Characterization of Amyloid Proteins AA and SAA as Apolipoproteins of High Density Lipoprotein (HDL)

An AA‐like protein with a molecular weight of 8600 complexed to high‐density lipoprotein (HDL) was demonstrated in several acute‐phase sera with high levels of SAA. The protein ‘apo AA’(to distinguish it from tissue AA) was isolated by elution from sodium dodecyl sulphate (SDS)‐polyacrylamide gel, and showed antigenic identity with purified tissue protein AA in double immunodiffusion. Normal HDL was shown to bind purified tissue AA in vitro. When the in vitro‐associated HDL‐AA complexes were given intravenously to mice during induction of amyloidosis, human AA was incorporated in the amyloid fibrils. Both apo AI and apo AII were shown to displace SAA from acute phase HDL when added to HDL‐SAA complexes in vitro. This might be of importanee in amyloidogenesis, as the liver and the small intestine, which are the main sites for AI and AII synthesis, are also sites of early amyloid deposition.