Unlocking the Secrets of Your Digestive System: A Deep Dive into the Biochemistry of Digestion and Absorption

Welcome to our article on the biochemistry of digestion and absorption. Here you will be diving deep into the complex processes that occur in our bodies to break down food and extract vital nutrients. This article is also useful for medical students because as healthcare professionals who want to take care of him/herself and their families, it is essential to have a comprehensive understanding of the complex processes that occur in our bodies to break down food and extract vital nutrients. The digestive system plays a crucial role in human physiology and a solid understanding of the biochemistry of digestion and absorption is essential for various healthcare professional exams such as BS Nursing, Paramedics, Nutrition, EATA NMDCAT, BS Biochemistry, Medical, and other competitive exams and entry tests.

Enzymes and Hormones of Digestive System
from Biochemistry Library of
Health, Education, and Skills

Introduction

The biochemistry of digestion and absorption begins in the mouth, where enzymes in our saliva start breaking down carbohydrates. As food moves through the esophagus and into the stomach, it is mixed with stomach acid and enzymes that further break down proteins. In the small intestine, bile and enzymes secreted by the pancreas aid in the digestion and absorption of fats, carbohydrates, and proteins.

The absorption process occurs mainly in the small intestine, where the walls are lined with finger-like projections called villi and microvilli. These structures increase the surface area for absorption, allowing for the efficient transport of nutrients into the bloodstream.

We will explore the biochemistry behind each step of the digestive process, including the role of enzymes, hormones, and other key players in the game. We will also discuss the importance of proper digestion and absorption for overall health and the potential consequences of disruptions in these processes. We hope you enjoy reading this article and gain a deeper understanding of the biochemistry of digestion and absorption!

What is Digestion?

The process of breaking down the larger food molecules into their simplest forms by the action of digestive juices, enzymes, and hormones secreted by various glands of the gastrointestinal tract is known as Digestion.

In most simple words, digestion is the breakdown of large food molecules (polymers) into smaller food molecules (monomers). But the main actors are interestingly some Chemicals such as Acids, Bases, Salts, Enzymes, Hormones, and some proteins. These chemicals broadly come from two sources i.e Digestive Juices, and Gastrointestinal Hormones, and that's basically what we are going to explore here in this journey of reading this post till the end.

Digestive juices, their enzymes, and hormones in them

Digestive juices are liquids produced by various organs of the digestive system that aid in the breakdown of food. They include enzymes and acids that help to break down carbohydrates, proteins, and fats in food.

Different types of digestive juices include Saliva, Gastric Juice, Pancreatic Juice, Bile, and Intestinal Juices.

I. Saliva

It is mainly secreted from three pairs of glands.

The Parotid Gland, its saliva is watery and rich in amylase.
The Sub-maxillary Gland or Sub-mandibular Gland, its saliva is rich in mucoproteins (Mucins: Which are a family of glycoproteins that are the major component of mucus), and
The Sub-lingual Gland.

Rate of Saliva Secretion

The rate of saliva secretion can vary greatly depending on a number of factors, including the individual's state of hydration, diet, and emotional state. On an adult can secrete between 0.5 to 1.5 liters of saliva per day or about 0.1 to 0.25 liters per hour. However, the rate of saliva secretion can be increased by certain stimuli such as food in the mouth, strong taste or smell, or even just thinking about food.

pH of freshly secreted Saliva

The pH of freshly secreted saliva is typically around 6.4 to 6.9, which is slightly acidic. This is due to the presence of lactic acid and other compounds produced by bacteria in the mouth. The pH of saliva can vary depending on a person's diet, oral hygiene, and other factors.

Chemical composition of saliva

Inorganic Constituents of Saliva include Na+, K+, Ca++, Mg++, Cl-, HCO3 (Bicarbonate), sulfate, phosphate, nitrate, iodide, and thiocyanate. The last one occurs mainly in smokers' secretion.
Organic constituents of saliva are Amylase (Ptyalin) and mucoprotein. Small amounts of Lactic Acid, Ascorbic Acid (Vitamin C), Glucose, Choline (an essential nutrient that is similar in structure to the B-vitamins), traces of enzymes like Phosphatase, Lipase, Carbonic Anhydrase, Lysozymes, Kallikrein, which causes secretion of Bradykinin (a potent vasodilator) and immunoglobulin A (IgA).

