CC Lipids and Lipoprotein

LIPIDS

These are commonly referred to as fats, composed mostly of carbon-hydrogen bonds.

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These are primary sources of fuel; they provide stability to cell membrane and allow for transmembrane transport.

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These are insoluble in blood and water, but soluble in organic solvents (chloroform and ether).

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They require special transport mechanisms (lipoproteins) for circulation in the blood.

PHOSPHOLIPID (Conjugated Lipid)

It is the most abundant lipids derived from phosphatidic acid.

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It originates in the liver and intestine.

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It is produced from the conjugation of two fatty and a phosphorylated glycerol.

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It is an amphipathic lipid, which means it contains polar hydrophilic (water-loving) head groups and non-polar hydrophobic (water-fearing) fatty acid side chains the saturated fatty acid content of plasma phospholipids is reported to be an independent risk factor for atherosclerosis.

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It is similar in structure to triglycerides, except that they contain two fatty acids

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In the lungs, it is produced by type II pneumocytes in the form lamellar bodies.

150 – 380 mg/dL (serum)

PHOSPHOLIPID (Conjugated Lipid) reference value

phosphatidic acid

PHOSPHOLIPID is derived from

liver and intestine

PHOSPHOLIPID originates from

type II pneumocytes

In the lungs, PHOSPHOLIPID is produced by _______ in the form lamellar bodies.

70%

Forms of Phospholipids:

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1\. Lecithin/ Phosphatidyl choline

20%

Forms of Phospholipids:

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Sphingomyelins

10%

Forms of Phospholipids:

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Cephalin

*Sphingomyelins*

It is the only phospholipids in membranes that is not derived from glycerol but from an amino alcohol

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It is essential component of cell membranes (RBC and nerve sheath).

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It accumulates in the liver and spleen of patients suffering from Niemann- Pick disease (lipid storage disorder).

sphingosine

*Sphingomyelins is derived from the amino alcohol*

Niemann- Pick

*Sphingomyelins* accumulates in the liver and spleen of patients suffering from _____ disease (lipid storage disorder).

CHOLESTEROL /3- hydroxyl -5, 6- cholestene

It is a unsaturated steroid alcohol containing four rings, and it has a single C-H side chain tail similar to fatty acid.

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It is amphipathic = hydroxyl group in the A- ring is the hydrophilic part of cholesterol.

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It is found on the surface of lipid layers; synthesized in the liver.

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It is almost exclusively synthesized by animals; not catabolized by most cells – does not serve as a source of fuel.

estrogen

CHOLESTEROL transport and excretion are promoted by

5 years

It should be measured in all adults 20 years of age and older at least once every _____ (healthy individuals).

< 200 mg/dL

CHOLESTEROL

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desirable

200 -239 mg /dL

CHOLESTEROL

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borderline

240 mg/dL

CHOLESTEROL

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 High Cholesterol

progestins, glucocorticoids, mineralocorticoids, androgens and estrogens.

CHOLESTEROL *Precursor of five major classes of steroids:*

D3

A small amount of cholesterol, after being converted to 7 – dehydrocholesterol, can also be transformed to vitamin ___ in the skin by irradiation from sunlight

Cholesterol Ester (CE)

__Forms of Cholesterol:__

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70 %

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It is found in plasma and serum.

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It is the cholesterol bound to fatty acid (hydrophobic form).

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Because it is not charged, it is classified as a neutral lipid and is not found on the surface of lipid layers but instead is located in the center of lipid drops, and lipoproteins, along with triglycerides.

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It undergoes esterification by LCAT.

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Excess cholesterol is re-esterified by the microsomal enzyme acyl: cholesterol acyl transferase (ACAT) and is stored until it is needed.

*Lecithin – Cholesterol Acyl Transferase (LCAT)*

It is normally present in human plasma; it catalyzes the esterification of cholesterol (HDL) by promoting the transfer of fatty acids from lecithin to cholesterol which results in the formation of lysolecithin and cholesterol ester.

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It is synthesized in the liver.

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It enables HDL to accumulate cholesterol as cholesterol ester.

Apo – 1

Activator of LCAT

Free Cholesterol (FC)

30%

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It is found in plasma, serum and RBCs.

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It is a polar non esterified alcohol.

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It is produced via lysosomal hydrolysis and becomes available for membrane, hormone, and bile acid synthesis.

lysosomal hydrolysis

Free Cholesterol (FC) is produced via

Total cholesterol (TC) concentration

__METHODS for cholesterol__

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measured rather that it forms

2 mg/dL/year

§  Serum total cholesterol increases at ______ between 45 to 65 years old.

