THE LIVER AND BILIRUBIN METABOLISM

The Liver and Bilirubin metabolism

·         Definitions 

•         Metabolism is the processes of the human body that provides energy, interconvert chemicals (requiring or providing energy) and help to carry out functions

•         Catabolism: degradation process that often produces energy or waste products

•         Anabolism: synthesis of new biochemicals for use in the body; often requires energy

•          Liver is the dark red organ in the upper part of the abdomen on the right side just beneath the diaphragm.

•         Bilirubin is the orange yellow pigment derived from the catabolism (break down) of haemoglobin

•         Jaundice is a yellow discoloration of the skin, sclerae (whites of the eyes) and mucus membranes caused by hyperbilirubinemia.

Other definitions:

·         Reticulo Endothelial System is a body defence system that consists variety of types of cells that have the ability to engulf or phagocytize substances such as foreign particles which include Liver, Bone marrow and Spleen

·         Biliverdin is a green bile pigment formed by catabolism of haemoglobin and is converted into bilirubin in the liver.

·         Haemoglobin is the iron containing protein attached to the red blood cells that transports oxygen from the lungs to rest of the body.

·         Unconjugated Bilirubin is free bilirubin that has not been conjugated with glucuronic acid.

·         Conjugated Bilirubin is bilirubin that has been taken up by liver cell and conjugated to form water solube bilirubin diglucuronide.

·         Urobilinogen is colourless compound formed in the intestine by the reduction of bilirubin.

·         Stercolinogen is a bilirubin metabolite and procursor of Stercobilin formed by the reduction of Urobilinogen

• Pre-hepatic jaundice is a condition in which a person’s skin and whites of the eyes are discoloured yellow due to an increased rate of haemolysis (breakdown of red blood cells). 
• Hepatic jaundice is a condition in which a person’s skin and whites of the eyes are discoloured yellow due to injury or disease of the Liver cells.
• Post-hepatic jaundice a condition in which a person’s skin and whites of the eyes are discoloured yellow due to biliary obstruction (obstruction of the bile flow). It is also called obstructive jaundice.
• Hyperbilirubinemia is increased levels of bilirubin in the blood 
• Porphyrin is an organic compound that contains four pyrrole rings.
• Pyrrole rins A porphyrin is an organic compound that contains four pyrrole rings. is a pentagon-shaped ring of four carbon atoms with a nitrogen atom at one corner (C₄H₅N).
• Icteric is relating to or affected with jaundice.
• Conjugation is a chemical reaction to add a molecule often to make a water soluble product.
• Oxidation is the addition of H+ sometimes in the presence of oxidation to increase the oxidation state of a molecule while Reduction is removing of H+ and addition of e- to decrease the oxidation state.

·         Hepatocyte: liver cells; the main site where liver function occurs

·         Biliary: pertaining to bile ducts or tubes draining bile away from the liver

·         Kupfer cells: immune defence cells in the liver

ACTIVITY: Aswer the following questions What is Bilirubin metabolism?

Answer: bilirubin metabolism is the end result of chemical reactions to degrade haeme from red blood cells to form conjugated bilirubin and reduce to urobilinogen to be easily excreted in faeces and urine.

Introduction to Bilirubin metabolism

•         An overview of the Liver

•         Bilirubin is mainly formed from the breakdown of haemoglobin in Reiculoendothelial cells of the Liver, Spleen and Bone marrow.

The structure and function of the liver 

•         Liver is the biggest and the most complex organ in the body with more than 500 functions. It lies immediately under the diaphragm, occupying most of the upper right part of the abdomen.

•         The liver is an "incredible major chemical and metabolic factory” in the human body,  producing many important chemical compounds needed to survive, such as bile, albumin, blood clotting factors, cholesterol, Vitamin E. Converts amino acids (proteins) and Plays a major role in maintaining normal blood sugar levels, an important source of energy for the brain, heart and muscles

The Structure.

•         The Liver is the largest gland in the body, i.e. In adults the liver weighs 2% of body mass.

•         Occupying the greater part the upper abdomen.

•         The liver is divided into right and left lobes by fossae for the gallbladder and the inferior vena cava. The right lobe of liver is the largest lobe, whereas the left lobe of liver is smaller wedge-shaped. The quadrate and caudate lobes are described as arising from the right lobe of liver, but functionally are distinct these are seen on the posterior side of the liver making it to have four lobes.