Parasympathetic and Sympathetic Stimulation of Saliva

The parasympathetic nervous system stimulates the production of saliva by activating the submandibular and sublingual glands via the release of the neurotransmitter Acetylcholine. This causes the glands to secrete more saliva, which then flows into the mouth to help with swallowing, digestion, and moistness.
The sympathetic nervous system does not stimulate the production of saliva. In fact, it has the opposite effect and tends to decrease saliva production. This is accomplished by releasing the neurotransmitter norepinephrine, which causes the blood vessels in the salivary glands to constrict, reducing blood flow and the amount of saliva produced. This decrease in saliva production is part of the body's "fight or flight" response.

Functions of Saliva

Lubrication

It is due to mucin present in saliva.
Saliva lubricates both Bolus and Pharynx and helps in swallowing the bolus.
A lubricated oral cavity makes talking comfortable.

Appetite Stimulation

Saliva stimulates taste buds which stimulate appetite.

Hygienic Action

Saliva maintains good oral hygiene.
Lysozymes present in saliva inhibit bacterial growth in the oral cavity and protect teeth from dental caries.

Digestion

Salivary Alpha-Amylase breaks down large molecules of carbohydrates like starch, glycogen, and dextrin and forms maltose (Glucose is almost negligible).

Note to remember: It should be remembered that Alpha Amylase breaks down carbohydrates into the smaller chain of sugar, but not into simple sugars e.g. Glucose because it is Beta-amylase, present in the pancreas and Small Intestines, which can break down the remaining sugars into simpler sugars like Glucose.

Excretion

Saliva excretes many substances that may get entry into the body. These include metals (lead), drugs, alkaloids (Morphine), and Antibiotics (penicillin, Streptomycin).

Regulation of Water Balance

In dehydration, the salivary secretion is suppressed causing dryness of the oral cavity and pharynx producing a sensation of thirst and forcing the person to drink water.

II. Gastric Juice

Gastric juice is a mixture of water, electrolytes, hydrochloric acid, enzymes, and mucus that is secreted by the stomach lining.
It is secreted from three types of cells in the stomach i.e.

Chief cells secrete Pepsinogen (the inactive form of Pepsin).
Parietal or Oxyntic cells secrete HCl and Intrinsic Factors.
Mucus cells secrete mucin.

The composition of Gastric Juice includes water, inorganic, and organic matter.

Inorganic constituents include HCl (most important), Na+, K+, and Phosphate (very less).
Organic constituents include Mucin, Pepsin, Gastric Lipase, traces of other enzymes, and intrinsic factor.

Gastric Juice is highly acidic. Its pH ranges from 1.3 to 2.6.
The amount of Gastric Juice secretion is about 2 to 3 Litres per day.

Properties and Functions of Gastric Juice constituents

a. Pepsin

Pepsin is an enzyme that is primarily responsible for the breakdown of proteins in food.
It is produced by the chief cells in the stomach and is activated by hydrochloric acid. Pepsin is a protease, which means that it breaks down proteins into smaller peptides by cleaving peptide bonds.
Pepsinogen, which is the inactive form of pepsin, is secreted by the chief cells in the stomach. When it comes into contact with hydrochloric acid in the stomach, it is converted to the active form of pepsin.
Pepsinogen is converted to pepsin by the action of acid on the molecule, specifically by the removal of a peptide fragment called the pepsinogen activation peptide.
Pepsin works best in an acidic environment, with a pH of 1-2, and it begins to denature and lose activity at pH greater than 4.
Pepsin is a powerful proteolytic enzyme. It is endopeptidase, therefore it hydrolyzes the central peptide bonds of proteins, which are then further broken down by other enzymes such as trypsin and chymotrypsin in the small intestine.
Pepsin has been found to have antimicrobial properties, which means that it can help to kill harmful bacteria present in the stomach.
Pepsin also aids in the absorption of some vitamins and minerals, such as iron, by breaking down the protein to which these nutrients are bound.
It's also worth noting that, pepsin can be found in the duodenum and other parts of the small intestine in small amounts.

