Bile pigments: how they are formed and what influences the process

Bile pigments are colored products of heme metabolism, primarily bilirubin and its derivatives. In clinical practice, the main pigment in this group is bilirubin, as its accumulation in the blood causes jaundice, and its transformation in the intestines produces the familiar color of feces and, to a lesser extent, urine. Modern sources emphasize that bilirubin and its metabolites impart the characteristic color to bile and feces, and when their metabolism is impaired, they become important diagnostic markers for diseases of the liver, biliary tract, and blood system. [1]

Biochemically, bilirubin is a metabolite of heme. Heme is a component of hemoglobin, myoglobin, cytochromes, and a number of other proteins. When these structures are broken down, the body must safely remove heme, as it is potentially toxic in its free form. Therefore, bilirubin formation is not a byproduct, but rather part of the normal heme utilization system. [2]

Most importantly, bilirubin exists in two main forms. The first is unconjugated, or indirect, bilirubin. It is poorly soluble in water and therefore circulates in the blood bound to albumin. The second is conjugated, or direct, bilirubin. It is already bound to glucuronic acid in the liver, becomes water-soluble, and can be excreted in the bile. This distinction underlies almost all clinical interpretations of hyperbilirubinemia. [3]

For practical medicine, the term "bile pigments" is also useful because it encompasses not only bilirubin itself but also its subsequent intestinal derivatives. After entering the intestine, bilirubin is converted to urobilinogen, and then to other colored compounds, including urobilin and stercobilin pigments. Therefore, the topic of bile pigments is essentially the topic of the complete metabolic fate of bilirubin, from red blood cell destruction to elimination from the body. [4]

From a clinical perspective, this is extremely important. If a patient has elevated total bilirubin, the physician must understand not just the fact of the increase, but at what stage of the chain the disruption has occurred: increased production, impaired hepatic uptake, slowed conjugation, the development of intrahepatic cholestasis, or the development of a mechanical obstruction to bile flow. Only this approach makes the topic of bile pigments truly useful for diagnostics. [5]

Table 1. The main bile pigments and their clinical significance

Pigment or metabolite	Where is it formed?	What does it mean?
Biliverdin	At the early stage of heme degradation	Intermediate green pigment
Unconjugated bilirubin	After restoration of biliverdin	Insoluble in water, carried with albumin
Conjugated bilirubin	In the hepatocyte after binding to glucuronic acid	Water-soluble, excreted in bile
Urobilinogen	In the intestines, under the influence of microbiota	Some is reabsorbed, some is excreted.
Urobilin	After further oxidation of urobilinogen	Participates in the coloring of urine
Stercobilin pigments	In the intestines and feces	They cause brown coloration of feces

The table is compiled based on modern reviews of bilirubin metabolism and guidelines on jaundice. [6]

How bile pigment formation begins: breakdown of red blood cells and heme catabolism

The majority of bilirubin is formed during the destruction of aging red blood cells. According to modern data, approximately 70%-90% of bilirubin originates from the heme of red blood cells, while classical reviews of physiology more often cite an estimate of approximately 80%. Red blood cells live for approximately 100-120 days, after which they are removed by cells of the reticuloendothelial system, especially in the spleen, liver, and bone marrow. [7]

After phagocytosis of the red blood cell, hemoglobin is broken down into globin, iron, and heme. Next, a key reaction is initiated: the enzyme heme oxygenase cleaves the heme ring. This is the rate-limiting step in the entire chain. The result is biliverdin, ferrous iron, and carbon monoxide. Important for modern biochemistry is that this is a strictly enzymatic and regulated process, not a simple "spontaneous breakdown" of hemoglobin. [8]

The next step is the conversion of biliverdin to bilirubin by biliverdin reductase. Biliverdin is green, and the resulting bilirubin is yellow-orange. This explains why the breakdown of blood in tissues can be accompanied by a sequential color change, for example, in the area of a hematoma. In normal physiology, this transition occurs primarily intracellularly in macrophages and cells of the reticuloendothelial system. [9]

Not all bilirubin comes solely from the hemoglobin of mature red blood cells. Some is formed from ineffective erythropoiesis in the bone marrow and from the breakdown of other heme-containing proteins, primarily myoglobin, cytochromes, catalase, and peroxidases. This is why hyperbilirubinemia can increase not only with overt hemolysis but also with other conditions associated with increased heme turnover. [10]

At this stage, bilirubin has already been formed but is not yet ready for excretion. It is hydrophobic, nearly insoluble in water, and potentially toxic, especially to nervous tissue. Therefore, the next vital step is its safe transport in the blood in a bound state. If this step is disrupted or if bilirubin is formed too quickly, unconjugated hyperbilirubinemia occurs. [11]

Table 2. Stages of early bilirubin formation

Stage	What's happening	Key result
Red blood cell aging	The erythrocyte is removed by cells of the reticuloendothelial system	Hemoglobin is released
Hemoglobin breakdown	Globin and heme are separated	Heme is subject to catabolism
Action of heme oxygenase	Heme is converted to biliverdin	Iron and carbon monoxide are released
Action of biliverdin reductase	Biliverdin is recovering	Unconjugated bilirubin is formed
Entry into the blood	Bilirubin binds to albumin	Safe transport to the liver is possible

The table is compiled from the NCBI Bookshelf and modern reviews of bilirubin metabolism. [12]

What happens to bilirubin in the blood and liver?