                    

                                 Macroscopic Structure of the Liver

Functions:

•         Eliminates various harmful chemicals from the body (remove or process alcohol, poisons and toxins). Treats about 1,300 ml of blood per minute, thereby acting as the body's "garbage disposal.

•         Performs numerous other functions such as regulating lipids or metabolizing prescribed and over the counter drugs, alcohol, and many other ingested chemicals such as caffeine, etc.  

•         Making bile which passes from the liver to the gut and helps to digest fats.

•         Helping to process fats and proteins from digested food and urea formation.

•         Storing glycogen (fuel for the body) which is made from sugars. When required, glycogen is broken down into glucose which is released into the bloodstream.

•         Synthesis, esterification and excretion of Cholesterol.

•         Regulation of blood volume and immune mechanisms.

•         Because about 1.3 to 1.5 liters of blood, passes in the liver within a minute, the liver is recognized as the blood enriched organ.

•         Making proteins that are essential for blood to clot (clotting factors).

•         The functional unit of the liver is the lobule

·         There are three major types of cells in the liver, the hepatocytes, the biliary epithelial cells and the Kupffer cells.

·         Liver lobules are hexagonal in outline and are formed by cubical-shaped cells; the hepatocytes arranged in pair, between them are columns of cells called sinusoids. Hepatic macrophages (Kupffer cells) are also present which function in destroy and ingest worn out cells and foreign particles in the liver.

·         A central vein is in the centre of each lobule. Central veins unite to form hepatic veins, which exit the liver on its posterior and superior surface and empty into the inferior vena cava.

·         Hepatic cord radiate out from the central vein of each lobule like the spokes of a wheel. The hepatic cords are composed of hepatocytes, the functional cells of the liver. The spaces between the hepatic cords are blood channels called hepatic sinusoids. The sinusoids are lined with a very thin, irregular squamous epithelium consisting of two cell populations; Endothelial cells and Hepatic phagocytic cells (kupffer cells).

·         A cleft-like lumen, the bile canaliculus lie between the cells within each cord

·         Canaliculi are small bile ducts that connect to larger bile ducts and ultimately the common bile duct for flow of bile to the duodenum, small intestine.

                                  

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The function of the hepatocytes 

The hepatocytes carries the metabolic functions and are responsible for;

• Bile production including bilirubin
• Storage of nutrients, vitamins, iron and copper
• Interconversion of nutrients (proteins, carbohydrates, fats, ketones)
• Detoxication of drugs and deamination of amino acids and ammonia forming urea
• Phargocytosis
• Sythesis of blood components  and coagulation factors ( haematopoiesis in foetal liver)

 Structure and function of Gallbladder

Structure of Gallbladder

·         The gallbladder is a pear-shaped sac lying on the visceral surface of the right lobe of the liver in a fossa between the right and quadrate lobes. It is about 8 cm long and 4 cm wide

·         The gallbladder has the following structures:

o   A rounded end (fundus of gallbladder), which may project from the inferior border of the liver,

o   A major part in the fossa (body of gallbladder), which may be against the transverse colon and the superior part of the duodenum;

o   A narrow part (neck of gallbladder) with mucosal folds forming the spiral fold.

The walls of the gallbladder is formed by three tunics which are

·         Inner mucous layer which folded into reggae and this allows it to expand.

·         The muscular layer which contains smooth muscles that allows the gallbladder to contract

·         The outer covering layer of the serosa

Function of the gallbladder;

·         It stores and concentrates the bile which is secreted by the liver

·         Concentration of the bile by up to 10 or 15 fold by absorption of water through the walls of the gall bladder

·         Release of stored bile

·         Gallbladder receives many small vessels from hepatic bed and cystic arteries a branch of right hepatic artery

·         Venous drainage is through the multiple small veins from the gallbladder bed.

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Organisation of the Biliary System 

·         Is the duct system for the passage of bile extends from the liver, connects with the gallbladder, and empties into the descending part of the duodenum. The connection of ducts begins in the liver parenchyma and continues until the right and left hepatic ducts are formed. These drain the respective lobes of the liver.

·         The two hepatic ducts combine to form the common hepatic duct, which runs, near the liver, with the hepatic artery proper and portal vein.

·         As the common hepatic duct continues to descend, it is joined by the cystic duct from the gallbladder. This completes the formation of the bile duct.

