Gripping abdominal pain and jaundice

Jaundice is due to an abnormally high concentration of bilirubin, one of the bile pigments in the bloodstream.

Macrophages take up old red blood cells and oxidise the haem of haemoglobin to bilirubin.

Bilirubin is then conjugated with glucuronic acid in the live, and excreted in the bile.

(A number of metabolites of drugs and other foreign compounds (xenobiotics), as well as physiological compounds such as steroid hormones, are conjugated with glucuronic acid in the liver and excreted in the bile.)

Conjugated bilirubin is one of the main compounds that colour faeces. If bilirubin is not excreted in the bile, the faeces will be pale coloured. The bilirubin accumulates in the bloodstream, giving the skin and whites of the eyes a yellow colour, and is excreted in the urine, giving it a dark colour.

Jaundice may occur as a result of:

• obstruction of the bile duct, so that bile cannot leave the gall bladder
• liver disease (e.g. the various types of viral hepatitis, or alcoholic liver disease)

In liver disease the liver has a reduced capacity to conjugate bilirubin, so that there will be unconjugated bilirubin in the circulation.

In obstructive jaundice the liver can conjugate bilirubin normally, but there is impairment of its excretion in bile, so that there will be conjugated bilirubin in the circulation.

A blood test showed that Ms GS had a high concentration of conjugated bilirubin in her circulation, suggesting obstructive jaundice. She was sent for ultrasound scan, which revealed the presence of gall stones, as shown on the right.

She then underwent cholecystectomy (surgical removal of the gall bladder); the excised gall bladder and the gall stones are shown below:

Chemical analysis of the gall stones showed that they were composed mainly of cholesterol, with a small amount of phospholipid and some calcium salts.

Apart from electrolytes, the main constituents of gall bladder bile are as follows (mmol /L):

The bile salts are conjugates of cholic acid and chenodeoxycholic acid with glycine and taurine. Normally the ratio of glycine : taurine conjugates is about 3 : 1.

They are synthesised in the liver by hydroxylation and side-chain oxidation of cholesterol, followed by conjugation with glycine or taurine.

The bile salts are amphiphilic molecules, with a hydrophobic planar hydrocarbon ring and a hydrophilic region provided by the hydroxyl groups and the glycine or taurine conjugated to the carboxylic acid group above the plane of the ring. In an aqueous medium the bile salts will form micelles on their own, and in bile they also emulsify the cholesterol that is secreted in the bile, so keeping it in solution. The phospholipids in bile also help to emulsify the cholesterol.

The function of the bile salts in the small intestine is to emulsify dietary lipids into mixed micelles that are small enough to be absorbed across the intestinal mucosa.

Cholesterol is extremely insoluble in water, and it needs only a relatively small increase in the content of cholesterol, or decrease in the concentration of bile salts, in the bile for it to crystallise, forming gall stones. In many cases gall stones are asymptomatic, since they are small enough to pass through the bile duct into the duodenum.

This is due to the presence of an excessive amount of unabsorbed triacylglycerol in her faeces. As a result of impaired absorption because of failure of bile secretion she was unable to form micelles small enough to be absorbed across the intestinal mucosa.

This is due to transient obstruction of the bile duct as the gall bladder contracts to secrete bile into the duodenum in response to stimulation by the presence of food in the duodenum

The function of the gall bladder is to concentrate and store bile until it is required for secretion.

The compounds that will form bile are synthesised in hepatocytes and are secreted into the bile caniculi, shown in green in the diagram on the right. These drain into bile ducts that enter the gall bladder.

Not all mammals have a gall bladder - for example the rat does not, although mice do. This suggests that the gall bladder is not essential for lipid digestion and absorption, although bile itself is.

In the absence of a gall bladder, what will be secreted into the duodenum is hepatic bile, which is less concentrated than gall bladder bile:

This means that without a gall bladder what will be secreted into the duodenum is a larger volume of more dilute bile. In the small intestine much water is reabsorbed anyway, so secretion of dilute bile will not lead to dehydration.

It seems likely that bile salts are reabsorbed from the small intestine and recycled. Interestingly, they are not absorbed from the micelles that contain monoacylglycerol, non-esterified fatty acids, phospholipids and cholesterol. Instead, as the other lipids are absorbed into intestinal epithelial cells from the micelles, the remnants for micelles that consist more or less completely of bile salt conjugates. These are absorbed in the terminal ileum.

Analysis of bile shows the presence of four bile acids, conjugated with glycine and taurine: chenodeoxycholic and cholic acids, which are synthesised from cholesterol by isolated hepatocytes, and deoxycholic acid, as well as a small amount of lithocholic acid, neither of which is synthesised by isolated hepatocytes.

When the glycine and taurine conjugates of chenodeoxycholic acid and cholic acid were incubated with a mixed culture of faecal bacteria, free chenodeoxycholic acid and cholic acid were found in the incubation medium, together with two new compounds: lithocholic acid and deoxycholic acid.