b. HCl

Hydrochloric acid (HCl) is a strong acid (has a pH of about 1-2) that is produced by the parietal cells in the stomach.
It plays a key role in biochemical digestion by creating a very acidic environment in the stomach that helps to break down food and kill any potentially harmful bacteria present in the food and prevent them from growing and reproducing (this activity is known as Antimicrobial Activity).
The acidity of HCl is necessary for the activation of the enzyme pepsin.
HCl also plays a role in the activation of other enzymes such as lipases that aid in the digestion of fats.
HCl also plays a role in the breakdown of certain minerals and vitamins, such as vitamin B12, which are bound to protein in food.
The acidity of HCl breaks down the protein, making these vitamins and minerals more accessible for absorption.

c. Mucin

Mucin is a type of glycoprotein that is produced by the mucous cells in the stomach lining.
It provides a protective barrier for the stomach lining and aiding in the lubrication and mixing of food.
Mucin is composed of a protein core and a carbohydrate coating. The carbohydrate coating is composed of a variety of sugars, including Sialic acid and N-acetylglucosamine. This carbohydrate coating is hydrophilic, which means that it attracts and holds water. This allows mucin to form a thick and viscous gel-like substance that coats the surface of the stomach lining.
The mucous layer, which is formed by mucin, also protects the stomach lining from the corrosive effects of stomach acid and enzymes.
It also helps to neutralize the acidity of chyme (partially digested food) before it is pushed into the small intestine.
Mucin also has antimicrobial properties. It has been found that mucin can bind to and inhibit the growth of certain types of bacteria, such as Helicobacter pylori, which is associated with stomach ulcers.

d. Gastric Lipase

Gastric lipase is an enzyme that is produced by the chief cells in the stomach and is released into the stomach lumen.
Its main function is to break down fats in the stomach, allowing them to be absorbed and utilized by the body.
This enzyme breaks down the triglycerides into smaller molecules called fatty acids and glycerol. These smaller molecules can then be absorbed by the small intestine and utilized by the body for energy.
The secretion of gastric lipase is stimulated by the presence of food in the stomach, particularly fats.
It is important to note that gastric lipase is less active than pancreatic lipase. Pancreatic lipase is more efficient in breaking down triglycerides and also it is less sensitive to acid. Therefore, gastric lipase plays a less important role in fat digestion than pancreatic lipase.
Gastric Lipase is destroyed by Trypsin.

e. Intrinsic Factor

Intrinsic Factor (IF) is a glycoprotein produced by the parietal cells in the stomach.
Its primary function is to aid in the absorption of vitamin B12.
When food containing vitamin B12 enters the stomach, IF binds to the vitamin, forming a complex. This complex is resistant to the acid and enzymes in the stomach and is able to pass through the stomach and into the small intestine.
In the small intestine, the IF-vitamin B12 complex binds to specific receptors on the surface of the ileal cells, where it is taken up and transported to the liver via the portal vein. In the liver, vitamin B12 is separated from the IF and is available for use by the body.
Deficiency of IF results in a deficiency of Vitamin B12, which eventually causes Megaloblastic Anemia. Since the Vitamin B12 deficiency is due to the lack of IF, it is called "Pernicious Anemia".
Vitamin B12 is essential for the production of red blood cells, and a deficiency can lead to anemia, nerve damage, and cognitive impairment. Treatment for pernicious anemia typically involves the administration of vitamin B12 via injection or nasal spray.

Mechanism of Regulation of Gastric Juice Secretion

Secretion of gastric juice continues under all circumstances.
Secretion is caused by a small amount of Gastrin (a Gastrointestinal Hormone), released from the Pyloric Antrum (a region of the stomach that connects the body of the stomach to the pyloric sphincter) by Vagal stimulation. Where Vagal stimulation refers to the activation of the vagus nerve, which is one of the longest nerves in the body and runs from the brainstem to the abdomen.
Secretion of Gastric Juice in response to a meal can be divided into the following three phases

1. Neural or Cephalic Phase

Neural sensory stimulation is caused by the sight, smell, and taste of food and it increases Gastric secretions.
A Vaso-Vagal reflex (also known as the vasovagal syncope) can cause a temporary loss of consciousness due to a sudden drop in blood pressure and heart rate) and also cause secretion of gastric juice.
The juice secreted in response to vagal stimulation is very rich in HCl, Pepsin, and Mucin.