·         The bile duct continues to descend, passing posteriorly to the superior part of the duodenum before joining with the pancreatic duct to enter the descending part of the duodenum at the major duodenal papilla

·         Therefore the bile either empties directly into the duodenum or is diverted for minutes up to several hours through the cystic duct into the gallbladder,

·         The clinical importance of this system is if there is a tumour of the head of pancreas will cause obstruction to the bile duct and cause a condition known as obstructive jaundice.

Bile Production, Circulation and Functions 

Bilirubin Metabolic Pathway from Haeme Molecule (tetrapyrole

Metabolism incudes

•      Anabolism  (synthesis))

•      Catabolism  (degradation)

Pathway is the steps to achieve metabolism

Tetra = 4

Haemoglobin Degradation

Bilirubin pathway: Haem degradation

•      Within  cells of the reticuloendothelial system (i.e., phagocytes) in spleen

•      Breakdown in haem proteins results in  daily supply of bilirubin

•      Most from haemoglobin in aged RBCs (~75%)

•      Remaining (15-20%) from destruction of RBC precursors in bone marrow, turnover of haemoproteins in other tissues

Bilirubin Formation:  Biliverdin is an intermediate product. Biliverdin: Green pigment; reduced to Bilirubin

ACTIVITY:  Aswer the following questions  Where is biliverdin formed?  Why is bilirubin bound to albumin?

Answer:  Biliverdin is formed in the reticulo-endothelial system such as spleen.

Answer:  Bilirubin is bound to albumin because it is lipophilic and hydrophobic so not water soluble unless bound to albumin for transport in blood.

Bilirubin

•      Yellow-orange pigment (tetrapyrole)

•      Major waste product of haeme metabolism

•      Photoisomers exist

•      Normal state; hydrophobic/lipophilic so transported bound to plasma proteins

•      Cis form: upon exposure to light; more hydrophilic

•      Secreted directly into bile for excretion without further modification so basis of phototherapy

Bilirubin Transport & Further Metabolism:

Transported to liver (from spleen) bound to albumin and when released in liver, it enters hepatocyte

On the smooth ER, it is conjugated with glucoronic acid with the enzyme (UDP-glucoronyl transferase) forming -> Mono- and di- glucoronide conjugates

Question for Students:  What are the components of the reaction to conjugate bilirubin?

(Work in pairs and have present their answer.)

Answer: Summary of Bilirubin Conjugation

Bilirubin  + Glucoronic acid  -------(UDP-glucoronyl transferase)à

Bilirubin diglucoronide and Bilirubin monoglucoronide

Bilirubin metabolism in Small Intestine:

“Conjugated” bilirubin secreted by the hepatocytes into the biliary canaliculi (small bile ducts connecting to larger bile ducts)

Bilirubin is reduced by anaerobic bacteria  in the gut to

Forms of urobilinogens (3 colorless tetrapyrroles) which are

–     Stercobilinogen

–     Mesobilinogen

–     Urobilinogen:

Oxidized to bile pigments found in faeces to:  Stercobilin, mesobilin, urobilin

Question for Students:  What forms of bilirubin are found in blood and in urine? (Work in pairs and then they present their answers.)

Answer: Forms of Bilirubin

Blood (Serum):

–     Total (unconjugated and conjugated)

–     Direct (conjugated)

Urine

–     Direct (conjugated)

–     Water soluble form

·         The liver produces in the hepatocytes and secretes a large amount of bile each day into the bile ducts. Continuous bile formation is an important function of the liver, and bile is used as a vehicle for the secretion of bile acids

·         Bile salts are quantitatively the major constituents of bile and are circulated in the enterohepatic circulation between the liver and the small intestine with high efficiency.

·         Synthesis of bile acids is a major route of cholesterol metabolism. Bile is synthesised through a series of chemical reactions in hepatocytes then secreted into minute bile canaliculi that originate between the hepatic cells. The mechanisms involved in the transcellularmovement of bile salts across hepatocytes is poorly understood

·         Half of the cholesterol produced in the body is used for bile acid synthesis.

·         90% of excreted bile acids are reabsorbed by active transport in the ileum and recycled in what is referred to as the enterohepatic circulation which moves the bile salts from the intestinal system back to the liver and the gallbladder.

·         Clinical significance is, since bile acids are made from endogenous cholesterol, the enterohepatic circulation of bile acids may be disrupted to lower cholesterol.  Gall stones form when there is a problem with synthesis or excretion of bile and they are mostly formed from cholesterol.

·         Bile constitutes of water, mineral salts, mucous, bile pigment (bilirubin), bile salts which are derived from the primary bile acids and cholesterol

·         Bile serves as a means for excretion of several important waste products from the blood. These include especially bilirubin, an end product of haemoglobin destruction, and excesses of cholesterol.

