A cholesteryl ester transfer complex in human plasma.

Immunoadsorption affinity chromatography has been used to define the structure of lipoproteins in human plasma containing lecithin:cholesterol acyltransferase (EC 2.3.1.43) (LCAT) and transfer protein (apo D). The whole of LCAT was absorbed by antibodies specific for apo D and for apo A-1, indicating that the enzyme is present in plasma exclusively as a complex with its cofactor (apo A-1) and product transfer protein (apo D). About 80% of apo D (but no LCAT) was removed by antibody to apo A-2, indicating the presence of most of apo D in the form of an enzyme-free complex will apo A-1 and apo A-2. After removal of LCAT with antibody to apo D, plasma was unreactive as a substrate with isolated LCAT, but substrate activity was generated by ultracentrifugal flotation with either intact or adsorbed plasma. The apparent stoichiometry of the complex with LCAT (LCAT:apo A-1:apo D) was 1.0:0.9:1.8; that of the complex containing apo A-1, apo A-2, and apo D was 3.9:2.2:1.0.