Bilirubin is the yellow breakdown product of normal heme catabolism. Heme is found in
hemoglobin, a principal component of red blood cells. Bilirubin is excreted in bile and urine,
and elevated levels may indicate certain diseases. It is responsible for the yellow color of
bruises, the background straw-yellow color of urine, the brown color of feces, and the
yellow discoloration in jaundice. It has also been found in plants.
Chemistry Bilirubin consists of an open chain of four
pyrrole-like rings. In heme, these four rings are connected into a larger ring, called a
porphyrin ring. Bilirubin can be "conjugated" with a molecule
of glucuronic acid which makes it soluble in water. This is an example of glucuronidation.
Bilirubin is very 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 is used in 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.
This 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
Unconjugated Erythrocytes generated in the bone marrow
are disposed of in the spleen when they get old or damaged. This releases hemoglobin,
which is broken down to heme as the globin parts are turned into amino acids. The heme
is then turned into unconjugated bilirubin in the reticuloendothelial cells of the spleen.
This unconjugated bilirubin is not soluble in water, due to intramolecular hydrogen bonding.
It is then bound to albumin and sent to the liver.
The measurement of direct bilirubin depends on its reaction with diazosulfanilic acid
to create azobilirubin. However, unconjugated bilirubin also reacts slowly with diazosulfanilic
acid, so that the measured indirect bilirubin is an underestimate of the true unconjugated
concentration. Conjugated
In the liver, bilirubin is conjugated with glucuronic acid by the enzyme glucuronyltransferase,
making it soluble in water: the conjugated version is also often called "direct" bilirubin.
Much of it goes into the bile and thus out into the small intestine. Though most bile
acid is resorbed 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: these give stool its characteristic
brown color. A trace of the urobilinogen is resorbed into the enterohepatic circulation
to be re-excreted in the bile: some of this is instead processed by the kidneys, coloring
the urine yellow. 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.
Furthermore, direct bilirubin tends to overestimate conjugated bilirubin levels due to unconjugated
bilirubin that has reacted with diazosulfanilic acid, leading to increased azobilirubin levels.
Urine Under normal circumstances, a tiny amount
of urobilinogen, if any, 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 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 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 feces. Instead the conjugated bilirubin is converted back into the unconjugated
form by the enzyme β-glucuronidase and a large proportion is reabsorbed through the
enterohepatic circulation. Associated health benefits
Recent research has indicated that in the absence of liver disease, individuals with
high levels of total bilirubin may experience various health benefits exceeding those with
lower levels of bilirubin. Studies have found higher levels of bilirubin in elderly individuals
are associated with higher functional independence. 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 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 heelstick, 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
Bilirubin is in one of two forms: Total bilirubin measures both BU and BC. 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 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, but typical [0.3 to 1.9 mg/dL]s for adults and [340 µmol/L]
for new borns:
Hyperbilirubinemia Hyperbilirubinemia results from a higher-than-normal
level of bilirubin in the blood. For adult this is any level above 17μmol/L and for
newborns 340 µmol/L and critical hyperbilirubinemia 425 µmol/L.
Mild rises in bilirubin may be caused by the following:
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
Sulfonamides are contraindicated in infants less than 2 months old as they increase unconjugated
bilirubin leading to kernicterus. Hepatitis
Chemotherapy Biliary stricture
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 Crigler–Najjar syndrome
Dubin–Johnson syndrome Choledocholithiasis.
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, blood
film examination or evidence of infective hepatitis.
Jaundice Jaundice may be noticeable in the sclera of
the eyes at levels of about 2 to 3 mg/dL, 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. See also
Biliary atresia Bilirubin diglucuronide
Biliverdin Crigler–Najjar syndrome
Gilbert's syndrome, a genetic disorder of bilirubin metabolism that can result in mild
jaundice, found in about 5% of the population. Hy's Law
Primary biliary cirrhosis Primary sclerosing cholangitis
References
External links Bilirubin: analyte monograph from The Association
for Clinical Biochemistry and Laboratory Medicine