Lecture 62

Question 1

porphyrins

Answer 1

• cyclic compounds that readily bind metal ions, usually ferrous (Fe2+) or ferric (Fe3+) iron forming metalloporphyrin (a prosthetic group)
• heme porphyrin: structure → one Fe2+ coordinated in the center of the tetrapyrrole ring of protoporphyrin IX; function → prosthetic group for hemoglobin, myoglobin, cytochromes in ETC, cytochrome p450 monooxygenase system, other enzymes
• porphyrins absorb visible light due to conjugated double bonds

pg 1573

Question 2

heme and hemoproteins

Answer 2

• hemoglobin and myoglobin must be in Fe2+ state to reversibly bind oxygen
• cytochrome, cyt p450, and catalase can be in either Fe2+ or Fe3+ form

pg 1574

Question 3

overview of heme biosynthesis

Answer 3

• tissue locations: liver (~15% CYP proteins → cytochrome p450 enzymes here) and RBC precursor cells in bone marrow (~85% → start with organelles, but lost with maturation)
• substrates: glycine (AA), succinyl CoA (a TCA cycle intermediate)
• subcellular localization of enzymes: mitochondria, cytosol
• stages of biosynthesis → 3 major stages: synthesis of pyrrol ring, conversion to a tetrapyrrole, incorporation of iron
• turnover rate: very rapid (~6-7 g of hemoglobin are synthesized per day to replace heme lost through normal turnover of erythrocytes)
• all metabolites are extremely neurotoxic

pg 1576

Question 4

heme biosynthesis: stage 1 → aminolevulinic acid synthase (ALAS)

Answer 4

• functions in the mitochondria converting glycine and succinyl CoA to δ-aminolevulinic acid (ALA)
• two isoforms with tissue-specific expression: ALAS1 in the liver (and other tissues), ALAS2 is present only in erythroid tissues
• require as a coenzyme PLP (from vitamin B6)
• rate-limiting committed step
• ALAS1 regulation: feedback inhibition by excess heme with Fe³⁺ (hemin) which… represses transcription of its gene, increases degradation of its mRNA, decreases import of the enzyme into mitchondria
• ALAS2 regulation: regulated ONLY by the availability of intracellular iron (want as much heme as possible)

pg 1577

Question 5

effect of drugs on ALAS1

Answer 5

• administration of any of a large number of drugs metabolized by the microsomal CYP monooxygenase system results in a significant increase in hepatic ALAS1 activity
• in response to the drugs, the synthesis of CYP proteins (requiring heme) increases, leading to an enhanced consumption of heme in the liver
• leads to an increase in the synthesis of ALAS1 and prompts a corresponding increase in the synthesis of ALA (accumulation leads to toxic adverse effects)

pg 1578

Question 6

x-linked sideroblastic anemia

Answer 6

• more than 50 loss-of-function mutations in ALAS2
• result in reduction in the amount of heme which prevents these cells from making enough hemoglobin
• reduced production of heme leads to a buildup of excess iron in the cells
• triggers increased uptake resulting in an overall iron overload

pg 1578

Question 7

heme biosynthesis: stage 1 → ALA dehydratase

Answer 7

• condenses 2 molecules of δ-aminolevulinic acid (ALA) to porphobilinogen (a pyrrol ring), releasing 2 water molecules
• inhibited by lead which contributes to the cause of anemia in lead poisoning

pg 1579

Question 8

heme biosynthesis: stage 2

Answer 8

• formation of the tetrapyrrole ring
• 4 molecules of porphobilinogen condense into a tetrapyrrole ring and undergo isomerization, decarboxylation, oxidation…
• ring formed in the cytosol and modified in the mitochondria
• eventually forms protoporphyrin IX

pg 1580

Question 9

clinical presentation of porphyria

Answer 9

• depends on the defective enzyme and respective intermediates that accumulate
• if the enzyme defect is prior to synthesis of the tetrapyrrole ring, manifestation includes abdmonial and neuropsychiatric signs
• if the enzyme defect is after the ring is formed, tetrapyrrole intermediates accumulate and account for observed photosensitivity → itching and burning skin when exposed to visible light, which is a consequence of oxidation of the accumulated intermediate and production of reactive oxygen species

