirubin = more severe!**

Question 29

What deficiency underlies all three inherited unconjugated hyperbilirubinemia syndromes?

Answer 29

UGT1A1 expression Crigler Najjar Type I: **Absent** Criggler Najjar Type II: ~**20**% activity Gilbert: ~**30**% activity

Question 30

1. Name two inherited syndromes that feature **_conjugated_ hyperbilirubinemia**. 2. What is the defect in each? 3. How are they inherited?

Answer 30

1. Dubin-Johnson Syndrome (**MOAT** defect) 2. Rotor Syndrome (molecular **mechanism** **unknown**) 3. Both are **autosomal recessive**

Question 31

What is the prognosis of the inherited conjguated bilirubinemias?

Answer 31

Both (Dubin-Johnson and Rotor) hav a **benign / mild** prognosis.

Question 32

Name three general causes of jaundice that are not inherited diseases. What type of bilirubin would be increased in each case?

Answer 32

1. Hemolytic (increased **indirect**) * RBC destruction exceeds conjugating ability of liver 2. Obstructive (increased **direct**) * Partial or complete blockage of the bile ducts 3. Hepatocellular (increased **indirect**) 1. Liver damage due to toxins, poisons, cardiac failure, and acute or chronic disease limit its conjugation ability

Question 33

1. What is major precursor of bile acids? 2. What is important about the relationship between bile acids and this precursor? 3. Where are bile acids synthesized?

Answer 33

1. **Cholesterol** 2. Bile acids are a **major mechanism for cholesterol excretion** from the body 3. The Liver

Question 34

Describe the passage of bile acids after their synthesis. Include sites in these locations: Liver Biliary tree & Gallbladder Small Intestine

Answer 34

* Secreted into **bile canaliculi** (channels between adjacent hepatocytes) * Travel through the **hepatic duct** and up the **cystic duct** into the gallbladder for storage * When needed, is carried through the **common bile duct** and excreted into the **duodenum** * (Wiki) 95% of bile acids are **reabsorbed** from the **ileum** back to the liver, where they can be **further modified** and secreted into the biliary tree to begin the process anew. * The remaining 5% exits the body in the feces.

Question 35

Name two major uses of bile acids

Answer 35

* Emulsify dietary triglycerides for more efficient hydolysis by pancreatic lipase * Facilitate absorption of fat-soluble vitamins from the intestine (e.g. Vitamin D)

Question 36

1. Name two **primary** bile acids. 2. What are they directly synthesized from? 3. What are they synthesized by?

Answer 36

1. Primary bile acids: * **Cholic** acid * **Chenodeoxycholic** acid 2. Cholesterol 3. Hepatocytes

Question 37

1. Name two **secondary** bile acids. 2. What are they synthesized from? 3. What are they synthesized by?

Answer 37

1. Secondary bile acids: * **Deoxycholic** acid * **Lithocholic** acid 2. Precursors: * Deoxycholic is synthesized from cholic * Lithocholic is synthesized from chenodeoxycholic 3. **Gut bacteria**

Question 38

1. By what mechanism are bile **_salts_** synthesized from bile acids? 2. Are primary or secondary bile acids converted into salts? 3. Where does this conversion take place?

Answer 38

1. Conjugation of bile salts with **glycine** or **taurine** 2. **Both** 3. Hepatocytes

Question 39

Why is the conjugation of bile acids into bile salts useful?

Answer 39

Salts have a lower pK_a value, which renders them more soluble in the small intestine (better emulsifying agents than bile acids.)

Question 40

What bacterial enzyme catalyzes the conversion of primary to secondary bile acids?

Answer 40

7alpha-hydrogenase

Question 41

What circular blood flow allows for efficient circling and reusing of bile acids?

Answer 41

**Enterohepatic** circulation

Question 42

About how much bile acid (in grams) can be typically stored in the gallbladder?

Answer 42

2-4g

Question 43

**Familial Hypercholesterolemia** 1. What enzyme deficiency causes FH? 2. How is it inherited? 3. How common is it?

Answer 43

1. Defect in gene encoding **LDL receptor** 2. Autosomal dominant with **gene dosage **effect * Homozygotes more severely affected than heterozygotes 3. Heterozygotes number 1 in 500 persons * One of the most common inborn errors of metabolism * Homozygotes are 1:1,000,000

Question 44

**Familial Hypercholesterolemia** 1. What is elevated in the plasma? 2. Where does cholesterol deposit? What are these deposits called?

Answer 44

1. **LDL** 2. Deposit sites: 1. Tendons & skin (**xanthomas**) 2. Arteries (**atheromas**)

Question 45

What is the treatment focused on in Familial Hypercholesterolemia?

Answer 45

**Reducing plasma LDL levels** (Direct link between high LDL and atherosclerosis)

Question 46

Can you name some examples of cellular functions that are up- or downregulated to control choelsterol levels?

Answer 46

* Synthesis of LDL receptors * Recycling of receptors * Synthesis of cholesterol * Storage of cholesterol esters * Synthesis of sterol hormones, vitamins, and bile acids from cholesterol

Question 47

Explain the "dual role" of the LDL receptor and its relation to FH.

Answer 47

1. LDLR **limits LDL production** by enhancing the removal of LDL's precursor, IDL. 2. LDLR **enchances LDL degradation** by mediating LDL cellular uptake. In FH, defective LDLR lacks these negative regulatory mechanisms, and so LDL levels increase markedly.

Question 48

Name three specific treatments for HF heterozygotes.

Answer 48

1. Bile acid binding resins 2. Statins 3. Low-fat, low-cholesterol diet

Question 49

Name two bile acid binding resins. Explain their mechanism of action. Used alone, how much can they lower LDL cholesterol levels?

Answer 49

1. Cholestyramine 2. Colestipol Bind bile acids in the intestine and **prevent their reabsorption**. Increased bile acid excretion prompts an increased conversion of cholesterol to bile acids. The resulting **cholersterol deficiency**, in turn, stimulates the liver to produce **more LDL receptors**. 10-20% decrease

Question 50

Recall the mechanism of action of statins. When combined with bile acid binding resins, how much can LDL cholesterol be decreased?

Answer 50

HMG-CoA reductase inhibitors. Decreased cholesterol synthesis promotes synthesis of LDLR. \>60%

Question 51

Alone, how much can a low-fat, low-cholesterol diet be expected to decrease LDL cholesterol levels?

Answer 51

10-20%