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The protein component of any lipoprotein complexes that is a normal constituent of plasma chylomicrons, HDL, LDL, and VLDL in humans.

- a. A-I an a. found in HDL and chylomicrons. It is an activator of LCAT and a ligand for the HDL receptor. A deficiency of this a. has been associated with low HDL levels and with Tangier disease.

- a. A-II an a. found in HDL and chylomicrons. It stabilizes HDL.

- a. A-IV an a. secreted with chylomicrons and also found in HDL. It participates in the catabolism of chylomicrons and VLDL. It is also required for activation of lipoprotein lipase.

- a. B apolipoproteins found in chylomicrons, LDL, VLDL, and IDL. Elevated in the plasma of individuals with familial hyperlipoproteinemia.

- a. B-100 an a. found in LDL, VLDL, and IDL. The ligand for the LDL receptor; absent in certain types of abetalipoproteinemia.

- a. B-48 an a. found in chylomicrons and chylomicron remnants. Retained in intestine of individuals with chylomicron retention disease.

- a. C-I an a. found in VLDL and chylomicrons. It modulates the interaction of a. E with VLDL.

- a. C-II an a. found in VLDL, HDL, and chylomicrons; an activator of lipoprotein lipase; a deficiency will result in accumulation of chylomicrons and triacylglycerols.

- a. C-III an a. found in VLDL, HDL, and chylomicrons. It inhibits a number of lipases.

- a. D an a. found in HDL. It forms a complex with LCAT and appears to be involved in the transport of bilin.

- a. E an a. found in VLDL, HDL, chylomicrons, and chylomicron remnants. Elevated in individuals with type III hyperlipoproteinemia. It has an important role in cholesterol transport.

apo·li·po·pro·tein .ap-ə-.lī-pō-'prō-.tēn, -.lip-ō-, -'prōt-ē-ən n any of the proteins that combine with a lipid to form a lipoprotein and that are now grouped into four classes designated A, B, C, and E and formerly into a fifth class D now considered part of A often followed by the letter designating the class or by the letter and a number expressed in Roman or Arabic numerals to indicate a specific member of the class <\apolipoprotein B is a major component of LDL>

any one of a family of proteins found in the various lipoprotein, which transport hydrophobic lipid molecules in the plasma. Apolipoproteins have a range of molecular weights and perform a variety of functions during the life cycle of the lipoproteins in which they are found. These include acting as ligands for the binding of enzymes (apolipoprotein B) and as cofactors for the action of other enzymes (apolipoproteins A and C).