heme degradation & jaundice

bilirubin (Bi)

water-insoluble product of heme degradation

• 85% from Hb - RE system (spleen & liver)

• 15% from Hb in RBCs, myoglobin, & cytochromes - extra-erythroid tissues (haptoglobin binds to free Hb released from RBCs away from spleen, transports it to spleen)

• transported in blood bound to serum albumin

MUST react with glucuronic acid in the liver (increases water solubility)

secreted into the bile

water-soluble products are excreted with urine & feces

heme oxygenase

STEP 1 in heme degradation - enzyme that breaks down methenyl bridge between 2 pyrrole rings in heme porphyrin in RE cells

• releases Fe²⁺ forming a green pigment biliverdin & produces CO

biliverdin reductase

STEP 2 in heme degradation - in addition to NADPH, enzyme that reduces biliverdin to form orange/yellow pigment bilirubin

• bilirubin = unconjugated, indirect, water-insoluble

albumin

STEP 3 in heme degradation - bilirubin is insoluble in plasma and thus transported to the liver bound to a this protein complex

bilirubin uptake by hepatocytes-facilitated transport system

STEP 4 in heme degradation

ligandin

STEP 5 in heme degradation - bilirubin is bound to this protein in hepatocytes to keep it water-soluble

UDP-glucuronyltransferase

STEP 6 in heme degradation - enzyme responsible for conjugating 2 mol of glucuronic acid (in SER) with bilirubin in the liver

bilirubin di-glucuronide

synthesized from the conjugation of 2 mols of glucuronic acid with bilirubin catalyzed by UDP-glucuronyltransferase

• bilirubin = conjugated, direct, water-soluble

van den Bergh reaction

used to measure bilirubin levels in blood

• a reagent (diazotized sulfanilic acid) reacts with bilirubin to form red products (measured colorimetrically)

direct bilirubin

water-soluble conjugated bilirubin reacts rapidly (in 1 step)

indirect bilirubin

water-insoluble unconjugated bilirubin reacts slowly,

• to facilitate the reaction, alcohol can be added to detach bilirubin from albumin (2 steps are required)

total bilirubin

direct (conjugated) + indirect (unconjugated)

MRP-2/MOAT (multi-drug Resistance Protein 2/multi-specific organic anion transporter)

STEP 7 of heme degradation - enzyme that actively transports conjugated bilirubin against a concentration gradient into the bile canaliculi → bile

• RATE-LIMITING ENERGY REQUIRING STEP

β-glucuronidase

STEP 7 of heme degradation - GI tract bacterial enzyme that hydrolyze and reduce conjugated bilirubin

• removal of glucuronic acid

• bilirubin broken down to urobilinogen

urobilinogen

colorless compound formed from the breakdown of unconjugated bilirubin in the GI tract via β-glucuronidase

• most (80%) is oxidized to stercobilin

• some (20%) is reabsorbed to portal blood to the liver

  • some (90%) is re-excreted into bile (enterohepatic urobilinogen cycle)

  • rest (10%) gets transported to the kidney and converted to urobilin

stercobilin

golden-brown pigment of feces

• formed from the oxidization of most (80%) of the urobilinogen

urobilin

yellow pigment of urine

• formed from 10% of the original 20% of urobilinogen that was reabsorbed in poral blood

hyperbilirubinemia

elevated level of bilirubin (conjugated, unconjugated, or both) due to increased production of bilirubin or decreased excretion from the organism

jaundice (icterus)

yellow color of the skin, nail beds, and sclerae caused by bilirubin deposition secondary to hyperbilirubinemia

• NOT a disease, a symptom of an underlying disorder

• appears if total bilirubin levels > 2-3mg/dL

kernicterus

toxic encephalopathy caused by elevated bilirubin levels above the binding capacity of albumin (> 25-30mg/dL), leading to diffusion into the basal ganglia

• some drugs can displace bilirubin from albumin

hemolytic jaundice (prehepatic)

cause:

• massive lysis of RBC (sickle cell anemia, G6PD deficiency, pyruvate kinase deficiency, malaria, ABO/Rh incompatibility) = increased production of bilirubin

pathobiochemistry:

• normal bilirubin production is 300mg/day - liver can conjugate

• hemolysis - bilirubin is produced faster than it can be conjugated

• more unconjugated bilirubin = more conjugated bilirubin = more urobilinogen

diagnosis:

• increased unconjugated & total bilirubin

• increased urobilinogen in urine, no bilirubin in urine

neonatal physiologic jaundice

pathobiochemistry:

• hemolysis of “old“ fetal RBCs (lifetime = 90 days) containing HbF to produce RBCs with HbA = increased production of bilirubin

• immature UDP-glucuronosyltransferase = decreased bilirubin conjugation

diagnosis:

• increased unconjugated & total bilirubin

• occurs after 1^st 24 hours of life

• usually resolves without treatment in 1-2 weeks

blue fluorescent light

treatment for neonatal physiologic jaundice

• converts bilirubin to more polar, water-soluble isomers = excreted in bile without conjugation

phenobarbital

treatment for neonatal physiologic jaundice

• activates UDP-glucuronosyltransferase

physiologic jaundice

not present in first 24 hours after birth

rate of bilirubin increase is < 0.5mg/dL/day

bilirubin peaks at 14-15mg/dL

direct bilirubin < 10% of total

resolves in 1 week in term infants & 2 weeks in preterm infants

pathologic jaundice

present in first 24 hours of life

rate of bilirubin increase is > 0.5mg/dL/day

bilirubin peaks at > 15mg/dL

direct bilirubin > 10% of total

persists beyond 1 week in term infants & 2 weeks in preterm infants

Crigler-Najjar Syndrome

type 1 = more severe / type 2 = less severe

cause:

• autosomal recessive

• absent UDP-glucuronosyltransferase = no bilirubin conjugation

diagnosis:

• increased unconjugated & total bilirubin without hemolysis

• kernicterus (some patients may not have neurologic signs until later in life)

• death within a few years

treatment:

• plasmapheresis

• phototherapy

• liver transplant

Gilbert Syndrome

benign/asymptomatic

cause:

• autosomal recessive

• reduced UDP-glucuronosyltransferase activity = decreased bilirubin conjugation

• impaired bilirubin uptake by liver from blood

diagnosis:

• asymptomatic or mild jaundice usually with stress, illness, or fasting

• increased unconjugated and total bilirubin without hemolysis

biliary tract obstruction (mechanical, posthepatic jaundice)

cause:

• extrahepatic cholestasis - gallstones, cholangiocarcinoma, pancreatic or liver cancer with obstruction of biliary system

pathobiochemistry:

• normal production of unconjugated bilirubin, normal conjugation, but inability to excrete bilirubin with bile → increased intrahepatic pressure → liver regurgitates conjugated bilirubin into blood → it is eventually excreted in urine (dark yellow-brown color)

diagnosis:

• increased conjugated & total bilirubin

• bilirubin in urine, no urine urobilinogen

• abdominal pain, nausea, vomiting

• pale stool, dark urine

Dubin-Johnson Syndrome

cause:

• autosomal recessive

• mutations in the gene encoding MRP-2

pathobiochemistry:

• impaired excretion of conjugated bilirubin to bile

diagnosis:

• increased conjugated & total bilirubin

• grossly black (dark) liver due to impaired excretion of epinephrine metabolites

Rotor Syndrome

cause:

• autosomal recessive

• defect in hepatic bilirubin uptake and excretion to bile

diagnosis:

• increased conjugated & total bilirubin

• no black (dark) liver

hepatic jaundice

cause:

• liver cell damage

  • cirrhosis & hepatitis (including viral); toxin-induced liver dysfunction (hepatotoxic agents - chloroform, arsphenamines, carbon tetrachloride, acetaminophen, amanita mushroom poisoning, etc.)

pathobiochemistry:

• decreased conjugation of bilirubin & decreased excretion of bilirubin into bile (intrahepatic cholestasis)

• decreased enterohepatic circulation of urobilinogen = more to enter the blood

diagnosis:

• increased conjugated, unconjugated & total bilirubin

• increased liver enzymes as ALT, AST (markers if liver damage)

• normal/increased urobilinogen in urine