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Mtabolism of LDL

 

VLDL is the precursor for IDL (intermediate density lipoprotein, or VLDL remnant).

IDL is then converted to LDL which represents the form by which cholesterol is transported from liver to the tissue.

 

Extra Edge
  1. Four major groups of lipoproteins are recognized: Chylomicrons transport lipids resulting from digestion and absorption. Very low density lipoproteins (VLDL) transport triacylglycerol from the liver. Low-density li­poproteins (LDL) deliver cholesterol to the tissues, and high-density lipoproteins (HDL) remove cholesterol from the tissues and return it to the liver for excretion in the process known as reverse cholesterol transport.
  2. Chylomicrons and VLDL are metabolized by hydrol­ysis of their triacylglycerol, and lipoprotein remnants are left in the circulation. These are taken up by liver, but some of the remnants (IDL), resulting from VLDL form LDL, which is taken up by the liver and other tissues via the LDL receptor.

 

  1. Lipoprotein (A)
    1. Lipoprotein (a) has nearly identical structure as LDL. However, it contains an additional apolipoprotein molecule, called apolipoprotein (a), which is covalently linked to apoB-100. Presence of lipoprotein (a) in blood in excessive amount increases risk of coronary heart disease.
    2. This could possibly be due to structural resemblance of this lipoprotein with plasminogen.
    3. These two proteins have about 80% sequence homology. Plasminogen is the precursor of a blood protease, plasmin that degrades fibrin, the major component of blood clots.
    4. Because of close structural resemblance, apolipoprotein (a) competes with plasminogen, theraby decreasing its conversion to plasmin.
    5. Reduced plasmin activity results in decreased degradation of clots. This, in turn, increases susceptibility of an individual to atherosclerosis. It is to be noted that apoproteins (a) is different from apoliprotein A.
  2. Hyperlipoproteins
    Disorder of plasma lipoproteins: include either hypo or hyperlipoproteinemia.
    These are either inherited defect in lipoprotein metabolism or associated disease such as diabetes mellitus, hypothyroidism or atherosclerosis.

Type

Primary disease

Basic defect

Serum Chol.

Serum TG

Elevated Lipoprotein

I

Familial Lipoprotein Lipase def.

LPG def*

N*

Increase*

Chylomicrons*

IIa

Familial Hyper cholesterolemia

Def of LDL receptors*

Increase*

N*

LDL*

IIb

Familial Mixed Lipoproteinemia

Unknown

Increase

Increase*

LDL Plus VLDL*

III

Familial type III Hyper Lipoproteinemia

Abnormal apo-E*

Increase

Increase*

---

IV

Familial Triglyceridemia

Diabetes associated

N*

Increase*

VLDL*

V

Familial combined hyperlipidemia

Def. of apoC*

N*

Increase*

Chylomicron Plus VLDL*





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