Cholestadienyl Monosulfonic Acid (green end color)

Liebermann Burchardt Reaction

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End product   

Cholestadienyl Disulfonic Acid (red end color)

Salkowski Reaction

           

End product

a.       Glacial Acetic Acid

b.      Acetic anhydride

c.       Concentrated H2SO4

__Color Developer Mixture (Liebermann Burchardt Reagent):__

5% to 15%

Bilirubin exceeding 5 mg/dL decreases cholesterol by

CDC reference method (Abell, Levy and Brodie Method)

It is uses hexane extraction after hydrolysis with alcoholic KOH followed by reaction with Liebermann – Burchardt color regent.

TRIGLYCERIDES/TRIACYLGLYCEROL (Neutral Fat)

It contains 3 molecules of fatty acid and one molecule of glycerol by ester bonds.

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It does not contain charged or hydrophilic groups – very hydrophobic and water insoluble.

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It is the main storage lipid in man (adipose tissue) – constitutes 95% of stored fat and the predominant form of glyceryl ester found in plasma.

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It allows the body to compactly store long carbon chains (fatty acids) for energy that can be used during fasting states between meals.

epinephrine and cortisol

The breakdown of TAG is facilitated by lipoprotein lipase (LPL)

95%

TAG constitutes ______% of stored fats

60g – 130g

An average person ingests, absorbs, resynthesizes, and transports about ______ of fat daily in the body, mostly in the form of triglycerides.

TAG Reference value

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normal

150-199 mg/dL

TAG Reference value

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borderline high

200-499 mg/dL

TAG Reference value

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High TAG

>500

TAG Reference value

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Very high TAG (acute and recurrent pancreatitis)

50%

Postural changes decrease TAG levels by almost ___ (upright to supine position).

ascorbic acid, bilirubin and hemolysis

TAG interference

2 mg/dL/year

TAG level increases at _____ between 45 to 65 years old.

Glycerol Kinase Method

It involves hydrolysis of triglycerides to free fatty acids and glycerol, followed by the phosphorylation of glycerol to glycerophosphate.

CDC reference method (Modified Van Handel and Zilversmith)

It is a time-consuming manual method which cannot be automated.

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It involves alkaline hydrolysis (saponification) using alcoholic KOH, solvent extraction with chloroform and the extract is treated with silicic acid (chromatography) to isolate TAG, and a color reaction with chromotropic acid, giving rise to a pink end color.

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Interfering substance are removed during the extraction (with chloroform) and adsorption (silicic acid chromatography) steps.

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The purpose of the silicic acid is to remove phospholipids from the chloroform extract. 

pink

CDC reference method (Modified Van Handel and Zilversmith) end color

FATTY ACIDS

These are linear chains of carbon – hydrogen bonds that terminate with a carboxyl group.

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These are mostly found as constituents of phospholipids or triglycerides.

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These are mainly derived from hydrolysis of triglycerides in adipose tissues.

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Only small amount is present in plasma (free unesterified form), most is bound to albumin.

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The polyunsaturated and cis- monosaturated fatty acids are not associated with elevated serum LDL cholesterol.

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*Example:* palmitic acid, stearic acid, oleic acid, linoleic acid, and arachidonic acid

albumin

FATTY ACIDS are mostly bounded with

9-15 mg/dL

FATTY ACIDS reference value

4-6 carbon atoms

FATTY ACIDS

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short chain

8-12 carbon atoms

FATTY ACIDS

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medium chain

 carbon atoms

FATTY ACIDS

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long chain

without double bonds

saturated fatty acids

with double bonds

unsaturated fatty acids

*Lipoprotein lipase (LPL)*

hydrolyzes TAG and cholesterol esters in lipoproteins; hydrolyzes TAG realizing fatty acid and glycerol; it is present on the surface of capillary endothelial cells in adipose tissues, cardiac and skeletal muscles

*Hepatic lipase*

hydrolyzes TAG and phospholipids from HDL; hydrolyzes lipids on VLDL and IDL.

*Lecithin Cholesterol Acyl Transferase (LCAT)*

catalyzes the esterification of cholesterol from HDL; enables HDL to accumulate cholesterol as cholesterol ester.

*Endothelial lipase*

hydrolyzes phospholipids and TAG in HDL

ATP

*binding cassette protein A1 (ABCA1)* – for efflux of cholesterol from peripheral cells into HDL

LIPOPROTEINS

These are large macromolecular complexes of lipids with specialized proteins known as apolipoproteins.

