Clinical diagnostic biochemistry - 8

Dr. Maha Al-Sedik2015

CLS 334

Transport of Fat: Lipoproteins

Chylomicrons

VLDL.

IDL.

LDL.

HDL

Lipoproteins

Lipids in plasma combined with apo lipoprotein and

transferred as lipoproteins.

Lipoprotein is arranged in the blood to be protein and

phospholipid to the outside and triglyceride and cholesterol to

the inside.

Spherical particles with a hydrophobic core (TG and esterified

cholesterol), and a hydrophilic surface (apoprotein, phospholipids

& free cholesterol).

Like Protein

Water soluble

Precipitated with cold ethanol

Can be separated by electrophoresis

LipoproteinsParticles found in plasma that transport lipids including cholesterol.

lipoprotein classes:• Chylomicrons: take lipids from small intestine to the blood.

• Very low density lipoproteins (VLDL): take endogenous lipid from

the liver.

• Intermediate density lipoproteins (IDL)

• Low density lipoproteins (LDL)

• High density lipoproteins (HDL)

LIPOPROTEINS

HDL

IDL

LDL

VLDL

Chylomicron

Density Electrophoresis

Apolipoproteins: The protein moiety on the surface of lipoproteins.

Major components of lipoproteins.

Classified by alphabetical designation (A to E)

Responsible for transport and redistribution of lipids among

cells.

Strengthen and maintain the lipoprotein structure.

Responsible for recognition of particle by receptors.

Cofactor and activator for enzymes involved in lipid metabolism.

Chylomicron contains the exogenous triglycerides and protein

( Apo B 48 , APO C II and APO E ).

It tends to float even without centrifugation.

High chylomicron leads to milky plasma.

It remains in its origin in electrophoresis.

Chylomicron

VLDL contains the endogenous synthetized cholesterol and

triglycerides and protein ( Apo B 100 , APO C II and APO E ).

It produces pre B band in electrophoresis.

High VLDL leads to turbid plasma.

VLDL

IDL contains equal amounts of cholesterol and triglycerides and

protein ( Apo B 100 , Apo C II and APO E ).

It produces slow pre B band in electrophoresis.

IDL is transformed to LDL by losing APO C II and APO E .

IDL

LDL contains mostly cholesterol ester , little triglycerides and

protein ( Apo B 100 ).

It produces B band in electrophoresis.

High LDL does not alter plasma clarity.

LDL enters the tissues through LDL receptor .

People with high levels of LDL are at high risk of developing

atherosclerosis.

LDL

HDL contains mostly 50 % protein and 20 % cholesterol , 30 %

phospholipid and traces of triglycerides.

It gives Apo C II and Apo E to the nascent chylomicron and

nascent VLDL.

It produces α band in electrophoresis.

High HDL does not alter plasma clarity.

High levels of HDL are good signs .

HDL

Exogenous Lipid Transport

Fatty acids are absorbed by the apical microvilli of mucosal cells.

Apo B48 is the structural protein of the chylomicron.

Now the chylomicron is called Nascent chylomicron.

Then it reaches the blood where it receives Apo C II and Apo E

from HDL.

Apo C II stimulates lipoprotein lipase enzyme.

Lipoprotein lipase digest the triglyceride in the chylomicron

transforming chylomicron to chylomicron remnants.

Chylomicron remnants enter the hepatocyte by Apo E receptor in

the hepatocyte.

Endogenous Lipid Transport

Endogenously synthesized cholesterol and triglyceride combine

with Apo B 100 to form VLDL.

VLDL from liver enters plasma.

VLDL combines with Apo E (binds to hepatocyte receptor) and

Apo C II (activates lipoprotein lipase).

LPL works on VLDL forming IDL.

IDL has two pathway, it may enter the hepatocyte through Apo E.

Or, IDL loses APO E and APO CII to form LDL.

LDL enters the tissues through LDL receptor ( APO B 100 ).

People with high level of LDL are at high risk of having diseases.

Laboratory investigations

for lipids

Important pre analytical considerations:

An individual's lipid and lipoprotein profile should be measured

only when the individual is in a metabolic steady state.

Subjects should maintain their usual diet and weight for at least 2

weeks before the determination of their lipids or lipoproteins.

Repeat sampling : The diagnosis should be confirmed on at least

2 specimens 2 – 4 weeks in between.

Subjects should not perform vigorous physical activity within the

24 hours before testing.

The patient should be fasting except for total cholesterol.

Sampling in sitting position: Individuals should be seated for at

least 5 minutes before.

specimen collection: The tourniquet should not be kept on more

than 1 minute during venipuncture.

Either serum or plasma should be used to measure total

cholesterol, triglyceride, and HDL cholesterol concentrations.

Blood specimens always should be considered potentially

infectious and therefore handled accordingly.

You should wait for at least 3 months after major surgery.

Laboratory investigations

for lipids

Standing plasma tests:

Observing plasma standing sample after 24 hours at 4oC:

Turbid or cloudy ---------------------------- VLDL

Layer of cream and clear below it ----------------------- Chylomicron

Clear ---------------------- LDL or HDL

Lipid profile:After fasting for about 12 hours.

Ultracentrifugation:Ultracentrifuge for 16 – 18 hours at 10 oc

( plasma lipoprotein free fraction ).

This method has been used as the standard by which the accuracy

of other methods are judged . Now it is only used in research.

Electrophoresis.

Chromatography:

Ion exchange and gel permeation chromatography are the most

common types. It is used only for research .

Total cholesterol = HDL + LDL + VLDL

VLDL = Triglyceride / 5

LDL = Total cholesterol – ( HDL + Triglyceride / 5 )

Friedewald formula

Reference: Burtis and Ashwood Saunders, Teitz fundamentals of Clinical Chemistry, 4th edition, 2000.

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