In cholesterol transpo which is not needed
Cholesterol in the diet is absorbed from the intestine, and in company with other lipids, are incorporated int "chylomicrons" and also to some extent 'VLDL'. Of the cholesterol absorbed, 80 to 90 percent in the lymph is esterified with long-chain FA. Esterification may occur in the intestinal mucosa. In man, the total plasmacholesterol varies from 150 to 250 mg% (average 200 mg%), rising with age, although there are wide variations between individuals. The greater pa is found in the'esterified' form and is transpoed as "lipoproteins" in plasma. Highest propoion of circulating cholesterol is found in LDL (b-lipoproteins) which carry cholesterol totissues and also in HDL, which takes cholesterol to liver from tissues for degradation (scavenging action). However, under conditions, where the VLDL arequantitatively predominant, an increased propoion of plasma cholesterol will reside in this fraction. Dietary cholesterol takes several days to equilibrate withcholesterol in plasma, and several weeks to equilibrate with cholesterol in tissues. Free cholesterol in plasma and liver equilibrates in matter of hours. In general, freecholesterol exchanges readily between tissues and lipoproteins, whereas cholesterol esters do not exchange freely. Some plasma cholesterol ester may be formed inHDL as a result of transesterification reaction in plasma between cholesterol and FA in position-2 of lecithin which is catalysed by the enzyme lecithin-cholesterol acyl transferase (LCAT).Cholesterol balance in tissues: Many factors will determine the cholesterol balance at the cellular level.(a) Increase of cholesterol in cells:Increased synthesis of cholesterol.Hydrolysis of cholesterol ester by the enzyme "cholesterol ester hydro-lyase".Uptake and delivery of cholesterol in cells by circulating LDL (uptake by specific receptors).Uptake of cholesterol containing lipoproteins by 'non-receptor' mediated pathway.Uptake of free cholesterol by cell membranes.(b) Decrease of cholesterol in cells:Efflux of cholesterol from cells to HDL (scavenging action).Esterification of cholesterol by the enzyme Acyl- CoA-cholesterol acyltransferase (ACAT).Utilisation of cholesterol for synthesis of steroid hormones, viz. glucocoicoids, mineralocoicoids, Gonadal hormones.In liver cells: formation of cholic acid.Formation of Vit D3.Ref: M.N. Chatterjea - Textbook of Biochemistry, 8th edition, page no: 434,435
Liver
Kidney
Intestine
Fat
Metabolism of lipid