Made by Intestinal Epithelial Cells

LIPOPROTEINS

5 types of lipoproteins: chylomicron, VLDL, IDL, LDL and HDL in order of increasing density and protein content.

Chylomicron  Made by intestinal epithelial cells.  high TAG content (85%)  large particles (80 - 500 nm)  rapid turnover in serum  released into lymphatic system and then to the general circulation (Except for mammary glands, liver and intestine are only tissues to secrete particulate lipid ). Particulate lipid cannot pass through capillary walls and therefore is released into lymphatic system and then to general circulation)  function: carry dietary TAGs (also cholesterol) from intestine to the liver and adipose tissue  chylomicron remnants left after release of fats return to and are taken up by liver.

VLDL - very low density lipoproteins (30 - 100 nm)  synthesized in liver  rapid turnover in serum  function: deliver endogenously synthesized fats (TAGs) to extrahepatic tissues (mainly muscle and adipose tissue)  Free fatty acids (FFA) are released by the action of lipases in the capillary walls and in tissues. Most of FFA is taken up into tissue although some is released into blood.  loss of TAGs and some protein results in increase in % cholesterol and VLDLs are converted to IDLs (also called VLDL remnants) and eventually to LDLs

IDL - intermediate density lipoproteins  formed from VLDL  converted to LDLs by further removal of TAGs, or taken up by liver by binding to the LDL receptor (required presence of ApoB-100)

LDL - low density lipoprotein (25 - 30 nm)  formed from VLDL as these loose TAGs to tissue (VLDL LDL LDL)  lipid core is predominantly cholesterol ester  bind to LDL receptor via ApoB-100 protein and are internalized and cholesterol made available to cells.  50 - 75% of LDLs are taken up by liver  function: primary mode of delivery of cholesterol to all tissues (primarily adipose as well as to liver) to be used as membrane component.  "bad" cholesterol: high levels of blood LDL correlate with risk for developing arteriosclerosis. (High intracellular cholesterol suppresses synthesis of the LDL- receptor leading to decreased uptake of LDLs. This can result in elevated levels of circulating cholesterol when dietary intake of cholesterol exceeds the body’s needs.)

HDL - high density lipoprotein (7.5 - 10 nm)  most soluble and protein-rich lipoprotein  secreted by liver as small protein-rich particle  function: scavenges surplus cholesterol from cell surfaces and other lipoproteins (chylomicron remnants, IDLs) and returns it to liver in the form of cholesteryl esters  converts cholesterol to cholesteryl esters (uses apolipoprotein lecithine-cholesterol acyl transferase)  "good" cholesterol, high levels of blood HDL correlate with low incidence of arteriosclerosis.  may also transfer cholesteryl esters to VLDLs and LDLs - allows excess cholesterol to be returned to liver by the LDL-receptor pathway as well as by the HDL-receptor pathway.