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Bile is a liquid solution that is produced and secreted by the liver. It is composed of bile salts, phospholipids, bile acids, conjugated bilirubin, cholesterol, water and electrolytes. The bile travels through the liver via many ducts and finally exits from the common hepatic duct. It is then transported to the gall bladder where it is concentrated and stored. The bile primarily functions to aid digestion of lipids and eliminate waste products from the body.
The bile acids in the bile are steroidal acids found in vertebrates and mammals. These bile acids conjugate with taurine or glycine to form bile salts. The bile salts are important for hepatobiliary (associated with liver, gallbladder and bile ducts) and intestinal digestion and homeostasis. The primary bile salts are synthesised in the liver by cholesterol in a series of enzymatic reactions. Secondary and tertiary bile salts are produced by alterations in the primary bile salts.
Structure of Bile Salts
The bile salts have a steroid nucleus with hydroxyl or other substituents. They also have an aliphatic side chain of varied lengths. In vertebrates, the carbon backbone is usually 24 carbons long whereas in predecessors it was seen to be 25-27 carbons long. In the bile, bile salts are found more predominantly at the physiological pH because pKa of glycine-conjugated salts ranges from 4-5 and that of taurine conjugated salts is 1.
Synthesis of Bile Salts
The bile salts are synthesised in the hepatocytes from cholesterol using upto 17 enzymes that are located in endoplasmic reticulum, mitochondria, peroxisomes and cytosol. The most common pathway for bile salt synthesis from cholesterol is called the classic or neutral pathway. The rate limiting step for the classic pathway is 7α-hydroxylation of cholesterol by microsomal cytochrome P-450 monooxygenase.
Alternative route for bile salt synthesis is called the acidic pathway. This pathway is initiated by a mitochondrial enzyme named 27-hydroxylase. The final step of bile salt is achieved in the liver where the full set of enzymes required for this conversion are found.
Chemical Properties of Bile Salts
The two major primary products of the complex biosynthetic pathway from cholesterol to bile salts are cholate (C) and chenodeoxycholate (CDC). Modification of the primary bile salt (dehydroxylation at C-7 of C or CDC) with bacterial enzymes results in the formation of secondary bile salts namely deoxycholate (DC) and lithocholate (LC). Epimerization at the C7 position of CDC leads to formation of tertiary bile salts called ursodeoxycholate (UDC).
UDC is the most hydrophobic salt followed by C, CDC, DC and LC. The taurine-conjugated bile salts are more hydrophilic than glycine ones.
Function of Bile Salts
Digestion of Lipids
The hydrophobic and hydrophilic properties of bile salts have the capability to form micelles at the lipid-water interface. The highly soluble nature of the bile salts prevents it from reabsorption in the small intestine. As a result, there is a high amount of bile acids/salts in the small intestine which helps in solubilisation of lipid molecules. The micelles formed by the bile acids help the lipases (enzymes that break down fat) in the digestion of lipids and brings them close to the intestinal brush border that aids in fat absorption.
Our body produces around 800 mg of cholesterol everyday, half of which (almost 400mg) is utilised for the production of bile acids. 12-18 g of bile acids are secreted in the intestines everyday after eating. Interestingly, the bile acid pool size is only 4-6 grams. This means that bile acids are recycled many times a day. Almost 95% of the bile acids are actively absorbed in the ileum, recycled and transported back to the liver for secretion in the gallbladder and biliary system. This is called the enterohepatic circulation of the bile acids.
- Eliminating cholesterol from the body
- Eliminating certain catabolites by driving the flow of bile
- Enabling absorption of vitamins by emulsifying fat-soluble vitamins
- Aiding the motility of bacteria flora found in the intestinal tract
- Regulating the activity of certain enzymes and ion channels
- Cholestasis: Impairment in the transport of bile salts by hepatocytes can lead to cholestasis. It is characterised by reduction in bile flow and retention of biliary constituents in various places such as the liver and serum. This causes biochemical, morphological and clinical changes in the body.
- Bile-acid diarrhoea: High concentration of bile acids in the colon can cause bile-acid diarrhoea. This condition arises due to malabsorption of bile acid in the ileum when it is not functioning properly or has been surgically removed.
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