High density lipoproteins (HDL) form a class of lipoproteins,
varying somewhat in their size and contents, that carry
cholesterol from the body's tissues to the liver. Because HDL
can remove cholesterol from atheroma within arteries, and
transport it back to the liver for excretion, they are seen as
"good" lipoproteins. When measuring cholesterol, any contained
in HDL particles serves as protection to the body's
cardiovascular health. (In contrast to "bad" LDL cholesterol.)
HDL are the smallest of the lipoproteins. They are the densest
because they contain the highest proportion of protein. These
proteins are called apolipoproteins. The liver synthesises these
lipoproteins as complexes of apolipoproteins and phospholipid,
which resemble empty flattened spherical protein particles. They
are capable of picking up cholesterol, carried internally, from
cells they interact with. A plasma enzyme called
lecithin:cholesterol acyltransferase (LCAT) converts the free
cholesterol into cholesteryl ester (a more hydrophob