pg 1580

Question 10

heme biosynthesis: stage 3 → ferrochelatase

Answer 10

• converts protoporphyrin IX to heme in the mitochondria by incorporating Fe²⁺
• inhibited by lead which contributes to the cause of anemia in lead poisoning

pg 1581

Question 11

lead poisoning

Answer 11

• ferrochelatase and ALA dehydratase (ALAD) are particularly sensitive to inhibition by lead
• protoporphyrin and ALA accumulate in urine
• ALAD deficiency porphyria is a very rare autosomal recessive acute hepatic porphyria

pg 1581

Question 12

porphyrias

Answer 12

• rare, inherited, or sometimes acquired, defects in heme synthesis, resultng in the accumulation and increased excretion of porphyrins or porphyrin precursors
• hepatic and erythropoetic
• hepatic: chronic (porphyria cutanea tarda) and acute (AIP, VP, HCP)
• erthyropoetic: ALL chronic → photosensitivity characterized by skin rashes and blisters that appear in early childhood (EPP and CEP)

pg 1582-1583

Question 13

chronic liver porphyria cutanea tarda

Answer 13

• most common → chronic disease of the liver; about 80% acquired
• clinical onset: typically during the fourth or fifth decade of life
• presentation: cutaneous symptoms (skin blisters) and urine that is red to brown in natural light (pink to red in fluorescent); photosensitivity
• can be caused by an autosomal dominant deficiency in uroporphyrinogen decarboxylase
• uroporphyrin accumulates in urine

pg 1584

Question 14

acute liver porphyrias

Answer 14

• characterized by: acute attacks of GI tract, neuropsychiatric, and motor symptoms that may be accompanied by photosensitivity
• accumulation of ALA and porphobilinogen cause abdominal pain and neuropsychiatric disturbances, ranging from anxiety to delirium
• condition can be induced by drugs metabolized by the cytochrome p450 system
• check urine levels of accumulated metabolites → ALA, porphobilinogen, coproporphyrin III, uroporphyrin III

pg 1585

Question 15

acute intermittent porphyria (AIP)

Answer 15

• acute autosomal dominant disease caused by a deficiency in hydroxymethylbilane synthase
• porphobilinogen and ALA accumulate in the urine
• urine darkens on exposure to light and air
• NOT photosensitive

pg 1585

Question 16

variegate porphyria (VP)

Answer 16

• acute autosomal dominant disease caused by a deficiency in protoporphyrinogen oxidase
• protoporphyrinogen IX and other intermediates prior to the block accumulate in the urine
• photosensitive

pg 1585

Question 17

hereditary coproporphyria (HCP)

Answer 17

• acute autosomal dominant disease caused by a deficiency in coproporphyrinogen III oxidase
• coproporphyrinogen III and other intermediates prior to the block accumulate in the urine
• photosensitive

pg 1585

Question 18

erythropoietic porphyrias

Answer 18

• result in photosensitivity characterized by skin rashes and blisters that appear in early childhood
• congenital erythropoietic porphyria (CEP): chronic autosomal recessive deficiency in uroporphyrinogen III synthase, uroporphyrinogen I and coproporphyrinogen I accumulate in urine, photosensitivity
• erythropoietic protoporphyria (EPP): chronic autosomal recessive and dominant deficiency in ferrochelatase; protoporphyrin accumulates in RBCs, bone marrow, and plasma; photosensitivity

pg 1586

Question 19

treatments for porphyrias

Answer 19

• acute porphyria attacks: patients require medical support, particularly treatment for pain and vomiting; severity of acute symptoms of the porphyrias can be diminished by IV injection of hemin and glucose (which decreases activity of ALAS1)
• photosensitivity: protection from sunlight, ingestion of β-carotene (provitamin A) that scavenges free radicals, and phlebotomy (removes porphyrins)

pg 1586

Question 20

heme degradation steps 1 and 2

Answer 20

1. senescent RBCs are a major source of hemeproteins
2. breakdown of heme to bilirubin occurs in macrophages of the mononuclear phagocyte system, particularly in the liver and spleen

