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The biosynthesis and metabolism of Cholesterol

There are 5 stages of biosynthesis of Cholesterol:

1) formation of Mevalonic acid

2) formation of active Isoprene

3) formation of Squalene

4) formation of Lanosterol

5) formation of Cholesterol

The Metabolism of Cholesterol:

Exogenic Cholesterol makes up approximately 500mg/d. but only 30% from this quantity is absorbed. Cholesterol is absorbed by Micelles. Then it is transported by Chylomicrons to liver. Almost half of Cholesterol of the body is synthesized (it is endogenic Cholesterol – 700-1000mg/d). thus, total pool of Cholesterol of the body makes up 1.2-1.5g. 90% of Cholesterol occurs in plasma membranes and is used for plastical purpose. 10% of Cholesterol occurs in blood plasma. LDL transfer Cholesterol from liver to extrahepatic tissues. Cholesterol is used for synthesis of Steroid’s hormones (incl. Sexual hormones, Glucocorticoids and Mineralocorticoids), vitamin D3. Unused Cholesterol is transported from extrahepatic tissues to liver by HDL. This Cholesterol is oxidized to bile acids.

Cholesterol occurs in bile and its makes up 0.5% from all substances of bile. Maxiaml solubility of Cholesterol of bile occurs in next relation: 5% of cholesterol:15% of Lecithine;80% of bile salts. The solubility of Cholesterol is decreased in deminishing of bile acids, Lecithine and water in bile. It results in the formation of bile stones.

About 500mg of Cholesterol per day is excreted by feces and 100mg through the skin.

“Pathologic chemistry of lipid’s metabolism”

The derangement of digestion and absorption of lipids

The reasons of derangement of digestion and absorption of lipids are next:

1) absence of bile;

2) derangement of entrance of pancreatic juice in a small intestine;

3) primary malabsorption;

4) receiving of a large amount of animal’s fats, for example mutton’s fat;

5) rapid intestinal peristalsis;

6) application of some drugs (for example, antibiotics – neomycin, chlortetracyclin);

7) hypocorticism;

8) excess of two-valent cations (calcium, magnesium) in the food and water. It promotes the formation of insoluble complexes of calcium and magnesium with FFA.

The derangement of digestion and absorption of lipids displays as fatty diarrhea.

A laboratory control includes the detection of: 1) total content of lipids in feces (in norm it is equal until 5g/day); 2) FFA of feces (in norm it is equal until 20mM/day); 3) stercobilin of feces (in norm it is equal 0.2-0.6g/day).

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