METABOLISM OF CHOLESTEROL AND COMPLEX LIPIDS.

 CLASS № 26 

METABOLISM OF CHOLESTEROL AND COMPLEX LIPIDS 

THEORETICAL PART 

1. Metabolism of cholesterol in the body.

2. Biosynthesis of cholesterol: main steps, scheme. Regulation of cholesterol synthesis. 

3. Initial reactions of cholesterol biosynthesis. 

4. Bile acids: representatives, structure, metabolism, biological functions. 

5. Metabolism of sphingolipids. Disorders of sphingolipid metabolism. 

6. Transport of lipids and fatty acids in the blood. Role of albumins. General characteristics of lipoproteins. 

7. Metabolism of lipoproteins: formation and utilization. Lipoprotein lipase. Role of apoproteins.

1. Metabolism of cholesterol in the body.

Cholesterol is a steroid, characteristic only of animal organisms.

The main place of its 

⦁ synthesis in the human body is the liver, where 50% of cholesterol is synthesized, 

⦁ in the small intestine only 15-20% of cholesterol is formed, 

⦁ the remainder is synthesised in the skin, adrenal cortex and gonads. 

⦁ Sources of formation of cholesterol and the ways of its use are illustrated in the figure

Cholesterol in human organism (total amount about 140 g) can be divided into three pools: 

- pool A (30 g) quickly metabolized – includes the intestinal wall cholesterol, blood plasma, liver or other organs, renewal occurs after 30 days (1 g / day); 

- pool B (50 g), slowly metabolized cholesterol of other organs and tissues;

- pool C (60 g), very slowly metabolized cholesterol of spinal cord and brain, connective tissue, its renewal takes years.

2. Biosynthesis of cholesterol: main steps, scheme. Regulation of cholesterol synthesis. 

A little more than half the cholesterol of the body arises by synthesis(about 700 mg/d), and the remainder is provided by the average diet. 

The liver and intestine account for approximately 10% each of total synthesis in humans. 

Virtually all tissues containing nucleated cells are capable of cholesterol synthesis, 

⦁ which occurs in the endoplasmic reticulum and the cytosolic compartments.

Acetyl-CoA Is the Source of All Carbon Atoms in Cholesterol

Cholesterol is a 27-carbon compound consisting of four rings and a side chain 

It is synthesized from acetyl-CoA by a lengthy pathway that may be divided into five steps: 

(1) synthesis of mevalonate from acetyl-CoA 

(2) formation of isoprenoid units from mevalonate by loss of CO2 

(3) condensation of six isoprenoid units form squalene 

(4) cyclization of squalene gives rise to the parent steroid, lanosterol; 

(5) formation of cholesterol from lanosterol.

(1) synthesis of mevalonate from acetyl-CoA  

(2) formation of isoprenoid units from mevalonate by loss of CO2  

(3) condensation of six isoprenoid units form squalene 

(4) cyclization of squalene gives rise to the parent steroid, lanosterol;  

(5) formation of cholesterol from lanosterol  

Regulation of cholesterol synthesis 

The rate limiting step: HMG-CoA reductase step.  

I: mevalonate, cholesterol, bile acids  

Cholesterol and metabolites repress transcription of the HMG-CoA reductase gene.

Fasting and/starvation also inhibit the enzyme and activate HMG-Co lyase to form ketone bodies.  

Feeding cholesterol reduces the hepatic biosynthesis of cholesterol by reducing the activity of HMG-CoA reductase.  

Intestinal cholesterol biosynthesis does not respond to the feeding of high cholesterol diets.  

A second control point appears to be at the cyclisation of squalene and conversion to lanosterol, but details of the regulation at this step is not clear. 

ROLE OF HORMONES  

Insulin increases HMG-CoA reductase activity.  

Thyroid hormones stimulate HMG-CoA reductase activity.  

Glucagon and/glucocorticoids: decreases the activity of HMG-CoA reductase and reduces the cholesterol biosynthesis.  

Role of cAMP:  

• HMG-CoA reductase may exist in active/inactive forms,  
• which is reversibly modified by phosphorylation/and dephosphorylation mechanisms,  
• which may be mediated by cAMP-dependent protein kinases.  
• cAMP inhibits cholesterol biosynthesis by converting HMG-CoA reductase to inactive form. 

