Metabolism 12

Question 1

Describe heme synthesis.

What are the tissues with highest rate of heme synthesis? (Explain why these tissues have the highest rates).

Answer 1

The tissues with the highest rate of heme synthesis are bone marrow (for incorporation into hemoglobin in erythrocytes)

Liver (for incorporation
into cytochromes, particularly cytochrome P-450 enzymes).

Question 2

Describe structure of heme.

Answer 2

pyrrole groups… p 3

Question 3

Describe the heme biosynthetic pathway.

Outline steps 1-8 and where they take place.

Answer 3

Step 1 the rate-limiting and regulated step; occurs in mitochondria): succinyl CoA + glycine (ALA synthase) to make ALA

Step 2 (cytosol): 
2 ALA's  (ALA dehydratase) to porphobilogen 

Step 3 (cytosol): 4 porphobilogens to hydroxymethylbilase

Steps 4 and 5 (cytosol)
generate uroporphrinogen III and copropophyrinogen III

Steps 6, 7, 8 (mitochondria)
protoporhyrinogen III -6 H to protoporhyrin III, add iron…Heme

Question 4

Describe the nomenclature/structural features:

Porphyrinogen
Porphyrins

How can you generate porphyrins from porhyrinogen?

Answer 4

a. Porphyrinogen
- no double bonds at the bridging carbons
- colorless

b. Porphyrins
- double bonds at the bridging carbons
- colored, highly fluorescent, photodegradable

c. Porphyrinogens can be non-enzymatically oxidized to porphyrins by light.

Question 5

Draw out a summary of biosynthesis of Heme with the enzymes at each step.

Describe the two ALA synthases. Where is each present?

How does regulation of heme biosynthesis differ in liver vs bone marrow?

Answer 5

p 5 …Heme provides negative feedback

a. There are two ALA synthases: ALAS1 is present in all tissues, while ALAS2 is specifically expressed in bone marrow erythroid cells.
b. The regulation of heme biosynthesis differs in bone marrow and liver.
c. In liver, heme inhibits its own synthesis by decreasing the activity of ALAS1 (as shown above). ALAS2 is not regulated by heme.

Question 6

What causes Porphyrias (disease)?

What causes the symptoms specifically?

Answer 6

Porphyrias are diseases caused by a partial deficiency of one of the enzymes involved in heme biosynthesis. Symptoms are caused by an increase in metabolic intermediates rather than a decrease in heme
production.

Question 7

Describe acute intermittent porphyria.

How is it inherited?
Penetrance?
Prevalence?

Answer 7

The disease is autosomal dominant with incomplete penetrance (most
who inherit the trait - 80% - never develop symptoms).
Prevelance: 1/20,000.

Question 8

What does a deficiency (50 percent of normal) of porphobilinogen deaminase (enzyme #3) lead to? What will be in higher concentration/lower concentration?

What causes this disease?
What clinical symptoms may manifest?

Answer 8

A deficiency (50% of normal) of porphobilinogen deaminase (enzyme #3) leads to increased levels of ALA and porphobilinogen (PBG) and a
somewhat lower concentration of heme in the liver. The disease
results (by an unknown mechanism) in nerve damage. Patients have intermittent acute attacks of severe abdominal pain, tachycardia, hypertension, muscle weakness, tremors, seizures, and psychiatric
symptoms such as agitation and hallucinations.

Question 9

A deficiency (50% of normal) of porphobilinogen deaminase (enzyme #3) leads to increased levels of ALA and porphobilinogen (PBG) and a
somewhat lower concentration of heme in the liver. The disease
results (by an unknown mechanism) in nerve damage. Patients have intermittent acute attacks of severe abdominal pain, tachycardia, hypertension, muscle weakness, tremors, seizures, and psychiatric
symptoms such as agitation and hallucinations.

What will slightly lower rate of synthesis of heme result in?

Answer 9

The slightly lower rate of synthesis of heme reduces the feedback inhibition of ALA synthase. The resulting increase in ALA synthase leads to more ALA and PBG, which exacerbates the disease.

Question 10

How might drugs like barbiturates, certain steroid hormones, and a low glucose diet interact with this disease?

Answer 10

Over 100 different drugs (e.g., barbiturates), alcohol, certain steroid hormones, and a low glucose diet, can induce the expression of ALA synthase; consequently, these conditions can precipitate or exacerbate
acute attacks.

Similar acute attacks also occur in other porphyrias such as hereditary coproporphyria and variegate porphyria. Patients with these two porphyrias may also exhibit skin sensitivity (see below).

Question 11

What does treatment of severe acute attacks entail?

Answer 11

Treatment of severe acute attacks includes glucose infusion and intravenous administration of heme to suppress ALA synthase.

Question 12

Describe Variegate porphyria.

1) How is this disease inherited? Penetrance? Prevalence? What effect do we see?
2) What enzyme is deficient and what does substances will increase?
3) How are heme and ALA synthase affected? What results?
4) Discuss the clinical consequences.

Answer 12

Variegate porphyria (a cutaneous porphyria)

1) This disease is autosomal dominant with incomplete penetrance (most never develop symptoms). Prevalence: 1/100,000 Finland; 1/333 South African whites (founder effect).
2) A deficiency (50% of normal) of protoporphyrinogen oxidase
(enzyme #7) leads to increased levels of protoporphyrinogen III
and coproporphyrinogen III in liver.
3) Since heme synthesis is reduced, ALA synthase is increased, leading to increased levels of ALA and PBG.
4) The protoporphyrinogen and coproporphyrinogen are deposited in the skin. Sunlight converts them to porphyrins. The porphyrins are then further degraded by light, a process that generates tissue-destroying singlet oxygen. Blistering and other skin lesions result.
5) Photosensitivity can also occur in other porphyrias in which porphyrinogens and porphyrins accumulate.