Intestinal bacteria seem to be able to deconjugate the conjugated bile acids, and metabolise them further, to lithocholic and deoxycholic acids.

The fact that these intestinal bacterial metabolites are secreted in bile is further evidence that the bile salts are reabsorbed from the small intestine, either before or after bacterial metabolism, and are recycled.

Although some 75 mmol of bile salts are secreted each day, the total body pool is only about 7.5 - 12.5 mmol; each molecule is secreted and reabsorbed some 6 - 10 time daily.

Chenodeoxycholic and cholic acid are generally referred to as primary bile salts, because they are synthesised in the liver; lithocholic and deoxycholic acids are referred to as secondary bile salts.

Gall stones in a young man

CD is a 20 year old man who went to see his GP because of persistent abdominal pain, diarrhoea and weight loss. On examination his abdomen was distended, and the pain was localised in the lower right of this abdomen. After a series of investigations he was found to have Crohn's disease affecting the terminal ileum. Treatment with steroids was ineffective in reducing the intestinal inflammation, and eventually he underwent surgery to remove the inflamed region.

Two years after the surgery he developed jaundice and complained of "gripping abdominal pain" after meals. An ultra-sound scan showed that he had a distended gall bladder and a number of gall stones.

The terminal ileum is where bile salts are reabsorbed. If it is resected then he will have severely impaired reabsorption of bile salts. To a limited extent his liver can compensate for this by synthesising more bile salts from cholesterol, but in the more or less complete absence of entero-hepatic recycling of bile salts this will not be enough to meet requirements. If the concentration of bile salts in his bile is too low, then cholesterol will crystallise, forming gall stones.

Triacylglycerol provides between 30 - 40% of energy intake. If he is deficient in bile salts then he will have impaired formation of lipid micelles in the small intestine, and hence impaired absorption of the products of lipid digestion. He will therefore be generally undernourished.

Familial hypercholesterolaemia is one of a group of disorders of plasma lipoprotein metabolism. It is a genetic condition and without treatment patients develop severe atherosclerosis at an early age, suffering frequent heart attacks, and commonly die young.

Until the development of the fibrate and statin families of drugs that inhibit synthesis of cholesterol, patients suffering from familial hypercholesterolaemia were treated with oral ion exchange resins to bind bile acids in the small intestine and prevent their reabsorption.

The first and rate-limiting enzyme of bile acid synthesis is cholesterol 7-alpha hydroxylase. The expression of the cholesterol 7-alpha hydroxylase gene in the liver is down-regulated by bile salts. If the concentration of bile salts in the liver falls, then gene expression increases, so that there is increased synthesis of bile salts from cholesterol.

If reabsorption of bile salts is inhibited by feeding the ion exchange resin, there will be less repression of cholesterol 7-alpha hydroxylase, and more cholesterol will be used to synthesise bile salts, to make up for the failure of entero-hepatic cycling. This reduces the total body burden of cholesterol, and so slows the development of atherosclerosis.

You will consider cholesterol and plasma lipoprotein metabolism in later exercises.

Key points from this exercise:

• Haem from old red blood cells is oxidised to bilirubin, which is conjugated with glucuronic acid in the liver, and excreted in the bile.
• Jaundice is due to abnormally high concentrations of bilirubin in the bloodstream; the urine is dark because of excretion of bilirubin, and faeces are pale because of the lack of bilirubin.
• Liver disease leads to jaundice because of failure to conjugate bilirubin, so that what is present in the bloodstream is unconjugated bilirubin.
• Gall stones may obstruct the bile duct, preventing the secretion of bile. In this case it is conjugated bilirubin that is present in the bloodstream.
• Bile contains bilirubin, cholesterol, phospholipids and bile salts.
• Bile salts are synthesised from cholesterol by 7-alpha hydroxylation and side-chain oxidation. They are conjugated with glycine or taurine.
• Bile salt conjugates are amphiphilic and can form micelles alone or together with other lipids. They are are essential for the absorption of the products of lipid digestion.
• Gall stones consist mainly of cholesterol, which is very insoluble. Only a small increase in the cholesterol content of bile, or a small reduction in the concentration of bile salts, results in crystallisation of cholesterol to form gall stones.
• There is considerable entero-hepatic recycling of bile salts; they are reabsorbed in the terminal ileum, commonly after deconjugation and further metabolism by intestinal bacteria, then stored in the gall bladder and resecreted. The small body pool of bile salts may undergo 6 - 10 cycles of secretion and reabsorption each day.
• Cholecystectomy does not impair lipid absorption - the function of the gall bladder is to concentrate bile and store it until required for secretion. In the absence of a gall bladder the more dilute hepatic bile is secreted into the duodenum.
• Impairment of the reabsorption of bile salts leads to increased synthesis from cholesterol - this can be exploited to treat familial hypercholesterolaemia by feeding ion exchange resins that bind bile salts and prevent their reabsorption.