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Indirect and direct bilirubin: origins, properties and metabolism

 

WHAT IS BILIRUBIN?

Red blood cells
Red blood cells (RBCs)

Bilirubin is an organic compound, which is also a powerful colorant. Bilirubin causes bile to have its characteristic greenish-yellow color. Its excessive accumulation in body tissues is the main cause of jaundice. Bilirubin is constantly being generated as a byproduct of red blood cells (RBC) renewal. This is an ongoing physiological process.

Bilirubin is excreted from the body in bile.

SOURCES OF BILIRUBIN

Source of bilirubin
Blood hemoglobin is the primary source of bilirubin

Blood hemoglobin is considered to be the primary source of bilirubin in human's body. It's hemoglobin where 85% of all bilirubin comes from. A small amount of bilirubin is produced by all the body's tissues.

Hemoglobin is located inside RBCs. RBC lifespan is about 120 days. Worn out cells are replaced by new RBCs grown in bone marrow matrix. Once an aged RBC breaks down, hemoglobin is released with further dissociation into simpler components and their transformation into another chemical compounds. Bilirubin is among the products of hemoglobin breakdown. Hemoglobin degradation process occurs mainly in Kupffer cells of liver(80%), as well as in bone marrow and spleen cells.

TYPES OF BILIRUBIN

Bilirubin undergoes a series of complex biochemical transformations from the point of generation out of hemoglobin up to final elimination from the body. Each step involves synthesis of numerous transitional compounds.

There are two bilirubin types that have high practical value:

  • indirect bilirubin
  • direct bilirubin

So-called "direct and indirect" bilirubin are often referred to as "conjugated and unconjugated" bilirubin.

Actually, these words are not quite synonyms. According to scientific terminology, it's more correct to use the latter variant. Nevertheless, the terms "indirect" and "direct" still remain prevalent due to the fact they have been used in medicine for over a century. In order to avoid confusion, it should be noted that within this article and its continuations just traditional terms are being used.

Total bilirubin does not exist as chemical compound, but represents a total amount of direct and indirect bilirubin.

Initially just indirect bilirubin is formed. During metabolic processes indirect bilirubin turns into direct one. Bilirubin transformation occurs in hepatic cells. Properties of two types of bilirubin are essentially different.

Over recent decades theoretical concepts of bilirubin have changed significantly. In particular, some new forms of bilirubin with unusual characteristics have been discovered, and at this stage bilirubin is no longer considered an entirely harmful agent for human's body.

INDIRECT BILIRUBIN

Bilirubin toxicity
Basal nuclei of brain are the most susceptible to indirect bilirubin toxic effect.

Indirect bilirubin is a toxic substance. Nervous tissue is the most susceptible to its toxic effect.

Indirect bilirubin is fat-soluble, but it does not dissolve in water. Since all biological fluids of the body are acqueous solutions, indirect bilirubin cannot be eliminated from the body without changing of its chemical structure.

Despite of being water-unsoluble, indirect bilirubin is still capable of circulating throughout the body. For this purpose it will form a temporary association with albumins - proteins of blood plasma. Figuratively speaking, bilirubin and huge albumin molecule are like a rider and a horse.

With high level of indirect bilirubin, it is mainly stored in tissues with high concentration of lipids that are nervous and fatty tissues.

Fat-solubility makes it possible for indirect bilirubin to infiltrate protective phospholipid membranes of nervous tissue, exposing it to the toxicant. Even modest increase in indirect bilirubin level is associated with disorders of central nervous system function (undue fatiguability, memory impairment etc.).

However, in adults content of bilirubin never reaches a value high enough to cause nervous tissue irreversible damage. But it can happen to a newborn. In hemolytic jaundice of newborns indirect bilirubin level can be dozens of times higher. And it is likely to cause a damage of basal nuclei of brain.

Because of high toxicity of indirect bilirubin our body has to keep its content in tissues within safe range. For that purpose indirect bilirubin is converted into another substance called direct bilirubin, that dissolves well in biofluids.

Such transformation takes place in hepatic cells called hepatocytes through chemical bonding between indirect bilirubin and glucuronic acid. This biochemichal reaction requires the presence of specific enzyme glucuronyltransferase (UGT).

Transformation of indirect bilirubin into direct is one of the "bottlenecks" of bilirubin metabolism, because it is limited by UGT contents in hepatic cells. UGT content may drop below the critical value (3% of the norm) in certain hereditary diseases. The most prevalent one is Gilbert's syndrome. UGT can also be blocked by numerous drugs.

High level of indirect bilirubin could be related to the following conditions:

  • hemolysis (rapid destruction of too many red blood cells)
  • blocking of indirect-to-direct bilirubin transformation

DIRECT BILIRUBIN

Transformation of bilirubin
Bilirubin transformation requires the presence of specific enthyme glucuronyltransferase (UGT)

So, each molecule of indirect bilirubin in hepatic cell joins up with one or two molecules of glucuronic acid. From now on we're dealing with the new substance called "direct bilirubin".

