Bilirubin Metabolism - Abali 2/22/16 Flashcards
bile metabolism: related disease
bilirubin metabolism: related disease
bile: cholestasis
bilirubin: bilirubinemia/jaundince/icterus
bilirubinemia
jauncice, icterus
increased bilirubin in circulation
- yellowing of eyes, hands
- if no eyes…might be hyperbetacarotenemia (high levels of beta-carotene : precursor of vit A)
key functions of heme in the body
- hemoglobin: oxygen transport in blood
- myoglobin: oxygen storage in muscle
- cytochrome c: e transport chain
- cytochrome P450s: toxin breakdown
- catalase: H2O2 breakdown
- Trp pyrrolase: oxidation of Trp
types of hemolysis
- proportions, location
extravascular (90%): macrophage phagocytosis in liver/spleen
and
intravascular (10%): in vasculature
extravascular hemolysis
- 90% of hemolysis
- RBCs phagocytized by macrophages in liver and spleen
- characterized by spherocytes (round cells - not discs)
why?
- senescent RBCs
- RBCs with membrane abnormalities (bound Ig)
- RBCs with physical abnormalities that prevent exit from spleen (sickle cell, thallasemia, PK deficiency)
*
intravascular hemolysis
- 100% of hemolysis
- RBC lysis within circulation
- characterized by fragmented cells called schistocytes
why?
- trauma
- complement fixation
- toxic damage to RBC
- Favism (G6PD deficiency)
extravascular erythrocyte destruction
- accounts for daily 1% normal RBC turnover
-
reticuloendothelial system is responsible
- monitors RBCs
- macrophages of liver, spleen, and bone marrow engulf RBCs before they lyse
salvage of intravascular hemolysis products
hemolysys releases RBC contents including Hb and heme
- Hb binds to haptoglobin
- Hb-Hp binds to receptor on macrophage, gets endocytosed
- heme binds to hemopexin
- heme-hemopexin binds to receptor on macrophage, gets endocytosed
in macrophage, Hb and heme undergo lysosomal degredation releasing…
- CO
- Fe+2 (recycled)
- heme → biliverdin → bilirubin
symptom: low/no haptoglobin in circulation
potentially hemolysis taking place somewhere → all haptoglobin is bound to hemoglobin that needs to be degraded, so none is free in circulation
heme degradation
in the macrophages of the reticuloendothelial system (liver, spleen, bone marrow)
same enzymes whether intra/extravascular hemolysis
heme [purple] → biliverdin [green]
- via heme oxygenase
- uses oxygen; gives off Fe+3, CO
biliverdin [green] → bilirubin [red-orange]
- via biliverdin reductase
bilirubin transport and modification in liver
released from site of production (macrophages of RES) into circulation
- it’s hydrophobic! need something for it to be bound to in circ: albumin
in liver,
- GST-B (glutathione S transferase B, aka ligandin) traps bilirubin in liver [“indirect”/unconjugated bilirubin]
-
conjugation occurs via UDP-GT (UDP gucuronyl transferase aka UGT1A1)
- attaches 2 glucoronic acid to bilirubin [“direct” bilirubin-monoglucuronide, “direct” bilirubin-diglucuronide]
- conjugated bilirubin moves into gallbladder → can be secreted into sm intestine
bilirubin in intestinal tract, fates
in colon
- bacterial enzymes deconjugate bilirubin-diglucuronide back to bilirubin
- bacterial enzymes take bilirubin → urobilinogen
- 10% urobilinogen reabs and sent back to liver via hepatic portal circ → central circ to kidney → oxidized to urobilin (yellow, gives urine its color), excreted in urine
- 90% unabsorbed, further modified to stercobilin, urobilin (gives red-brown color to feces), excreted in feces
urobilinogen: antibiotics and urine concentration
oral admin of broad spectrum antibiotics drastically drops production of urobilinogen
high urine concentration of urobilinogen associated with hemolytic disorders/hepatocellular disease
direct vs. indirect bilirubin
direct = conjugated
indirect = unconjugated
named based on test, which makes use of van den Bergh rxn in which bilirubin is coupled with diazonium salt to make colored complex
-
direct bilirubin: fraction which complexes with diazonium salt in absence of an accelerator
- approximates conjugated bilirubin (normal : 0-0.3 mg/dL)
- total bilirubin: bilirubin which complexes with diazonium salt in presence of an alcohol accelerator
-
indirect bilirubin: diff between total and direct bilirubin
- approximates unconjugated bilirubin (normal : 0.3-1.9 mg/dL)
why is direct bilirubin supposed to be near-absent in circulation?