Question 13

Describe lead poisoning. What enzymes are affected? What will be elevated?

Answer 13

Lead can inhibit three enzymes of the heme biosynthetic pathway:
ALA dehydratase, coproporphyrinogen oxidase, and ferrochelatase.

Consequently, lead poisoning results in elevated levels of ALA,
coproporphyrinogen, and protoporphyrin III (Zn). The latter, measured in erythrocytes, can serve as a marker for lead ingestion over the previous 3
months.

Question 14

Describe heme catabolism.

What is the half life of erythrocyte?

Draw flow chart.

Answer 14

The erythrocyte, containing hemoglobin, has a half-life of 120 days.

Eventually, the erythrocyte is taken up by the phagocytic cells of the reticuloendothelial system and destroyed. Both the protein (globin) and the heme group of hemoglobin are degraded while the iron is reutilized:

Question 15

Describe the catabolism of heme.

Answer 15

Catabolism of heme in phagocytic cells of the reticuloendolthelial system (monocyte-macrophage system in spleen, bone marrow, and liver)

p 9

heme oxygenase to biliverdin-
biliverdin reductase

Question 16

What happens to unconjugated bilirubin?

Answer 16

unconjugated bilirubin- This form of bilirubin, which is insoluble, is also called “indirect bilirubin.”)

Unconjugated bilirubin is carried in the plasma as a complex with albumin and is delivered to the liver where it is taken up by active transport and conjugated.

Question 17

Describe the conjugation of bilirubin in the liver.

What happens to conjugated bilirubin?

Answer 17

Two glucuronic acid groups (from UDP-glucuronate) are attached to the unconjugated bilirubin via the propionic acid side chains:

(bilirubin diglucuronide or
“direct bilirubin”) (soluble)

Conjugated bilirubin is actively secreted into the bile canaliculus.

In the intestine, bilirubin diglucuronide is deconjugated by bacterial flora and oxidized to colored stercobilins, which give the stool its brown color.

Question 18

What gives stool its brown color?

Answer 18

In the intestine, bilirubin diglucuronide is deconjugated by bacterial flora and oxidized to colored stercobilins, which give the stool its brown color.

Question 19

What happens to urobilinogen?

Answer 19

Some of the urobilinogen is absorbed, scavenged by the liver, and re-excreted in the bile, or excreted in the urine. Urobilin, an oxidized form of urobilinogen, gives urine its characteristic yellow color.

Question 20

Draw a summary of heme/bilirubin and where reactions take place.

Answer 20

p 11

Question 21

Clinically, when does someone have hyperbilirubinemia?

Answer 21

elevated bilirubin in serum (above 1 mg/dL).
– can be either the conjugated or unconjugated form, or both,
depending on the condition.

– The elevated bilirubin can diffuse into tissues, making them appear yellow. This is called jaundice, and is detectable in the sclerae of the eyes when serum bilirubin reaches 2-2.5 mg/dL.

Question 22

Describe the clinical consequences of hyperbilirubinemia.

Conjugated hyperbilirubinemia?
Unconjugated hyperbilirubinemia?

At what concentrations can free unconjugated bilirubin enter the brain? What results?

Answer 22

1) Conjugated hyperbilirubinemia is benign.
2) Unconjugated hyperbilirubinemia is also benign at concentrations less than 25 mg/dL, which is the capacity of albumin to bind it.
3) At concentrations greater than 25 mg/dL, free unconjugated bilirubin can enter the brain and cause toxic encephalopathy (kernicterus).

Question 23

What is hemolysis? What results?

Answer 23

• • increased destruction of erythrocytes.
• • results in increased blood unconjugated bilirubin.

jaundice results…

Question 24

Describe bilary obstruction.

What results?
How is urine and feces affected/what might they look like?

Answer 24

Biliary obstruction (e.g., bile duct obstruction) (see figure below)
– conjugated bilirubin is not delivered to the intestine and spills
over into blood.
– results in increased blood conjugated bilirubin.
– the urine is dark.
– the feces can be chalky white because of the absence of
stercobilins.

Figure on p 12

Question 25

Describe the clinical consequences of hepatitis or cirrhosis.

Answer 25

– liver damage; results in decreased conjugation and excretion of bilirubin.

– results in mixed hyperbilirubinuria, i.e., increased blood unconjugated & conjugated bilirubin.

Question 26

Describe neonatal “physiological jaundice”

Answer 26

Neonatal “physiological jaundice” (see figure below)
– fragile erythrocytes.
– immature hepatic system of the newborn results in decreased
uptake, conjugation, and excretion of bilirubin. Bilirubin is also reabsorbed from the intestine.

– results in increased blood unconjugated bilirubin.

If it gets high enough, unconjugated bilirubin causes
kernicterus.

Figure p 13

Question 27

How is neonatal physiological jaundice treated?

Answer 27

Blue light can convert the insoluble and blood-brain barrier-permeable unconjugated bilirubin to isomers that are more soluble (e.g., lumirubin) and can’t get into the brain. These isomers are excreted in the urine or bile.

Newborns with dangerous physiological jaundice can be placed under blue-green emitting lights (the higher green wavelengths penetrate deeper into the skin) covered with a diaper and a blindfold. Blood traversing the skin capillaries will be exposed to the light.