In contrast to indirect bilirubin, this one is more "convenient" for the body, because:

  • it's only slightly toxic
  • its high solubility makes its disposal an easy task.

Under normal circumstances all direct bilirubin is excreted with bile into the intestine. With its high level in blood plasma it is also excreted via the kidneys in urine.

Presence of bilirubin in urine is a symptom of certain diseases of liver or bile ducts.

Each hepatic cell is attached to a blood capillary by one pole, and to bile capillary by another. It receives an indirect bilirubin through one pole from blood capillary, but does not return a direct bilirubin to blood circulatory system. Instead, direct bilirubin is being transported through opposite pole directly into bile capillary bypassing a bloodstream. Neutralization of many other substances in liver follows the same pattern.

Hence, under normal circumstances there is very little direct bilirubin in blood.

Numerous liver diseases (hepatitis, cirrhosis) are associated with high level of direct bilirubin, while indirect bilirubin level can remain normal or be slighly elevated.

Why is it so?

Transportation of direct blirubin from hepatic cell to bile capillary functions like pump: it goes in an unnatural direction from lower to higher concentration. Thus it has considerable energy demands. Then the disturbances of hepatic cells in some diseases entail an energy deficit in them. "Pump" is no more working.

Meanwhile even weakened cells partially or totally retain the ability to synthesise direct bilirubin, because this prosess is much less energy-intensive. Direct bilirubin is accumulated in hepatic cell first, and then spontaneously finds its way out of there right to a blood capillary.

What is more, in liver diseases slowing or stopping of bile flow in bile capillaries (cholestasis) is not unusual. Bile becomes so concentrated, that hepatic cells fail to add to it any amount of bilirubin.

Obstruction of bile ducts results in the same conditions.

BILIRUBIN DERIVATIVES: UROBILINOGEN AND STERCOBILIN

Bilirubin derivatives: urobilinogen and stercobilin
Bilirubin derivatives: urobilinogen and stercobilin

As we learned from previous section, direct bilirubin goes with bile through bile ducts into the intestine. But at this stage its metabolism is not over yet. In the intestine resident microorganisms subject it to further biochemical transformations. They synthesize a series of products from direct bilirubin, that share similar structure and properties. All of them are absolutely harmless for the body. They are referred to as urobilinoids, since urobilinogen is their parent compound. In the intestine it is converted into another substance called stercobilin. Stercobilin is responsible for characteristic colour of feces. Feces discoloration is an important diagnostic sign of acute hepatitis or obstructive jaundice, in which bile does not reach the intestine.

About half of urobilinogen is absorbed back into the bloodstream in the intestine. In fact, a venous network of the intestine is arranged in such a way that all venous blood with absorbed substances collects in a portal vein and passes through the liver. In there urobilinogen is being caught by hepatic cells. Then the liver handles urobilinogen in a very simple way: it reverts to become direct bilirubin once again and travels with bile into the intestine.

Hence, a part of bilirubin circulates throughout the liver and the intestine all the time, systematically turning into urobilinogen and back into bilirubin.

Under normal circumstances, only a small portion of urobilinogen (< 1%) is exreted via the kidneys in urine. That small quantities of urobilinogen cannot be detected by standard lab tests.

Following are conditions characterized by high concentration of urobilinogen in urine:

  • hemolysis: massive breakdown of RBCs → excess bilirubin → excess bilirubin derivatives
  • hepatitis: liver's ability to metabolize urobilinogen into bilirubin is reduced

The opposite is also possible, whereby there is no urobilinogen in urine AT ALL, because, for some reason, bile does not enter the intestine. No bilirubin in intestine → no urobilinogen in urine. It may be due to:

  • total blockage of extrahepatic bile ducts (so-called obstructive jaundice)
  • cholestasis in severe cases of acute hepatitis: bile flow in bile capillaries stops completely
in more detail

Urobilinogen in urine

The appearance of urobilinogen in urine is always the sign of a serious disease which may be liver disease or hemolytic anemia. In some conditions (obstructive jaundice) the absence of urobilinogen may be a symptom as well

DIFFERENCE BETWEEN DIRECT AND INDIRECT BILIRUBIN

Indirect Bilirubin Direct Bilirubin
Origins RBCs breakdown product (85%) synthesised out of indirect bilirubin
Sources of generation liver (Kupffer cells), bone marrow, spleen liver (hepatic cells)
Toxicity high low
Solubility practically unsoluble in biological fluids dissolves well in blood plasma, bile, urine
Excretion cannot be excreted in its initial form via bile ducts, also via kidneys (with high levels)
Negative effects (with elevated levels) toxic effects on nervous tissue formation of bilirubin gallstones (when overabundant in bile)
The main causes of buildup
  • hemolysis (massive destruction of red blood cells)
  • some hereditary liver diseases (Gilbert's syndrome etc)
  • certain medications overdose
  • liver diseases (hepatitis, cirrhosis etc)
  • obstruction of biliary tract (cholelythiasis etc)