1) Protein components of lipoproteins which remain after the lipids to which the proteins are bound have been removed; they play an important role in lipid transport and metabolism.

2) The protein component of lipoproteins.

3) 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).