• bilirubin appears to function at low levels as an antioxidant; heme oxidized to biliverdin which is then reduced by biliverdin reductase generating bilirubin
• heme oxygenase releases the iron and CO from heme to form biliverdin (tetrapyrrole ring is open)

pg 1588

Question 21

heme degradation steps 3 and 4

Answer 21

• # 3: unconjugated bilirubin is transported through the blood (complexed to albumin - highly blood soluble) to the liver
• # 4: bilirubin is taken up via facilitated diffusion by the liver and conjugated with glucuronic acid
• bilirubin converted to biliburin-albumin complex which is carried through the blood to the liver

pg 1589

Question 22

heme degradation steps 5-10

Answer 22

• # 5: conjugated bilirubin (bilirubin diglucuronide) actively secreted into bile and then the intestine
• # 6: in intestine, glucuronic acid is removed by bacteria → resulting bilirubin converted to urobilinogen
• # 7: some urobilinogen reabsorbed from gut and enters portal blood
• # 8: portion of urobilinogen participates in enterohepatic urobilinogen cycle
• # 9: remainder of urobilinogen transported by blood to kidney where it is excreted as yellow urobilin which gives urine its color
• # 10: urobilinogen is oxidized by intestinal bacteria to brown stercobilin (feces color)

pg 1590

Question 23

deficiency of bilirubin UDP-glucuronosyltransferase

Answer 23

• Crigler-Najjar syndrome (I and II) → most severe is type I
• Gilbert syndrome → mild and often diagnosed version
• symptoms: high levels of unconjugated bilirubin can be very toxic to nervous tissue

pg 1591

Question 24

jaundice

Answer 24

• NOT a single disease, but a condition that refers to the yellow color of skin, nail beds, and sclerae caused by deposition of bilirubin, secondary to increased bilirubin levels in the blood (hyperbilirubinemia)
• symptom of an underlying disorder
• blood bilirubin levels in a healthy adult below ~1 mg/dL → jaundice begins to appear above 2-3 mg/dL
• total bilirubin measured in methanol, direct is only conjugated and measured in water

pg 1592

Question 25

hemolytic jaundice

Answer 25

* any number of conditions that result in hemolytic anemia which overwhelms the ability of liver to conjugate bilirubin * increased serum *unconjugated bilirubin (UCB)* ## Footnote pg 1593

Question 26

hepatocellular jaundice

Answer 26

* any liver damage results in a defect in conjugation and/or secretion * may lead to increased serum levels of both *unconjugated and conjugated bilirubin* ## Footnote pg 1593

Question 27

obstructive jaundice

Answer 27

* blocking the bile duct leading to accumulation of conjugated bilirubin in the liver * increased serum *conjugated bilirubin (CB)* due to its release from liver ## Footnote pg 1593

Question 28

neonatal jaundice

Answer 28

* a transient, physiologic jaundice seen in most newborns during the first postnatal week * presents with a rise in serum levels of *unconjugated bilirubin* (very toxic to infants) ## Footnote pg 1593

Question 29

neonatal jaundice

Answer 29

1. activity of the enzyme that conjugates bilirubin with glucuronic acid (*bilirubin UDF-glucuronosyltransferase*) is low in newborns and especially low in premature babies 2. serum levels of bilirubin rise after birth in full-term infants, although usually not to dangerous concentrations 3. serum levels of bilrubin in premature infants may rise to toxic levels * increased unconjugated bilirubin ## Footnote pg 1595

Question 30

neonatal jaundice treatment

Answer 30

phototherapy with blue fluorescent light which reacts with the bilirubin and makes it more soluble, so that is it able to be filtered by the kidneys and excreted into the urine ## Footnote pg 1595