2. Gastric Phase

Starts when food enters the stomach and stimulates the secretion of gastric juice. The effect is through local neurons.
The effect is more profound in response to water-soluble products of protein digestion (peptides and amino acids), and alcohol.
These products cause the release of a hormone called Gastrin, from G-Cells (Entero-chromaffin Cells) of the Pyloric Antrum.
Gastrin enters the blood, reaches gastric glands, and stimulates Parietal and Chief cells to secrete a juice rich in Pepsin and HCl.

3. Intestinal Phase

Occurs when chyme (partially digested food) from the stomach enters the first part of the duodenum.
Digested lipids, proteins, and low pH of chyme (<2) cause secretion of three gastrointestinal hormones i.e. Secretin, Gastric Inhibitory Polypeptide (GIP), and Chole-cytokinin-pancreo-zymin (CCK-Pz).
Acidic chyme with these three hormones inhibits the gastric phase and initiates the intestinal phase of digestion.
Secretin causes Pancreas to secrete Pancreatic Juice rich in bicarbonate. It also inhibits the secretion of HCl by inhibiting parietal cells.
GIP along with secretin inhibits HCl secretion by the stomach.
CCK-Pz is the predominant hormone controlling this phase. It stimulates the gall bladder to contract and release the hepatic and pancreatic enzymes, stored in it, into the duodenum.
This phase represents only 10% of gastric secretion.

III. Pancreatic Juice

Pancreatic Juice is a clear solution containing water, inorganic, and Organic matter i.e

Inorganic Constituents of Pancreatic Juice include mainly bi-carbonate (HCO3-), Cl-, Na+, and small amounts of Phosphate, Ca++, and Mg++.
Organic Constituents of Pancreatic Juice include enzymes i.e. Trypsin, Chymotrypsin, Carboxypolypeptidase, Elastase, Collagenase, Pancreatic Lipase, Pancreatic Amylase, Deoxy-ribo Nuclease, Ribo-Nuclease, Phospholipase A2, and Cholesterol Ester Hydrolase. Their detail is given below

a. Trypsin

Trypsin plays a critical role in the digestion of proteins and the maintenance of homeostasis in the body through its enzymatic and regulatory functions.
It is secreted in a Zymogen form called Trypsinogen, which is converted to the active form by the action of the Enterokinase enzyme, and then by Trypsin itself (Autocatalysis).
Being Endopeptidase it hydrolyzes the central peptide linkage of protein.
It activates Chymotrypsinogen to Chymotrypsin.
Trypsin is also involved in the process of blood coagulation, by activating the serine protease factor Xa.
Trypsin has been shown to have anti-inflammatory properties and has been studied for its potential therapeutic use in treating certain inflammatory conditions.

b. Chymotrypsin

It is secreted in the Zymogen form called Chymotrypsinogen and is then converted to the active form Chymotrypsin, by the action of Trypsin.
After activation by Trypsin, Chymotrypsin specifically cleaves peptide bonds on the carboxyl side of large hydrophobic amino acids such as phenylalanine, tyrosine, and tryptophan.

c. Carboxypolypeptidase

It is an Exopeptidase and Hydrolyzes the terminal peptide linkage of protein, at the carboxyl end.
Carboxypeptidase plays a critical role in the digestion of proteins in the small intestine by breaking down large polypeptides into smaller peptides and amino acids that can be easily absorbed by the body.
It also helps to break down and remove any undigested proteins from the gut, preventing their absorption and the potential development of allergies.
It also has been found to have a role in the regulation of insulin and other hormones by cleaving off specific amino acids from the peptide hormone precursors.

