Apoproteins are lipid-binding proteins present in chylomicrons, absorbed by enterocytes within the small intestines and colonocytes in the colon. Following initial absorption, they are then exocytosed into the lymphatic system and into venous blood along with Apoprotein A, which is found in HDL cholesterol or High-density lipoprotein cholesterol.
Apoprotein A facilitates cholesterol uptake by the liver and other tissues, allowing HDL cholesterol to help transport other cholesterol out of the blood plasma, hence being considered a "healthy cholesterol". Furthermore, a high protein to lipid ratio in HDL cholesterol due to apoprotein A increases its density.
Apoprotein B, found in LDL cholesterol or Low-density lipoprotein cholesterol, combines with receptors that bring LDL cholesterol into other cells of the body. A low protein to lipid ratio in LDL cholesterol due to apoprotein B decreases its density.
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