Unconjugated bilirubin is nearly insoluble in water, so it is transported in plasma in close association with albumin. This association limits its filtration in the glomeruli, reduces deposition in tissues, and helps deliver the pigment to the liver. Normally, free unconjugated bilirubin is present in very low levels in the blood, which protects tissues from its toxic effects. [13]

When the albumin-bilirubin complex reaches the liver, bilirubin is separated and transported to the sinusoidal surface of the hepatocyte. Recent studies indicate that both passive diffusion and specialized transport systems are involved in this uptake. More recent reviews specifically highlight the role of organic anion transport proteins 1B1 and 1B3, which help uptake bilirubin from the blood into the liver cell. [14]

The main step in bilirubin "neutralization" occurs within the hepatocyte: conjugation with glucuronic acid. This reaction is performed by the enzyme uridine diphosphate glucuronosyltransferase 1A1. First, bilirubin monoglucuronide is formed, then bilirubin diglucuronide. After this, the molecule becomes water-soluble and ready for active excretion in bile. Without this reaction, normal bilirubin elimination is impossible. [15]

After conjugation, bilirubin must leave the hepatocyte through the canalicular membrane. Multidrug resistance protein 2 plays a key role here. It transports conjugated bilirubin into the bile canaliculi. If this excretion is impaired, conjugated bilirubin begins to accumulate in the blood. In cholestasis or obstruction, some conjugated bilirubin can return to the bloodstream through other transport pathways, such as multidrug resistance protein 3. [16]

Clinically, the liver stage is particularly important because it is here that the boundary between indirect and direct hyperbilirubinemia is formed. Impaired uptake and conjugation often leads to a predominance of the unconjugated fraction, while impaired tubular excretion and bile flow lead to an increase in the conjugated fraction. This is why bilirubin biochemistry is so closely linked to the diagnosis of liver and biliary diseases. [17]

Table 3. Hepatic stage of bilirubin metabolism

Stage	What does the liver do?	What happens if there is a violation?
Blood Capture	The hepatocyte receives bilirubin from the sinusoidal side	Possible growth of indirect fraction
Intracellular binding	Bilirubin is retained and directed to the endoplasmic reticulum	Preparation for conjugation is deteriorating
Conjugation	Uridine diphosphate glucuronosyltransferase 1A1 makes bilirubin water-soluble	Unconjugated hyperbilirubinemia occurs
Tubular secretion	Conjugated bilirubin is excreted into bile	Conjugated hyperbilirubinemia occurs
Re-entry into the blood	Compensatory pathways return part of the conjugated bilirubin to the plasma	Urine darkens and stool becomes lighter in cholestasis

The table is compiled based on StatPearls, Merck Manual and modern reviews on bilirubin transport. [18]

What happens to bile pigments in the intestines?

After secretion into bile, conjugated bilirubin enters the duodenum and then moves through the intestines along with bile. At this stage, it is already water-soluble and no longer requires albumin. However, its journey doesn't end there: bilirubin then actively participates in intestinal microbial metabolism. [19]

The intestinal microbiota plays a key role here. Bacterial enzymes remove glucuronide residues and reduce bilirubin to urobilinogen. Modern reviews separately describe the involvement of bacterial beta-glucuronidases and microbial bilirubin reductase. In other words, without the intestinal microbiota, the normal bile pigment pathway would be impossible. [20]

Some urobilinogen then remains in the intestines and is converted into more oxidized pigments, which give the stool its brown color. The other portion is reabsorbed into the blood and returns to the liver through the portal system. This cycle is called enterohepatic circulation. Normally, the liver re-extracts a significant portion of these compounds and re-excretes them with bile. [21]

A small amount of urobilinogen enters the systemic circulation, is filtered by the kidneys, and, after oxidation, contributes to the color of urine. This is why, with normal bile pigment metabolism, urine has its usual yellow hue. However, unconjugated bilirubin itself is usually absent from the urine because it is bound to albumin and is not filtered by the glomeruli. Conjugated bilirubin appears primarily in the urine when it begins to circulate in excess in the blood. [22]