d. Elastase

It specifically hydrolyzes Elastin, a protein that gives elasticity to connective tissue, and thus, it has a role in the maintenance of lung and skin elasticity.
In addition to its role in protein digestion, elastase has also been shown to have a number of other biological functions. For example, it is used in treating conditions such as cystic fibrosis, a genetic disorder affecting the lungs and pancreas.

e. Collagenase

Collagenase is a proteolytic enzyme that breaks down collagen (a major component of connective tissue such as skin, tendons, and bones). Collagen is a tough and fibrous protein that is resistant to degradation by most proteases, but collagenase is able to cleave the peptide bonds that hold the collagen molecules together.
Collagenase plays a critical role in the normal process of tissue remodeling and repair, allowing the body to break down and replace damaged collagen fibers.
However, an overproduction or abnormal activation of collagenase can lead to the destruction of healthy tissue, which is seen in certain pathological conditions such as osteoarthritis, rheumatoid arthritis, and periodontal disease.

f. Pancreartic Lipase

Pancreatic lipase is an enzyme that is produced and secreted by the pancreas.
Its primary function is to break down triglycerides into smaller molecules that can be absorbed by the body. This process occurs in the small intestine, where the lipase is able to interact with dietary fats.
Pancreatic lipase also has a secondary role in the absorption of fat-soluble vitamins, such as A, D, E, and K.
It is also important in the digestion of dietary fats and in the maintenance of normal blood lipid levels.

g. Pancreatic Amylase

Pancreatic amylase breaks down carbohydrates, specifically starch and glycogen, into simpler sugars such as glucose. This process occurs in the small intestine, where the amylase is able to interact with dietary carbohydrates.
Pancreatic amylase also plays a role in the digestion of dietary carbohydrates and in the maintenance of normal blood sugar levels.
Additionally, it is important in the metabolism of carbohydrates and in the absorption of nutrients.

h. Deoxyribonuclease

Deoxyribonucleases (DNases) play a key role in the digestion and absorption of DNA in certain organisms.
In the human digestive system, DNases are produced by the pancreas and are released into the small intestine to cleave DNA in the food we consume. This is especially important for the absorption of dietary nucleotides, which are the building blocks of DNA.
DNases break down DNA in the small intestine by cleaving the phosphodiester bonds between the nucleotides.
This process releases individual nucleotides, which can then be absorbed by the small intestine and used for various metabolic processes in the body.

i. Ribonucleases

Ribonucleases (RNases) cleave the phosphodiester bonds in ribonucleic acid (RNA) molecules.
They play important roles in many biological processes, including RNA metabolism, protein synthesis, and defense against viral and bacterial invasion.
In the human digestive system, RNases are produced by the pancreas and are released into the small intestine to cleave RNA in the food we consume. This is especially important for the absorption of dietary nucleotides, which are the building blocks of RNA.

j. Phospholipase A2

Phospholipase A2 (PLA2) catalyzes the hydrolysis of the sn-2 ester bond in phospholipids, releasing fatty acids and lysophosphatidylcholine.

What is SN2 Ester Bond?

SN2 (Second Order Nucleophilic Substitution) is a type of reaction mechanism in which a nucleophile (a molecule or ion with a lone pair of electrons) attacks the carbon atom of an ester bond, resulting in the substitution of the leaving group (such as a halide ion) with the nucleophile.

PLA2 is found in various tissues, including the pancreas, and is produced by many cells in the body, including immune cells, epithelial cells, and neurons.
In the human digestive system, PLA2 is produced by the pancreas and is released into the small intestine to cleave phospholipids in the food we consume.
PLA2 breaks down phospholipids in the small intestine by cleaving the sn-2 ester bond in the phospholipids. This process releases Fatty Acids and Lyso-phosphatidyl-choline, which can then be absorbed by the small intestine.

k. Cholesterol Ester Hydrolase

Cholesterol Ester Hydrolase (CEH) is an enzyme that catalyzes the hydrolysis of cholesterol esters to free Cholesterol and Fatty Acids.
In the gut, CEH is produced by Enterocytes and is secreted into the lumen where it acts on cholesterol esters present in dietary lipids.
The hydrolysis of these esters results in the release of free cholesterol, which can be absorbed by enterocytes and transported to the liver for further metabolism.
CEH also plays a role in the regulation of cholesterol metabolism by controlling the levels of free cholesterol in Enterocytes.
CEH deficiency leads to the accumulation of cholesterol esters in enterocytes and the liver, which can lead to dysfunction in the gut and liver.