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Main purpose: To transport TAG and cholesterol to sites of energy storage and utilization

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Cholesterol and triglycerides travel in plasma not as free-floating molecule, but as part of water- soluble complexes called *lipoproteins*.

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Vitamin E (fat – soluble vitamin) depends upon chylomicrons for absorption and relies upon VLDL and LDL for delivery tissues.

Apolipoprotein

It helps to keep the lipids in solution (solubility) during circulation through the blood stream.

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It aids in the solubilization of the lipids and also in their transfer from the gastrointestinal tract to the liver, which contains specific receptors for apolipoproteins.

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It interacts with specific cell-surface receptors and direct the lipids to the correct target organs and tissues in the body – present on the surface of lipoprotein particles.

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It contains a structural motif called an “amphipathic helix” – ability of proteins to bind to lipids.

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It maintains the structural integrity of the lipoprotein (LPP) complex.

amphipathic helix

Apolipoprotein contains a structural motif called an

Chylomicron (CM)

It is the largest and the least dense of the lipoprotein particles.

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It is produced in the intestine from dietary fat; completely cleared within 6 to 9 hours post prandial.

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It transports exogenous / dietary TAG to liver, muscles and fat depot.

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*Major composition:* 90% TAG (non-fasting plasma) + 1-2% protein

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*Apolipoproteins:* Apo B-48, Apo A-1, Apo C and Apo E

Apo B-48, Apo A-1, Apo C and Apo E

Chylomicron (CM) *Apolipoproteins*

Chylomicron (CM) density

Very Low-Density Lipoprotein /Pre-Beta Lipoprotein (VLDL)

It is secreted in the liver.

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It transports endogenous TAG from the liver to muscle, fat depots and peripheral tissues.

*Major composition:* 65% TAG (fasting plasma) +6-10% protein +16%CE

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*Apolipoproteins:* Apo B-100, Apo C and Apo E

Apo B-100, Apo C and Apo E

Very Low-Density Lipoprotein /Pre-Beta Lipoprotein (VLDL) *Apolipoproteins*

0\.95-1.006 kg/L

Very Low-Density Lipoprotein /Pre-Beta Lipoprotein (VLDL) density

High Density Lipoprotein / Alpha Lipoprotein (HDL)

It is the smallest lipoproteins but the densest (5-12nm).

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It is produced in the liver and intestine (nascent disk-shaped particles)

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It transports excess cholesterol from the tissues and return it to the liver *(reverse cholesterol transport)* – HDL maintains the equilibrium of cholesterol in peripheral cells by the reverse cholesterol transport pathway.

It transports effectively the lipids to the liver and more cardioprotective.

The phospholipids content of is more important than cholesterol or even apolipoprotein.

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C*DC reference method:* ultracentrifugation, precipitation with heparin – MnCI2 and Abell- Kendal assay (3step method)

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*Major composition:* 30% phospholipid + 45-50% protein +20% CE

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*Apolipoproteins:* Apo A-I, Apo A-II, Apo C

Apo A-I, Apo A-II, Apo C

Low Density Lipoprotein / Beta Lipoprotein (LDL) *Apolipoproteins:*

40mg/dL (cutoff level)

High Density Lipoprotein / Alpha Lipoprotein (HDL) Reference value

high risk for CHD

 >60m mg/dL

High HDL

1\.063 – 1.21 kg/L

HDL density

Low Density Lipoprotein / Beta Lipoprotein (LDL)

It is synthesized in the liver

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It is the major end product from the catabolism of VLDL.

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It constitutes about 50% of the total LPP in the plasma- the major source of cholesterol for tissues.

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It transports cholesterol to the peripheral tissues – it carries most of the circulating cholesterol and transports cholesterol to hepatic and extrahepatic tissues, where it is taken samples, LDL contains the cholesterol that is not present in HDL or VLDL.

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It is the primary target of cholesterol lowering therapy; primary marker for CHD risk.

50% CE +18% protein and phospholipid

Low Density Lipoprotein / Beta Lipoprotein (LDL) major component

Beta Quantification

Low Density Lipoprotein / Beta Lipoprotein (LDL)

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*Research method:*

Intermediate Density Lipoprotein (IDL)

It is a product of VLDL catabolism –VLDL remnant.

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It is converted to LDL – “subclass of LDL”

It migrates either in the pre beta or beta region (electrophoresis).

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Defective clearance of IDL in type 3 hyperlipoproteinemia is probably due to deficiency of A po E-III.

Apo B-100

Intermediate Density Lipoprotein (IDL)

*Major apolipoprotein*

1\.006 – 1.019kg/L

Intermediate Density Lipoprotein (IDL)

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*Density*