Types and Causes of Hyperbilirubinaemia 

·      Activity: Buzzing

 3 questions:

·      What is kernicterus? 

·      What urine and serum bilirubin results are expected in hepatitis? 

·      What urine and serum bilirubin results are expected in gall stones?

 Answer:  Kernicterus is encephalopathy (disorder of the brain) related to increased bilirubin that leads to permanent brain damage

Answer: In hepatitis you expect to see Increased urine bilirubin, Increased total and direct serum bilirubin and Indirect bilirubin and direct serum bilirubin are both elevated

Answer: gall stones are post –hepatic so you expectedto see Increased urine bilirubin.

In a complete obstruction, urobilinogen is absent from the urine but urinalysis methods may not detect decreased urobilinogen levels

Types of Jaundice:

1.      Haemolytic jaundice (Pre hepatic)

2.      Hepatic jaundice (Intra hepatic jaundice)

3.      Obstructive jaundice (Post hepatic jaundice)

Jaundice is classified into three categories, depending on which part of the physiological mechanism the pathology affects. The three categories are:

•         Pre-hepatic jaundice (Haemolytic jaundice): The pathology is occurring prior the liver

•         Hepatic (Non obstructive hepatic jaundice): The pathology is located within the liver

•         Post-Hepatic (Obstructive jaundice): The pathology is located after the conjugation of bilirubin in the liver

1.0 Pre-hepatic jaundice (Haemolytic jaundice):

•         The word “pre-hepatic” would mean “before the liver”. Indeed, pre-hepatic jaundice has nothing to do with the liver. The liver is fine.  The problem has to do with excessive bilirubin production as a result of hemolysis - undue destruction of red blood cells. As mentioned above, bilirubin is derived from hemoglobin contained in red blood cells. The spleen and liver constantly monitor the circulation, removing old, damaged, or abnormal red blood cells from the circulation.

•         If the liver and spleen remove too many red blood cells from the circulation, too much bilirubin will be produced.  The liver will shift into overdrive, removing as much of the bilirubin as it can.

•         Eventually, the liver’s capacity to remove this excessive amount of bilirubin is exceeded, and bilirubin levels build up in the bloodstream, staining the tissues yellow.  Serum bilirubin would be increased from normal due to unconjugated bilirubin but urine bilirubin should be negative and urobilinogen increased.

2.0 Hepatic jaundice (Intrahepatic Jaundice)

•      Hereditary

        Gilbert’s syndrome (Small increase in unconjugated bilirubin)

–     due to decreased hepatic uptake and

–     due to decreased conjugation

•      Secondary

–     Interference of uptake in liver by drugs

•      Acetominophen, Paracetamol, alcohol

–     Or Infection

•      Viral Hepatitis

–     Reduced Excretion of Hepatic Bilirubin (Excretion into bile is rate-limiting step) and may be caused by hepatocellular damage

–     Leads to increase in conjugated bilirubin

–     Uptake and conjugation in liver is also effected

–     Increase in unconjugated bilirubin

•      Secondary causes include:

•      cirrhosis, hepatitis, cholestasis induced by synthetic steroids.

Serum total and conjugated bilirubin would be increased as would urine bilirubin and urobilinogen.

3.0  Posthepatic Jaundice (Obstructive jaundice)

–     Caused by an obstruction of the biliary tree such as gall Stones and cancer of liver or pancreas

•      Increased Conjugated bilirubin

–     other biliary metabolites, (bile acids) accumulate

–     Hyperbilirubinemia is proportional to obstruction

•      Characterized by pale colored stools, dark urine due to high amounts of urine bilirubin but negative urobilinogen.  Serum bilirubin would be increased from normal and mostly conjugated bilirubin.

Jaundice is the Yellow coloration of sclera, skin resulting from increased serum bilirubin levels (25-50mg/L) [hyperbilirubinemia] while  Icterus is the yellow colouration of serum or urine.

·         Gilbert disease: hereditary intrahepatic jaundice due to due to decreased hepatic uptake and decreased conjugation

·         Non haemolytic jaundice, it is failure of bilirubin trasportation  from sinusoid membrane to microsome region

·         Criger – najar disease.  It is adificiency of UDP- glucolonyl Transferase

·         Dubin Johnson. It is intrahepatic cholestasis which is caused by viral hepatitis,toxic cirrhosis and necrosis