Mechanism of regulation of the Pancreatic Juice Secretion

Secretion of Pancreatic Juice is regulated by Neurologenic and Hormonal mechanisms.

1. Neurogenic Mechanism

The Enteric Nervous System (ENS), which is a network of nerves that controls the function of the gut, plays a role in the regulation of pancreatic juice secretion through the release of neurotransmitters.
Acetylcholine, a neurotransmitter released by the Enteric nerves, stimulates the pancreas to release enzymes. It does this by binding to muscarinic receptors on the Acinar cells of the pancreas, which leads to an increase in intracellular calcium ions, resulting in the release of enzymes.
Substance P, another neurotransmitter, is released by the Enteric nerves and stimulates the pancreas to release bicarbonate. It does this by binding to Neurokinin-1 receptors on the duct cells of the pancreas, which leads to an increase in intracellular cyclic Adenosine Monophosphate (cAMP), resulting in the release of bicarbonate.
Sympathetic stimulation of the pancreas results in the inhibition of secretion.
Vagotomy inhibits secretion.

2. Hormonal (Humoral) Mechanism

a. Secretin

It is a gastrointestinal hormone, which is secreted from the Duodenum and the upper part of the small intestine.
It is released in response to HCl, which is present in food coming from the stomach.
The pancreatic Juice secreted in response to secretin is watery, and more alkaline but deficient in enzymes.
Secretin relaxes the lower part of the Esophageal Sphincter, decreases the activity of the Pyloric Antrum, increases HCO3- secretion in bile, increases secretion of duodenal glands, and increases the motility of the small intestine.

b. Cholecystokinin-Pancreozymin (CCK-Pz)

This gastrointestinal hormone is secreted from the upper part of the small intestine and duodenum.
It is released in response to proteins and fatty acids present in the food.
The pancreatic juice secreted in response to this hormone is rich in enzymes.
CCK-Pz causes contraction of the Gall Bladder.

What is CCk-PZ?

CCK-PZ (Cholecystokinin-Pancreozymin) is a hormone that plays a role in the regulation of pancreatic juice secretion.
It is a combination of two hormones:

Cholecystokinin (CCK): It is released in response to the presence of fats and proteins in the small intestine, and it stimulates the pancreas to release enzymes that help to digest these nutrients.
Pancreozymin (PZ), on the other hand, is released in response to the presence of partially digested food in the small intestine and stimulates the pancreas to release enzymes that help to break down carbohydrates, proteins, and fats. PZ also promotes the release of bicarbonate from the pancreas, which neutralizes the acidity of the chyme and helps to protect the duodenum from acid damage.

Both of these hormones are produced by specialized cells in the duodenum and jejunum, the first and second parts of the small intestine, respectively.