This intestinal stage is of great diagnostic importance. If bilirubin does not enter the intestine due to severe biliary obstruction, the stool loses its normal dark color, and the urine darkens. If conjugation is impaired, however, the picture is different: the stool may remain colored, but bilirubin is not detected in the urine. Therefore, the color of stool and urine remains a valuable clinical guide. [23]

Table 4. Intestinal fate of bilirubin

Stage	What's happening	Clinical significance
Entry into the duodenum	Conjugated bilirubin is excreted in bile.	Normal bile flow maintains the color of stool
Deconjugation and reduction	Microbiota produces urobilinogen	Normal stage of intestinal metabolism
Oxidation in the intestines	Fecal pigments are formed	The stool turns brown
Reabsorption of part of urobilinogen	Enterohepatic circulation occurs	The liver re-extracts metabolites
Renal excretion of a small portion	Urinary pigments are formed	Normal urine color is maintained

The table is compiled according to the Merck Manual and modern reviews of the role of intestinal microbiota in bilirubin metabolism. [24]

How disturbances at different stages of bile pigment formation cause jaundice

Jaundice occurs when bilirubin accumulates in the blood faster than the body can neutralize and eliminate it. According to current data, it becomes noticeable at a total bilirubin level of approximately 2-3 milligrams per deciliter, which corresponds to approximately 34-51 micromoles per liter. However, the pathogenic significance lies not in the occurrence of jaundice itself, but in the specific fraction predominating and at what stage of the chain the disruption occurs. [25]

If the problem begins before the liver, unconjugated hyperbilirubinemia usually predominates. This occurs with hemolysis, ineffective erythropoiesis, resorption of large hematomas, and some congenital disorders. In this situation, the liver may be anatomically normal, but it simply has to process an excessive flow of bilirubin. The Merck Manual identifies this as a classic mechanism for increased production. [26]

If uptake and conjugation in the liver are impaired, the indirect fraction also increases predominantly. This is how hyperbilirubinemia develops in Gilbert's syndrome and Crigler-Najjar syndrome. In Gilbert's syndrome, the disorder is usually mild, paroxysmal, and benign, and jaundice is often triggered by starvation, dehydration, illness, or menstruation. In Crigler-Najjar syndrome, the enzyme deficiency is much more severe and can lead to dangerous bilirubin encephalopathy. [27]

If bilirubin is already conjugated in the liver, but its excretion into bile is impaired, conjugated hyperbilirubinemia occurs. This occurs with intrahepatic cholestasis, biliary obstruction, hepatocellular dysfunction, and some hereditary syndromes, such as Dubin-Johnson and Rotor syndromes. Since conjugated bilirubin is water-soluble, it may appear in the urine, causing it to darken. At the same time, the stool often becomes lighter in color because less pigment enters the intestines. [28]

In practice, jaundice analysis is always based on this logic. The physician evaluates total, direct, and indirect bilirubin, urine and stool color, liver enzymes, signs of hemolysis, and bile duct imaging data. Therefore, the formation of bile pigments is not an abstract biochemical analysis, but the basis for diagnosing hemolytic, hepatic, and cholestatic jaundice. [29]

Table 5. What disturbances at each stage are caused by different types of hyperbilirubinemia?

Level of violation	What suffers	Which faction grows more often?	Typical examples
To the liver	Excessive production of bilirubin	Unconjugated	Hemolysis, ineffective erythropoiesis, hematomas
Liver seizure	Entry of bilirubin into the hepatocyte	More often unconjugated	Certain drug-induced and hereditary disorders
Conjugation	Function of uridine diphosphate-glucuronosyltransferase 1A1	Unconjugated	Gilbert's syndrome, Crigler-Najjar syndrome
Tubular excretion	Excretion of bilirubin into bile	Conjugated	Cholestasis, Dubin-Johnson syndrome
Bile outflow	Bile flow into the intestines	Conjugated	Stone, stricture, tumor obstruction

The table is compiled according to Merck Manual, StatPearls and MedlinePlus. [30]

Special clinical situations: newborns and hereditary bilirubin metabolism syndromes

Newborns have fundamentally different bilirubin metabolism than adults. They have a higher red blood cell mass per unit of body weight, a shorter red blood cell lifespan, a more immature hepatic conjugation system, and more active enterohepatic circulation. Therefore, physiological jaundice in newborns is extremely common and, in most cases, is transient. [31]

In newborns, unconjugated hyperbilirubinemia is particularly dangerous because unconjugated bilirubin can penetrate the blood-brain barrier and damage nerve tissue. StatPearls emphasizes that the immaturity of uridine diphosphate-glucuronosyltransferase plays a central role in newborns, and severe forms of hyperbilirubinemia can lead to bilirubin encephalopathy. Therefore, in infants, the same biochemical pathway has a much higher clinical cost. [32]

