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| Identifiers | |
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3D model (JSmol)
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| ChEBI | |
| ChEMBL | |
| ChemSpider | |
| ECHA InfoCard | 100.010.218 |
PubChem CID
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| UNII | |
CompTox Dashboard (EPA)
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| Properties | |
| C33H36N4O6 | |
| Molar mass | 584.673 g·mol−1 |
| Supplementary data page | |
| Refractive index (n), Dielectric constant (εr), etc. | |
Thermodynamic
data |
Phase behaviour solid–liquid–gas |
| UV, IR, NMR, MS | |
Bilirubin is a yellow compound that occurs in the normal catabolic pathway that breaks down heme in vertebrates. This catabolism is a necessary process in the body's clearance of waste products that arise from the destruction of aged or abnormal red blood cells. First the hemoglobin gets stripped of the heme molecule which thereafter passes through various processes of porphyrin catabolism, depending on the part of the body in which the breakdown occurs. For example, the molecules excreted in the urine differ from those in the feces. The production of biliverdin from heme is the first major step in the catabolic pathway, after which the enzyme biliverdin reductase performs the second step, producing bilirubin from biliverdin.
Bilirubin is excreted in bile and urine, and elevated levels may indicate certain diseases. It is responsible for the yellow color of bruises and the yellow discoloration in jaundice. Its subsequent breakdown products, such as stercobilin, cause the brown color of faeces. A different breakdown product, urobilin, is the main component of the straw-yellow color in urine.
It has also been found in plants.
Structure and function
Bilirubin consists of an open chain tetrapyrrole. It is formed by oxidative cleavage of a porphyrin in heme, which affords biliverdin. Biliverdin is reduced to bilirubin. After conjugatation with glucuronic acid, bilirubin is excreted.
Bilirubin is structurally similar to the pigment phycobilin used by certain algae to capture light energy, and to the pigment phytochrome used by plants to sense light. All of these contain an open chain of four pyrrolic rings.
Like these other pigments, some of the double-bonds in bilirubin isomerize when exposed to light. This isomerization is relevant to the phototherapy
of jaundiced newborns: the E,Z-isomers of bilirubin formed upon light
exposure are more soluble than the unilluminated Z,Z-isomer, as the
possibility of intramolecular hydrogen bonding is removed. Increased solubility allows the excretion of unconjugated bilirubin in bile.
Some textbooks and research articles show the incorrect geometric isomer of bilirubin. The naturally occurring isomer is the Z,Z-isomer.
Function
Bilirubin is created by the activity of biliverdin reductase on biliverdin, a green tetrapyrrolic bile pigment that is also a product of heme catabolism.
Bilirubin, when oxidized, reverts to become biliverdin once again. This
cycle, in addition to the demonstration of the potent antioxidant
activity of bilirubin, has led to the hypothesis that bilirubin's main physiologic role is as a cellular antioxidant.
Metabolism
Heme metabolism
Unconjugated
The
measurement of unconjugated bilirubin is underestimated by measurement
of indirect bilirubin, as unconjugated bilirubin (without
glucuronidation) reacts with diazosulfanilic acid to create azobilirubin which is measured as direct bilirubin.
Conjugated
In the liver, bilirubin is conjugated with glucuronic acid by the enzyme glucuronyltransferase,
making it soluble in water: the conjugated version is the main form of
bilirubin present in the "direct" bilirubin fraction. Much of it goes
into the bile and thus out into the small intestine. Though most bile acid is reabsorbed in the terminal ileum to participate in enterohepatic circulation, conjugated bilirubin is not absorbed and instead passes into the colon.
There, colonic bacteria deconjugate and metabolize the bilirubin into colorless urobilinogen, which can be oxidized to form urobilin and stercobilin.
Urobilin is excreted by the kidneys to give urine its yellow color and
stercobilin is excreted in the faeces giving stool its characteristic
brown color. A trace (~1%) of the urobilinogen is reabsorbed into the enterohepatic circulation to be re-excreted in the bile.
Although the terms direct and indirect bilirubin are used
equivalently with conjugated and unconjugated bilirubin, this is not
quantitatively correct, because the direct fraction includes both
conjugated bilirubin and δ bilirubin (bilirubin covalently bound to albumin, which appears in serum when hepatic excretion of conjugated bilirubin is impaired in patients with hepatobiliary disease).