IV. Bile

Bile is a yellow-green fluid produced by the Parenchyma cells of the liver and stored in the gallbladder. Thus it is the secretion and excretion of the liver.
Ingestion of food causes the release of CCK-PZ which in turn causes contraction and emptying of gall bladder releasing bile into the 1st part of the duodenum.
Composition of Bile
It is composed of water, and following organic and inorganic matter.
Inorganic Constituents include Na+, Cl-, HCO3-, and K+.
Organic Constituents include Bile salts, Bile pigments (Golden yellow color Bilirubin and green-colored Biliverdin), mucin, and lipids (neutral fat, fatty acids, phospholipids, and cholesterol).
The pH of bile is alkaline (pH 7.8 to 8.6) due to the presence of HCO3-.
a. Bile Acids/ Bile Salts
The functions of bile are mainly because of the bile acids and bile salts present in it.
Primary Bile Salts: Cholic Acid and Chenodeoxycholic Acid are the primary bile acids synthesized by the liver from cholesterol.
The liver conjugates these two acids with Glycine and Taurine to form Glycocholic Acid and Glyco-cheno-deoxy-cholic acid, Taurocholic acid, and Tauro-cheno-deoxy-cholic acid.
In bile, these acids are present in the form of their corresponding sodium salts are known as "Bile salts".
Normally 90% of the bile salts are reabsorbed from the ileum, into the bile. This is known as the Entero-Hepatic Circulation of bile salts.
Functions of Bile salts
Bile salts act as emulsifiers, coating the fats and breaking them down into smaller droplets called Micelles. This process, called Emulsification, increases the surface area of the fats and makes them more accessible to the enzymes.
Bile salts also aid in the absorption of fatty acids and cholesterol by forming mixed micelles with these molecules. These mixed micelles are small enough to be absorbed by the enterocytes (intestinal cells) and are transported to the liver for further metabolism.
Bile salts also play a role in the absorption of fat-soluble vitamins, such as vitamins A, D, E, and K, by forming mixed micelles with these vitamins and facilitating their absorption.
They are strong Choleretic agents (substances that increase the production and/or flow of bile).
b. Bile Pigments
Bilirubin and to a lesser extent Biliverdin are the two bile pigments.
Bile pigments are formed from the breakdown of red blood cells and are responsible for the yellow-green color of bile.
They are not directly involved in the digestion or absorption of food.
Bilirubin (a breakdown product of hemoglobin) is further metabolized in the liver by the enzyme Bilirubin Uridine Diphosphate-Glucuronosyltransferase (BUGT) and conjugated with Glucuronic acid, forming Bilirubin Glucuronide.
This conjugated form is water-soluble and can be easily excreted by the liver into the bile and then into the intestine.
From there, it is eliminated from the body in the feces, giving them its characteristic brown color.
Bile pigments also play a role in maintaining the health of the liver by serving as indicators of liver function.
Elevated levels of bilirubin in the blood can be a sign of liver disease or hemolytic anemia, while low levels of bilirubin can indicate a blockage of the bile ducts. In this way, bile pigments can serve as biomarkers for liver health and disease.
c. Cholesterol
It is an excretory product of bile.
The amount of cholesterol excreted in the bile has no relation with the plasma cholesterol level.
The amount of cholesterol to bile salts is 1:20 to 1:30. Gallstones occur when the ratio of cholesterol to bile salts is increased.
d. Alkaline Phosphatase
It serves no function in the bile.
Raised level indicates obstruction to the biliary flow, which may be intrahepatic or extrahepatic. The condition is known as Cholestasis.
Factors affecting the bile secretion
Bile secretion is affected by several factors, including:
Diet: Due to the presence of fats in the diet Cholecystokinin (CCK) stimulates the release of bile.
Hormonal: The hormone secretin increases the flow of bile. Insulin and Glucagon also have an effect on bile secretion by regulating the metabolism of glucose, which in turn affects the bile flow.
Neural: The Enteric nervous system and the sympathetic nervous system regulate bile secretion through the release of neurotransmitters such as Acetylcholine and Noradrenaline.
Gallbladder Contractility: Factors that affect gallbladder contractility, such as certain drugs and medical conditions, can affect bile secretion.
Liver function: Bile is produced by the liver and any dysfunction in the liver can affect the production and release of bile.
V. Intestinal Juice (Succus Entericus)
Intestinal juice is made up of the secretions of two types of glands i.e Brunner's gland in the duodenum and the Intestinal gland or Crypts of Liberkuhn, present throughout the small intestine.
Secretion from the Brunner's gland has more mucus and HCO3-, which protects the duodenal mucosa from the injurious effects of gastric contents reaching the duodenum.
Intestinal juice has a higher content of HCO3- than blood.
Inorganic constituents of intestinal juice are almost the same as those present in other body fluids.
Organic constituents of intestinal juice include cellular debris mucus and enzymes.
Enzymes of Intestinal Juice
a. Peptidases:
Include Aminopolypeptidase, Tripeptidase, and Prolinase.
Aminopolypeptidase: This enzyme breaks down peptides (smaller chains of amino acids) into individual amino acids.
Tripeptidase: This enzyme breaks down tripeptides (chains of 3 amino acids) into individual amino acids.
Prolinase: This enzyme specifically breaks down the amino acid proline, which is found in many proteins. Proline is known to be resistant to many proteases, thus the function of prolinase is to break down this amino acid specifically.
All of these enzymes are located on the brush border of the small intestine and work in concert with other enzymes and the action of enzymes secreted by the pancreas to break down proteins into small molecules that can be easily absorbed by the body.
They complete the digestion of proteins, that have been partially digested by gastric and pancreatic juices.
b. Dissacharidases
Disaccharidases are a group of enzymes that break down disaccharides into their respective individual components.
These enzymes play an important role in the digestion and absorption of carbohydrates in the small intestine. For examples
Sucrase: breaks down the disaccharide sucrose (table sugar) into glucose and fructose.
Lactase: breaks down the disaccharide lactose (found in milk) into glucose and galactose.
Maltase: breaks down the disaccharide maltose (found in grains) into glucose.
c. Dextrinase
Hydrolyze dextrin into low molecular weight carbohydrates.
d. Phospholipases
Phospholipases play a role in the digestion and absorption of phospholipids, which are a type of fat found in many foods, particularly in cell membranes.
They hydrolyze Phospholipids into Lecithins and Cephalins.
There are three types of Phospholipases: Phospholipase A1, Phospholipase A2, and Phospholipase C.
Phospholipase A1: breaks down phospholipids at the sn-1 position, releasing a fatty acid and a Lysophosphatidic acid.
Phospholipase A2: breaks down Phospholipids at the sn-2 position, releasing a fatty acid and a Lysophosphatidylcholine.
Phospholipase C: breaks down Phospholipids releasing diacylglycerol and Phosphocholine.
e. Polynucleotidase, Nucleotidases, and Nucleosidases
All these enzymes play a role in the digestion and absorption of nucleic acids, which are the building blocks of DNA and RNA.
Polynucleotidases: These enzymes break down polynucleotides, which are chains of nucleotides, into individual nucleotides.
Nucleotidases: These enzymes break down nucleotides, which are the building blocks of DNA and RNA, into their individual components: a nitrogenous base, a sugar, and a phosphate group.
Nucleosidases: These enzymes break down nucleosides, which are composed of a nitrogenous base and sugar (Ribose), into their individual components: a nitrogenous base and a sugar.
Functions of Intestinal Juice