Gilbert's syndrome is the most common of the hereditary syndromes. This benign condition is characterized by decreased uridine diphosphate-glucuronosyltransferase 1A1 activity and intermittent elevations of the indirect bilirubin fraction. Typically, other liver function tests are normal, and treatment is not required. Classic precipitating factors include fasting, dehydration, intercurrent illnesses, and menstruation. [33]

A much more severe variant is Crigler-Najjar syndrome. In type 1, enzyme activity is virtually absent, while in type 2, it is sharply reduced. This leads to severe unconjugated hyperbilirubinemia, often within the first days of life. Severe forms are dangerous due to damage to the central nervous system and require early detection and treatment. [34]

A separate group consists of Dubin-Johnson and Rotor syndromes. In these syndromes, it's not conjugation that is affected, but rather the transport of conjugated bilirubin. Therefore, laboratory tests show a predominance of the direct fraction; bilirubin may appear in the urine, but significant cholestasis is usually absent. These syndromes are important as a differential diagnosis in patients with chronic benign conjugated hyperbilirubinemia. [35]

Table 6. Main hereditary and age-related variants of bilirubin metabolism disorders

State	The main defect	Which fraction is increasing?	Key feature
Physiological jaundice of newborns	Immature conjugation and increased enterohepatic circulation	More often unconjugated	Very common in the first days of life
Gilbert's syndrome	Decreased activity of uridine diphosphate-glucuronosyltransferase 1A1	Unconjugated	Benign course, provoked by stress and starvation
Crigler-Najjar syndrome type 1	Almost complete absence of the enzyme	Sharply unconjugated	High risk of encephalopathy
Crigler-Najjar syndrome type 2	Partial enzyme activity	Unconjugated	The course is milder than with type 1
Dubin-Johnson syndrome	Impaired excretion of conjugated bilirubin	Conjugated	Benign jaundice, bilirubin in urine is possible
Rotor syndrome	The transport and redistribution of conjugated bilirubin is impaired	Conjugated	Similar to Dubin-Johnson syndrome, but without liver pigmentation

The table is compiled from StatPearls, Merck Manual and reviews of hereditary hyperbilirubinemia. [36]

FAQ

What exactly is the main bile pigment in humans?
The main clinically significant bile pigment is bilirubin. It is formed during the catabolism of heme, then undergoes liver processing and is subsequently converted into intestinal and urinary pigments. [37]

Are bile pigments and bilirubin the same thing?
Not quite. Bilirubin is the main representative of this group, but the concept of bile pigments usually also includes its derivatives, which arise after hepatic and intestinal metabolism. [38]

Where in the body is bilirubin primarily produced?
The bulk is formed during the breakdown of aging red blood cells in the spleen, liver, and bone marrow, where heme is converted to biliverdin and then to bilirubin. [39]

Why doesn't unconjugated bilirubin appear in urine?
Because it's poorly water-soluble and bound to albumin in the blood, preventing it from passing through the kidney filter. Conjugated bilirubin appears primarily in urine. [40]

Why does urine darken and stool lighten with cholestasis?
Because conjugated bilirubin begins to enter the blood and is excreted by the kidneys, while less of it enters the intestines. This reduces the production of fecal pigments. [41]

Why is jaundice so common in newborns?
Because they have a higher bilirubin load, a shorter red blood cell lifespan, an immature conjugation enzyme, and more active enterohepatic circulation. [42]

Is Gilbert's syndrome a liver disease?
It is a hereditary benign disorder of bilirubin conjugation in which the indirect fraction is elevated, but serious chronic liver damage usually does not occur. [43]

Can bilirubin alone determine the cause of jaundice?
No. For a proper interpretation, it is necessary to evaluate the total, direct, and indirect fractions, liver enzymes, signs of hemolysis, urine, feces, and, if necessary, visualization of the bile ducts. [44]

Conclusion

The formation of bile pigments is a multi-step process that begins with the breakdown of heme, continues with the transport of unconjugated bilirubin with albumin, its hepatic uptake, conjugation, and tubular excretion, and culminates in intestinal metabolism and partial enterohepatic circulation. Biochemically, this is one of the most elegantly organized pathways for the detoxification of a potentially toxic heme breakdown product. [45]

For clinical practice, the key is different: each link in this pathway can break down in its own way. This is why one patient presents with hemolytic jaundice, another with Gilbert's syndrome, a third with cholestasis, and a newborn develops a specific age-related hyperbilirubinemia. Understanding the formation of bile pigments makes laboratory bilirubin values logical and allows for a more accurate interpretation of symptoms, rather than simply stating the presence of jaundice. [46]