Possible mechanisms in the regulation of cholesterol synthesis by HMG-CoA reductase.

3. Initial reactions of cholesterol biosynthesis. 

4. Bile acids: representatives, structure, metabolism, biological functions. 

BILE ACIDS  

• Primary bile acids: synthesised in the liver from cholesterol.  

They are: Cholic acid(found in the largest amount in most mammals), Chenodeoxycholic acid.  

• Secondary bile acids: are produced in intestine from the primary bile acids by the action of intestinal bacteria.  

They are: Deoxycholic acid Lithocholic acid 

The primary bile acids are synthesized in the liver from cholesterol.  

These are cholic acid (found in the largest amount in most mammals) and chenodeoxycholic acid (Figure 26–7).  

The 7α-hydroxylation of cholesterol is the first and principal regulatory step in the biosynthesis of bile acids and is catalyzed by cholesterol 7α-hydroxylase, a microsomal cytochrome P450 enzyme–designated CYP7A1 (see Chapter 12).  

A typical monooxygenase, it requires oxygen, NADPH, and cytochrome P450.  

Subsequent hydroxylation steps are also catalyzed by monooxygenases.  

The pathway of bile acid biosynthesis divides early into one subpathway leading to cholyl-CoA, characterized by an extra α-OH group on position 12, and another pathway leading to chenodeoxycholylCoA (Figure 26–7).  

A second pathway in mitochondria involving the 27hydroxylation of cholesterol by the cytochrome P450 sterol 27hydroxylase (CYP27A1) as the first step is responsible for a significant proportion of the primary bile acids synthesized.  

The primary bile acids (Figure 26–7) enter the bile as glycine or taurine conjugates. Conjugation takes place in liver peroxisomes. 

In humans, the ratio of the glycine to the taurine conjugates is normally 3:1.  

In the alkaline bile (pH 7.6-8.4), the bile acids and their conjugates are assumed to be in a salt form—hence the term “bile salts.” 

Primary bile acids are further metabolized in the intestine by the activity of the intestinal bacteria.  

Thus, deconjugation and 7αdehydroxylation occur, producing the secondary bile acids, deoxycholic acid, and lithocholic acid. 

Functions of Bile Acids (Bile Salts)  

• Emulsification of fats  
• Accelerate the action of pancreatic lipase  
• Bile salts form ‘micelles’ with fatty acids, monoand diacyl glycerols and also TAG which are made water soluble and helps absorption.  
• They aid in the absorption of fat soluble vitamins.  
• They stimulate intestinal motility. 

Bile salts keep cholesterol in solution in gallbladder bile.  

In the absence of bile salts, cholesterol may get precipitated producing gallstones. 

5. Metabolism of sphingolipids. Disorders of sphingolipid metabolism 

Biosynthesis of ceramide 

Sphingolipidoses (Lipid storage diseases)  

A group of inherited diseases that are often manifested in childhood.

Treatment  

There is no effective treatment for many of these diseases.  

Recently some success has been achieved with enzymes that have been chemically modified to ensure binding to receptors of target cells, e.g. to macrophages in the liver in order to deliver βglucosidase (glucocerebrosidase) in the treatment of Gaucher’s disease.  

A recent promising approach is substrate reduction therapy to inhibit the synthesis of sphingolipids.  

Gene therapy for lysosomal disorders is currently under investigation. 

6. Transport of lipids and fatty acids in the blood. Role of albumins. General characteristics of lipoproteins. 

Lipids are insoluble in aqueous medium, therefore their transport in the body is performed by lipoprotein (LP, complex of lipids with proteins).  

There are exogenous and endogenous types of lipid transport:  

exogenous – transport of lipid, received from food, and  

endogenous – transport of lipids synthesized by the body.  

There are several types of LP, but they all have a similar structure  

 hydrophobic core and a hydrophilic layer on the surface.  

The hydrophilic layer is formed by proteins, called apoproteins (Apo),  

- amphiphilic molecules and lipids (phospholipids and cholesterol).  

The hydrophilic groups of the molecules are directed towards the aqueous phase and the hydrophobic groups are a central part of LP in which the lipids are transported.  

TRANSPORT OF CHOLESTEROL  

Cholesterol in the diet is absorbed from the intestine, and are incorporated into chylomicrons and also to some extent VLDL.  

