Bilirubin


Bilirubin is a red-orange compound that occurs as the reduction product of biliverdin, a breakdown product of heme.
It is further broken down in the colon to urobilinogen, most of which becomes stercobilin, causing the brown color of feces. Some unconverted urobilinogen, metabolised to urobilin, provides the straw-yellow color in urine.
Although bilirubin is usually found in animals rather than plants, at least one plant species, Strelitzia nicolai, is known to contain the pigment.

Structure

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 conjugation with glucuronic acid, bilirubin is water-soluble and can be 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. Consistent with this, animal studies suggest that eliminating bilirubin results in endogenous oxidative stress. Bilirubin's antioxidant activity may be particularly important in the brain, where it prevents excitotoxicity and neuronal death by scavenging superoxide during N-methyl-D-aspartic acid neurotransmission.

Metabolism

Bilirubin in plasma is mostly produced by the destruction of erythrocytes. Heme is metabolized into biliverdin and then into bilirubin inside the macrophages.
Bilirubin is then released into the plasma and transported to the liver bound by albumin, since it is insoluble in water in this state. In this state, bilirubin is called unconjugated.
In the liver, unconjugated bilirubin is up-taken by the hepatocytes and subsequently conjugated with glucuronic acid. In this state, bilirubin is soluble in water and it is called conjugated bilirubin.
Conjugated bilirubin is excreted into the bile ducts and enters the duodenum. During its transport to the colon, it is converted into urobilinogen by the bacterial enzyme bilirubin reductase. Most of the urobilinogen is further reduced into stercobilinogen and is excreted through feces.
A lesser amount of urobilinogen is re-absorbed into portal circulation and transferred to the liver. For the most part, this urobilinogen is recycled to conjugated bilirubin and this process closes the enterohepatic circle. There is also an amount of urobilinogen which is not recycled, but rather enters the systemic circulation and subsequently the kidneys, where it is excreted. Air oxidizes urobilinogen into urobilin, which gives urine its characteristic color.
In parallel, a small amount of conjugated bilirubin can also enter the systemic circulation and get excreted through urine. This is exaggerated in various pathological situations.

Toxicity

Hyperbilirubinemia

is a higher-than-normal level of bilirubin in the blood. Hyperbilirubinemia may refer to increased levels of conjugated, unconjugated or both conjugated and unconjugated bilirubin. The causes of hyperbilirubinemia can also be classified into prehepatic, intrahepatic, and posthepatic.
Prehepatic causes are associated mostly with an increase of unconjugated bilirubin. They include:
Intrahepatic causes can be associated with elevated levels of conjugated bilirubin, unconjugated bilirubin or both. They include:
  • Neonatal hyperbilirubinemia, where the newborn's liver is not able to properly process the bilirubin causing jaundice
  • Hepatocellular disease
  • * Viral infections
  • * Chronic alcohol use
  • * Autoimmune disorders
  • Genetic syndromes:
  • * 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
  • * Dubin–Johnson syndrome
  • * Crigler–Najjar syndrome
  • Pharmaceutical drugs
  • * Sulfonamides are contraindicated in infants less than 2 months old 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.
Post-hepatic causes are associated with elevated levels of conjugated bilirubin. These include:
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

Hemoglobin acts to transport oxygen which the body receives to all body tissue via blood vessels. Over time, when red blood cells need to be replenished, the hemoglobin is broken down in the spleen; it breaks down into two parts: heme group consisting of iron and bile, and protein fraction. While protein and iron are utilized to renew red blood cells, pigments that make up the red color in blood are deposited into the bile to form bilirubin. Jaundice leads to raised bilirubin levels that in turn negatively remove elastin-rich tissues. 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.
Jaundice is classified, depending upon whether the bilirubin is free or conjugated to glucuronic acid, into conjugated jaundice or unconjugated jaundice.

Kernicterus

Unbound bilirubin levels can be used to predict the risk of neurodevelopmental handicaps within infants. Unconjugated hyperbilirubinemia 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 hyperbilirubinemia, 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. In addition, recent studies point towards high total bilirubin levels as a cause for gallstones regardless of gender or age.

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. While the poor solubility and potential toxicity of bilirubin limit its potential medicinal applications, current research is being done on whether bilirubin encapsulated silk fibrin nanoparticles can alleviate symptoms of disorders such as acute pancreatitis. In addition to this, there have been recent discoveries linking bilirubin and its ε-polylysine-bilirubin conjugate, to more efficient insulin medication. It seems that bilirubin exhibits protective properties during the islet transplantation process when drugs are delivered throughout the bloodstream.

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 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 a bilirubinometer which shines light onto the skin and calculates the amount of bilirubin by analysing how the light is absorbed or reflected. This device is also known as a transcutaneous bilirubin meter.
Bilirubin is found in two forms:
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 can still be part of the "direct" bilirubin fraction. Likewise, not all conjugated bilirubin is readily available in solution for reaction or detection and therefore would not be included in the direct bilirubin fraction.
Total bilirubin measures both BU and BC. Total bilirubin assays work by using surfactants and accelerators 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.