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Chapter: Biochemistry: Lipid Metabolism

Biosynthesis of phospholipids

Biosynthesis of phospholipids
All tissues synthesize phospholipids, but at different rates.

Biosynthesis of phospholipids


All tissues synthesize phospholipids, but at different rates. In all tissues except liver, phospholipids are synthesized, utilised and degraded in situ; while in liver large proportion of the phospholipids after synthesis is transferred to the plasma and as a matter of fact, liver is practically the sole source of plasma phospholipids. Phospholipids are mainly synthesized from glycerol as detailed below.


1.  Biosynthesis of Lecithins


Lecithins are otherwise called as phosphatidyl choline. The choline component of lecithins is derived by the stepwise methylation of ethanolamine which in turn is formed by the decarboxylation of serine. Serine is derived from the pathways of both carbohydrate and protein metabolism as shown below.


2.  Degradation of phospholipids by enzymes


Degradation of phospholipids are effectively catalysed by a group of hydrolytic enzymes known as phospholipases. Phospholipases are classified into four major types namely phospholipase A1,A2, C and D. The classification is based on their cleavage specificity.


3.  Lysolecithin formation


Lecithin is hydrolysed by the enzyme lecithinase. The lecithinases are also known as phospholipases. When lecithin is acted upon by the enzyme phospholipase A2 it is converted into lysolecithin and a free fatty acid.

Lysolecithin may also be formed by an alternative route involving lecithin cholesterol acyl transferase. This enzyme is synthesized in liver and found in plasma in appreciable amounts. This enzyme transfers the fatty acid moiety from the lecithin to the cholesterol to form cholesterol ester. Lysolecithin accounts much of the cholesterol ester in plasma.


4.  Effects of lysolecithin


Lecithinases are specific enzymes which hydrolyse lecithins to form free fatty acids, glycerol, phosphoric acid and choline. A lecithinase in cobra venom readily split off an unsaturated fatty acid in lecithin producing “lysolecithin”. As the name implies, lysolecithin is a potent red blood cell hemolysing agent. This is the explanation of the toxicity of snake venom, bee-sting and certain poisonous spiders.


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