The greater part of cholesterol is found in the esterified form and is transported as lipoproteins in plasma. 

Highest proportion of circulating cholesterol is found in LDL which carry cholesterol to tissues and also in HDL, which takes cholesterol to liver from tissues for degradation (scavenging action).  

Free cholesterol exchanges readily between tissues and lipoproteins,  

whereas cholesterol esters do not exchange freely. 

Some plasma cholesterol ester may be formed in HDL as a result of transesterification reaction in plasma between cholesterol and FA in position-2 of lecithin which is catalysed by the enzyme lecithincholesterol acyl transferase (LCAT) 

Transport of cholesterol between the tissues in humans 

Cholesterol balance in tissues: 

FACTORS THAT INFLUENCE CHOLESTEROL LEVEL and SYNTHESIS 

7. Metabolism of lipoproteins: formation and utilization. Lipoprotein lipase. Role of apoproteins. 

Apoproteins perform several functions:  

• form the structure of lipoproteins (e.g. B-48 – basic protein of chylomicrons (ChM), B-100 – the main protein of VLDL, LPID, LDL);  

• interact with receptors on the cell surface, determining what tissue is captured by this type of lipoproteins (apoprotein B-100, E);  

• are enzymes or enzyme activators acting on lipoproteins (C-II – activator of lipoproteinlipase, A-I – activator lecithin: cholesterol acyltransferase). 

When exogenous TAG resynthesize in enterocytes, they with phospholipids, cholesterol and proteins form ChM, and are secreted into the lymph first, and then enter the blood.  

In the lymph and blood HDL transfer apoproteins E (apoE) and C-II (apoC-II) into ChM, thus converting into "mature" ChM.  

ChM is quite large, so after a fatty meal they give blood plasma opalescent, like milk.  

Getting into the circulatory system, ChM quickly undergo catabolism, and disappear within a few hours.  

Decay time depends on hydrolysis of TAG in ChM by the action of lipoprotein lipase (LPL). 

This enzyme is synthesized and secreted by adipose and muscle tissues, cells of the mammary glands.  

Secreted LPL binds to the surface of endothelial cells of the capillaries of the tissues where it is synthesized.  

Regulation of secretion has tissue specificity.  

In adipose tissue LPL synthesis is stimulated by insulin, allowing the entry of fatty acids synthesis and storage in the form of TAG.  

In diabetes, when insulin deficiency is noted, the level of LPL is reduced.  

As a result, blood accumulates a large amount of PhL.  

In muscle, where LPL is involved in the delivery of fatty acids to oxidation between meals, insulin inhibits the formation of the enzyme. 

On the ChM surface there are two factors necessary for LPL activity – apoC-II and PhL.  

ApoC-II activates the enzyme and PhL is involved in binding the enzyme to the surface of ChM.  

As a result of LPL action on ChM TAG breaks down into fatty acids and glycerol. 

Thereafter, fatty acids are transported into the tissue, where they can be deposited in the form of TAG (adipose tissue) or used as an energy source (muscles).  

Glycerol is transported in the blood to the liver where in postabsorptive period may be used for the synthesis of fats. 

As a result of LPL activity the amount of neutral fats in ChM decreases by 90%, particle sizes decrease as well; apoC-II is transferred back to HDL. The formed particles are called residual XM(remnant).  

They contain PhL,Ch, fat-soluble vitamins, apoB-48 and apoE. 

Residual ChM enter hepatocytes that have receptors that interact with these apoproteins. Under the action of lysosomal enzymes, proteins and lipids are hydrolyzed, and then recycled.  

The fat-soluble vitamins and exogenous ChL are used in the liver or transported to other organs 

At endogenous transport, sinthesized in the liver TAG and PhL form VLDL, which

includes apoB100 and apoC. VLDL is the main form of transport for endogenous TAG.  

Once in the blood, VLDL gets apoC-II and apoE from HDL and converts into LPL.  

In this process, VLDL is first converted to LPID, and then LDL.  

Cholesterol becomes the main lipids of LDL and is transferred to the cells of all tissues.  

Formed during hydrolysis fatty acids enter the tissue and glycerol is transported to the liver by blood, which can be used again for the synthesis of TAG.