Amyloid protein SAA is associated with high density lipoprotein from human serum

Human serums contain a protein antigenically related to protein AA, the principal protein of a major class of amyloid substance. The serum antigen, SAA, occurs mainly in a high molecular weight form, 1 to 2 X 10(5). This work shows that the bulk of the SAA sediments at density 1.12 g/cm3 and floats at density 1.21 g/cm3, as does the high density lipoprotein HDL3. SAA is associated with the apolipoproteins ApoA-I and ApoA-II. The total cholesterol:total protein ratio of the fraction with density 1.12-1.21 g/cm3 is 0.2:1, consistent with that of SAA constituent of the HDL3 fraction into low molecular weight species of the order of 13,000. The quantity of SAA may vary from 0.1% or more of the total protein of HDL3.