Approximately 50% of IDL /VLDL remnant goes for conversion to LDL . This is one of its fate. The LDL that is generated is full of cholestrol esters and the raito of cholesterol esters to the Triacylglycerols is high . Saying this, if the extra hepatic cell desire the cholesterol it can take up LDL . How is the LDL taken up by the extra hepatic cell ?Refer to the diagram and read the text . Apo B- 100 which is present on the LDL molecule is recognized by the LDL receptors present on the extra heptic cell. Along with the LDL , LDL receptors are also endocytosed . These receptors need to go back to the surface in order for the cell to take up more cholesterol. So,the receptor are recycled and then reach the surface . If any receptor is destroyed , that receptor after being re-synthesized reaches the surface. Now the lysosomes approximate to the LDL molecule & the enzymes in the lysosomes, that is, Lysozymes diffuses to the central core and converts the cholesterol esters in to free cholesterol
Remember, cell cannot utilize the cholesterol esters ,these are the free cholesterol which is utilized for various processes.
Now,what happens when the free cholesterol pool becomes in excess . The cell has devised 4 mechanism to drive out the free excess cholesterol/ convert it into storage form.These are the 4 mechanisms-
1) The free cholesterol is acted upon by Acyl CoA Cholesterol Acyl transferase enzyme which converts it back into cholesterol esters which is the storage form.
2) Free cholestrol decreases the re- synthesis of LDL receptors .Thereby, it decreases the surafce LDL receptors and upatake of receptors decreases
3)The free cholestrol decreases its own concentration by product inhibition
4)If all the mechanisms fail,then the cell restores to one last mechanism. Here, Liver comes to the rescue and takes up all the excess free cholesterol . This process requires HDL (high Density Lipoprotein) and ATP
To see how cell takes up cholestrol in a step – by -step manner click h/ ;bere