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In this article we will discuss about Choline:- 1. Introduction to Choline 2. Chemistry of Choline 3. Absorption and Storage 4. Requirements 5. Functions 6. Deficiency Symptoms 7. Metabolism.
Contents:
- Introduction to Choline
- Chemistry of Choline
- Absorption and Storage of Choline
- Requirements of Choline
- Functions of Choline
- Deficiency Symptoms of Choline
- Metabolism of Choline
1. Introduction to Choline:
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a. In 1934, Best and Huntsman discovered that choline deficiency produces fatty liver in rats.
b. Choline is very essential for life processes. It is an important metabolite although it cannot be classified as a vitamin. Furthermore, the choline requirement is more than most substances and the deficiency symptoms are suggestive of vitamin deficiency diseases. Hence, it is considered as vitamin.
2. Chemistry of Choline:
a. Choline is hydroxyethyl tri-methyl-ammonium hydroxide. The chemical structure is given (Fig. 15.27).
b. It is a strong base and a colourless crystalline compound.
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c. It is soluble in water.
d. Dilute solutions of choline (less than 4 per cent) are stable to heat.
3. Absorption and Storage of Choline:
Choline in the Free State present in food is easily absorbed from the small intestine through the portal vein into the general circulation. Excess ingestion of choline than the requirements is not stored in the tissues. Part of it is converted into phospholipids but a greater part is metabolised in the liver into trim ethylamine which is excreted in the urine.
4. Requirements of Choline:
Choline requirements of human being are not yet established. However, the diets commonly consumed contain large amounts of choline.
Normal level of free choline in plasma in male adults:
About 4.4 mg/ml.
5. Functions of Choline:
a. Acetyl choline is a chemical mediator of parasympathetic and other types of activity in the nervous system.
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It is formed as follows:
Acetyl cholinesterase is an enzyme present in many tissues which can hydrolyze acetylcholine to choline and acetic acid. Red blood cells can synthesize acetyl choline. Both choline acetylase and acetyl cholinesterase are present in red cells.
Choline acetylase is also present in brain, skeletal muscle, spleen and placental tissue. When choline acetylase becomes inactive due to drug inhibition or lack of substrate, the red cells lose their selective permeability and undergo hemolysis.
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b. Choline is a constituent of phospholipids which are essential constituents of all cells in the body.
c. Choline takes part in trans-methylation reactions in the formation of methionine from homocystine.
Choline 
Betaine
Betaine + homocystine → Methionine + di-methylglycine
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d. It prevents accumulation of fat in the liver of many animals.
e It is essential for growth of many animals.
f. Its deficiency reduces egg formation in the hen.
6. Deficiency Symptoms of Choline:
a. Deficiency of choline causes fatty liver in rats.
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b. In the young growing rat there is hemorrhagic degeneration of the kidneys and hemorrhage into the eyeballs and other organs and ultimately leads to cirrhosis.
c. In chicks, its deficiency causes slipped tendon disease in which there is a defect at the tibiotarsal joint of the bird.
7. Metabolism of Choline:
A. Biosynthesis:
a. Serine is decarboxylated to ethanol amine in presence of pyridoxal phosphate.
b. Ethanol amine is progressively methylated to choline by the incorporation of one- carbon fragment into a methyl group of methionine.
Serine → Ethanol amine → choline.
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Catabolism:
a. Choline is first oxidized to betaine aldehyde and then to betaine. Choline acts as a methyl donor only after oxidation to betaine.
b. After loss of methyl group in the use of homocystine to methionine, betaine is converted to di-methyl-glycine. Di-methyl-glycine produces N-hydroxy-methyl sarcosine after oxidation.
c. Sarcosine is then formed by transferring hydroxymethyl to tetrahydrofolic acid.
d. Sarcosine is then converted to glycine by oxidation.
e. Glycine is converted to serine by the addition of a hydroxymethyl group derived from formylated tetrahydrofolic acid derivatives.