Furthermore, direct bilirubin tends to overestimate conjugated
bilirubin levels due to unconjugated bilirubin that has reacted with
diazosulfanilic acid, leading to increased azobilirubin levels (and
increased direct bilirubin).
Urine
Under normal circumstances, only a very small amount, if any, of urobilinogen, is excreted in the urine.
If the liver's function is impaired or when biliary drainage is
blocked, some of the conjugated bilirubin leaks out of the hepatocytes
and appears in the urine, turning it dark amber. However, in disorders
involving hemolytic anemia,
an increased number of red blood cells are broken down, causing an
increase in the amount of unconjugated bilirubin in the blood. Because
the unconjugated bilirubin is not water soluble, one will not see an
increase in bilirubin in the urine. Because there is no problem with the
liver or bile systems, this excess unconjugated bilirubin will go
through all of the normal processing mechanisms that occur (e.g.,
conjugation, excretion in bile, metabolism to urobilinogen,
reabsorption) and will show up as an increase in urine urobilinogen.
This difference between increased urine bilirubin and increased urine
urobilinogen helps to distinguish between various disorders in those
systems.
Toxicity
Unconjugated hyperbilirubinaemia in a newborn can lead to accumulation of bilirubin in certain brain regions (particularly the basal nuclei) with consequent irreversible damage to these areas manifesting as various neurological deficits, seizures, abnormal reflexes and eye movements. This type of neurological injury is known as kernicterus. The spectrum of clinical effect is called bilirubin encephalopathy. The neurotoxicity of neonatal hyperbilirubinemia manifests because the blood–brain barrier has yet to develop fully,
and bilirubin can freely pass into the brain interstitium, whereas more
developed individuals with increased bilirubin in the blood are
protected. Aside from specific chronic medical conditions that may lead
to hyperbilirubinaemia,
neonates in general are at increased risk since they lack the
intestinal bacteria that facilitate the breakdown and excretion of
conjugated bilirubin in the faeces (this is largely why the faeces of a
neonate are paler than those of an adult). Instead the conjugated
bilirubin is converted back into the unconjugated form by the enzyme β-glucuronidase
(in the gut, this enzyme is located in the brush border of the lining
intestinal cells) and a large proportion is reabsorbed through the enterohepatic circulation.
Health benefits
In the absence of liver disease, high levels of total bilirubin confers various health benefits. Studies have also revealed that levels of serum bilirubin are inversely related to risk of certain heart diseases.
Blood tests
Bilirubin
is degraded by light. Blood collection tubes containing blood or
(especially) serum to be used in bilirubin assays should be protected
from illumination. For adults, blood is typically collected by needle
from a vein in the arm. In newborns, blood is often collected from a
heel stick, a technique that uses a small, sharp blade to cut the skin
on the infant's heel and collect a few drops of blood into a small tube.
Non-invasive technology is available in some health care facilities
that will measure bilirubin by using an instrument placed on the skin
(transcutaneous bilirubin meter)
Bilirubin (in blood) is in one of two forms:
| Abb. | Name(s) | Water-soluble | Reaction |
| "BC" | "Conjugated bilirubin" | Yes (bound to glucuronic acid) | Reacts quickly when dyes (diazo reagent) are added to the blood specimen to produce azobilirubin "Direct bilirubin" |
| "BU" | "Unconjugated bilirubin" | No | Reacts more slowly, still produces azobilirubin, Ethanol makes all bilirubin react promptly, then: indirect bilirubin = total bilirubin – direct bilirubin |
Note: Conjugated bilirubin is often incorrectly called "direct
bilirubin" and unconjugated bilirubin is incorrectly called "indirect
bilirubin". Direct and indirect refer solely to how compounds are
measured or detected in solution. Direct bilirubin is any form of
bilirubin which is water-soluble and is available in solution to react
with assay reagents; direct bilirubin is often made up largely of
conjugated bilirubin, but some unconjugated bilirubin (up to 25%) can
still be part of the "direct" bilirubin fraction. Likewise, not all
conjugated bilirubin is readily available in solution for reaction or
detection (for example, if it is hydrogen bonding with itself) and
therefore would not be included in the direct bilirubin fraction.
Total bilirubin (TBIL) measures both BU and BC. Total bilirubin
assays work by using surfactants and accelerators (like caffeine) to
bring all of the different bilirubin forms into solution where they can
react with assay reagents. Total and direct bilirubin levels can be
measured from the blood, but indirect bilirubin is calculated from the
total and direct bilirubin.
