Joe_24
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Heys guys,

My lecturer has confused me in regards to lipoprotein metabolism, and any help would be appreciated.

Low density lipoproteins (the bad ones) are cholesterol rich where they take it to the peripheral tissues and deposit it. High density lipoproteins (the good ones) remove cholesterol from peripheral tissues and take it back to liver so it can be excreted out.

So my lecturer seems to think LDL go to peripheral tissues (adipose tissues) and pick up cholesterol to become HDL which can then return to the liver?
To me it is counter intuitive and just doesn't make any sense as they each have the complete opposite actions - why would they transform into the other?
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macpatgh-Sheldon
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Hi,

With due respect to your lecturer, you are not the only one he/she has confused - the lecturer himself/herself is confused.com .

The attachment of LDL to cholesterol does NOT produce HDL. Yes, LDL does supply the cell membrane with cholesterol [this is part of the structure of the unit membrane [the lipid bilayer]], and the LDL is synthesized by the RER (of course, as it has a protein part) then is attached to its lipid part in the Golgi, and transported to the cell membrane by an endosome, which becomes a secondary lysosome.

HDL, on the other hand, is known as a "cholesterol scavenger" and after entering a cell by a different mechanism than does LDL (latter enters by endocytosis), i.e. HDL binds to a hepatic cell surface receptor called SR-BI (scavenger receptor class B type I) and offloads its lipid part to the hepatocyte for degradation (the liver is the only organ that can dispose of appreciable amounts of cholesterol [by conversion to bile (cheno)acids]).

In short, HDL tends to lower tissue cholesterol [and serum cholesterol] while LDL tends to raise these.

Hence the reason why HDL cholesterol is called in lay terms, good cholesterol and LDL is called bad cholesterol.

A routine blood test in a person over 40 or in a patient after symptoms, family history or signs suggestive of myocardial ischaemia (IHD) will include a lipid profile, of which one part is the total cholesterolDL cholesterol ratio. A lower value for this is a good sign, and the upper limit of normal is about 5 [might vary slightly from hospital to hospital based on assay methods].

Hope this clarifies matters somewhat.

M
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macpatgh-Sheldon
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2nd line in penultimate paragraph should read: "total cholesterol to HDL cholesterol ratio" NOT a smiley haha!
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