Since the pH of intestinal juice is alkaline, it helps in the neutralization of the acid present in the chyme coming from the stomach.
The mucus secreted by Brunner's gland and globet cells causes lubrication of the small intestine and also buffers the HCl present in the chyme.
Juice contains intrinsic factor, which helps in the absorption of Vitamin B12.
IgA present in the juice protects against bacteria.
Enzymes present in intestinal juice help in the digestion of food.
Mechanism of Regulation of Intestinal Secretion

The secretion of intestinal juices is regulated through a complex biochemical mechanism involving hormones, neural signals, and local factors.
Hormonal regulation: The secretion of intestinal juices is regulated by hormones such as secretin, cholecystokinin, and gastrin. Secretin, produced by the duodenum, stimulates the pancreas to secrete bicarbonate and enzymes, while cholecystokinin, also produced by the duodenum, stimulates the pancreas to secrete enzymes and the gallbladder to release bile. Gastrin, produced by the stomach, stimulates the release of gastric acid.
Other hormones which cause increased secretion are Gastric Inhibitory Peptide (GIP) and Vasoactive Intestinal Peptide (VIP)
Neural regulation: The enteric nervous system, which is a network of nerves within the gut, controls the secretion of intestinal juices through the release of neurotransmitters such as acetylcholine and gastrin-releasing peptide.
Local factors: The presence of food in the stomach and small intestine also plays a role in the regulation of intestinal juice secretion. For example, the presence of fat in the small intestine stimulates the release of bile from the gallbladder. Similarly, the presence of acid in the stomach stimulates the release of bicarbonate from the pancreas to neutralize the acid.

Biochemistry of Digestion and Absorption of Nutrients