Indirect bilirubin is fat-soluble and direct bilirubin is water-soluble.
Measurement methods
Originally, the Van den Bergh reaction was used for a qualitative estimate of bilirubin.
This test is performed routinely in most medical laboratories and can be measured by a variety of methods.
Total bilirubin is now often measured by the
2,5-dichlorophenyldiazonium (DPD) method, and direct bilirubin is often
measured by the method of Jendrassik and Grof.
Blood levels
The
bilirubin level found in the body reflects the balance between
production and excretion. Blood test results should always be
interpreted using the reference range provided by the laboratory that
performed the test. The SI units are umol/L. Typical ranges for adults are:
- 0-0.3 mg/dl - Direct (conjugated) bilirubin level
- 0.1-1.2 mg/dl - Total serum bilirubin level
| μmol/l = micromole/litre | mg/dl = milligram/ decilitre | |
| total bilirubin | <21 nbsp="" span=""> | <1 .23="" span=""> |
| direct bilirubin | 1.0–5.1 | 0–0.3 0.1–0.3 0.1–0.4 |
Hyperbilirubinemia
Hyperbilirubinemia
is a higher-than-normal level of bilirubin in the blood. For adults,
this is any level above 170 μmol/l and for newborns 340 µmol/l and
critical hyperbilirubinemia 425 µmol/l.
Mild rises in bilirubin may be caused by:
- Hemolysis or increased breakdown of red blood cells
- Gilbert's syndrome – a genetic disorder of bilirubin metabolism that can result in mild jaundice, found in about 5% of the population
- Rotor syndrome: non-itching jaundice, with rise of bilirubin in the patient's serum, mainly of the conjugated type
Moderate rise in bilirubin may be caused by:
- Pharmaceutical drugs (especially antipsychotic, some sex hormones, and a wide range of other drugs)
- Sulfonamides are contraindicated in infants less than 2 months old (exception when used with pyrimethamine in treating toxoplasmosis) as they increase unconjugated bilirubin leading to kernicterus.
- Drugs such as protease inhibitors like Indinavir can also cause disorders of bilirubin metabolism by competitively inhibiting the UGT1A1 enzyme.
- Hepatitis (levels may be moderate or high)
- Chemotherapy
- Biliary stricture (benign or malignant)
Very high levels of bilirubin may be caused by:
- Neonatal hyperbilirubinaemia, where the newborn's liver is not able to properly process the bilirubin causing jaundice
- Unusually large bile duct obstruction, e.g. stone in common bile duct, tumour obstructing common bile duct etc.
- Severe liver failure with cirrhosis (e.g. primary biliary cirrhosis)
- Crigler–Najjar syndrome
- Dubin–Johnson syndrome
- Choledocholithiasis (chronic or acute).
Cirrhosis may cause normal, moderately high or high levels of bilirubin, depending on exact features of the cirrhosis.
To further elucidate the causes of jaundice or increased bilirubin, it is usually simpler to look at other liver function tests (especially the enzymes alanine transaminase, aspartate transaminase, gamma-glutamyl transpeptidase, alkaline phosphatase), blood film examination (hemolysis, etc.) or evidence of infective hepatitis (e.g., hepatitis A, B, C, delta, E, etc.).
Jaundice
Jaundice may be noticeable in the sclera of the eyes at levels of about 2 to 3 mg/dl (34 to 51 μmol/l), and in the skin at higher levels. For conversion, 1 mg/dl = 17.1 µmol/l.
Jaundice is classified, depending upon whether the bilirubin is
free or conjugated to glucuronic acid, into conjugated jaundice or
unconjugated jaundice.
Urine tests
Urine bilirubin may also be clinically significant.
Bilirubin is not normally detectable in the urine of healthy people. If
the blood level of conjugated bilirubin becomes elevated, e.g. due to
liver disease, excess conjugated bilirubin is excreted in the urine,
indicating a pathological process. Unconjugated bilirubin is not water-soluble and so is not excreted in the urine. Testing urine for both bilirubin and urobilinogen can help differentiate obstructive liver disease from other causes of jaundice.
History
Bilirubin was discovered by Rudolf Virchow in 1847.) It is not always distinguished from hematoidin, which one modern dictionary defines as synonymous with it
but another defines as "apparently chemically identical with bilirubin
but with a different site of origin, formed locally in the tissues from
hemoglobin, particularly under conditions of reduced oxygen tension."