macrophages that bring bilirubin to liver bring it in circulation as indirect bilirubin
conjugation to direct bilirubin occurs in the liver
heme oxygenase
enzyme found in macrophages of RES (liver, spleen, bone marrow)
converts heme to biliverdin
- produces Fe+3 and CO (only rxn that produces CO)
- uses NADPH
biliverdin
heme to biliverdin + CO, catalyzed by heme oxygenase in RES macrophages
- biliverdin and CO are cytoprotective
- biliverdin = antioxidant
- CO = signaling mol, vasodilator
biliverdin reductase
catalyzes biliverdin to bilirubin in RES macrophages
- uses NADPH
- bilirubin produced is released from macrophage into circ bound to albumin
reactions catalyzed by UDP-GT
consequence of reduced UDP-GT activity
UDP-GT conjugates bilirubin with glucoronic acid in liver
- bilirubin + UDP-glucoronic acid → BMG
- BMG + UDP-glucoronic acid → BDG (conjugated bilirubin)
under normal conds, the second reaction is rate limiting step
- if there’s a reduction in UDP-GT activity (ex. in genetic conds), you’ll see buildup of BMG
bilirubin transport
- into hepatocytes
- into bile canaliculi
- efflux from hepatocytes
- reuptake by hepatocytes from circ
influx
-
unconj bilirubin into hepatocytes: OATP1B1
- acted on by GST-B to prevent auto-efflux
- reabs conj bilirubin, into hepatocyltes: OATP1B1, OATP1B3
efflux
- into bile canaliculi: MRP2
- sinusoidal efflux: MRP3 [uses ATP]
OATP = organic anion transporting polypeptide
jaundice overview
French jaune = yellow
= icterus = hyperbilirubinemia
- consequence of malfunction in heme degradation
-
not a disease!!! indicator that something else might be wrong
- can be a serious problem in newborns
jaundice
- clinical features
- biochemical features
clinical features
- yellowing of skin and eyes
- due to affinity of elastin for bilirubin
- reason why you DO NOT see yellowing of eyes in hyperbetacarotenemia! (sclera x carotene)
biochemical features
- accumulation of bilirubin in hydrophobic compartments of periph tissues
- usually in cytoplasmic membranes
- bilirubin is harmless in older children and adults, not in newborns
- has neurotoxic props in newborns, leads to KERNICTERUS
causes of jaundice
prehepatic
- hemolysis : autoimmune or abnormal Hb : rare
intrahepatic
- infection
- chemicals/drugs
- genetic errors : bilirubin metab [4 syndromes]
- genetic errors : specific proteins (Wilson’s disease/Cu toxicity)
- autoimmune : chronic active hepatisis
- neonatal : physiologic : common
posthepatic
- intrahepatic bile ducts
- posthepatic bile ducts
physiological jaundice
newborns achieve adult levels of UDP-GT in approx 1 month
- premature babies have less to begin with, so it takes them longer to catch up
- buildup of unconjugated/indirect bilirubin in the catchup period is normal for all babies, but if a baby starts with a lower stock of UDP-GT, the increased buildup can reach dangerous levels
kernicterus
condition in newborns in which bilirubin accumulates in parts of the brain (kern = nucleus), causes neurotoxicity
yellow-staining of deep nuclei of brain
symptoms
- early
- extreme jaundice
- absent startle reflex
- poor feeding/sucking
- extreme lethargy
- midstage
- high pitched cry
- arched back, neck hyperextended
- bulging fontanel
- seizures
- late state (full neurological syndrome)
- high freq hearing loss
- intellectual disability
- muscle rigidity
- speech difficulty
- seizures
- movement disorder
treatment for neonatal jaundice
bilirubin builds up in part bc it’s so insoluble: making it more soluble would allow it to be excreted in urine
phototherapy (blue light oxidizes bilirubin, makes it into more soluble products: yellow photoisomers and colorless oxidation pdts)
- photoisomers → excreted in bile (main pathway)
- oxidation products → excreted in urine (minor pathway)
intrahepatic causes of jaundice due to genetic errors in bilirubin metabolism
- Rotor’s syndrome
* absence of OATP1B1, OATP1B3 (influx of reabs conj bilirubin) - Gilbert’s syndrome [1 in 20]
* defect in UDP-GT : normal protein, but less of it - Crigler-Najjar syndrome
* defect in UDP-GT : either no protein or less protein - Dubin-Johnson syndrome
* absence of MRP2 (efflux of conj bilirubin from hepatocyte into bile canaliculus)
Criggler-Najjar syndrome
Type 1
- complete absence of UDP-GT gene
- no enzyme activity
- jaundice: severe, causes kernicterus
Criggler-Najjar syndrome
Type 2
- mutation in UDP-GT gene
- some enzyme activity
- jaundice: benign
Dubin-Johnson syndrome
- mutation in MRP2
- diffuse deposition of course, dark-brown granular pigment in hepatocytes
- some enzyme activity
- jaundice: only symptom
*appears during adolescence/early childhood
*more common in Iranians, Morrocan Jews of Israel
Rotor syndrome
- biallelic inactivation mutation of OATP1B1, OATP1B3
- no enzyme activity
- jaundice: intermittent; mild conjugated/unconjugated jaundice
*appears shortly after birth/in childhood
Gilbert syndrome
- mutation in promoter of UDP-GT gene → reduced transcription
- some enzyme activity
- jaundice: mild, with physiologic stress
*appears during/after adolescence
*common (2-10% of pop)
types of jaundice overview
- prehepatic: hemolytic
-
intrahepatic
- neonatal
- hepatocellular
-
posthepatic
- obstructive
hemolytic jaundice
prehepatic
- massive hemolysis : heme released in quantities that overwhelm liver’s capacity to degrade it
- see increased indirect and increased direct bilirubin
- buildup in blood and periph tissues
neonatal jaundice
- esp in premature infants
- low levels of hepatic UDP-GT at birth
- low levels of GST-B
- increased levels of indirect bilirubin
obstructive jaundice
- obstruction of bile duct, prevents passage of conjugated bilirubin into intestines
- feces have characteristic pale clay color due to absence of stercobilin
hepatocellular jaundice
more complicated
- damage to liver cells (via hepatitis, liver disease, alcohol) leading to decrease in normal fx
- cant handle even normal loads
- both liver uptake and conjugation can be affected
- increased unconjugated bilirubin
liver function tests:
AST, ALT, alkaline phosphatase
AST/ALT will usually work in tandem BUT
- ALT > AST in viral hep
- AST > ALT in alcoholic hep
alk phos
highest levels in biliary obstruction, infiltrative liver disease
do bilirubin wrapup questions
