High density serum lipoprotein underwent serologic and physicochemical alterations on aging during storage at 0°C. for 1 month, as judged by decrease of diffusion coefficient and increase of C' fixation. Ultracentrifugation, dialysis, and high concentrations of sodium chloride did not cause these changes. A protein sedimenting at density 1.24 in the ultracentrifuge reacted with antiserum to high density lipoprotein. Probably it was the protein portion of α lipoprotein dissociated from the lipide during ultracentrifugation. Although the antiserum to high density lipoprotein did not react with low density lipoprotein prepared from normal serum, it reacted with similarly prepared lipoproteins from the serum of a patient with biliary cirrhosis.
The following human low density lipoproteins were prepared: ß-lipoproteins of densities greater than 1.040 (A, B,C) a ß-lipoprotein of – S 1·063 = 5 (D), a lipoprotein of – S 1·063 = 19 (E), and a lipoprotein of – S 1·063 = 70 (F). Data are presented which show the immunochemical homogeneity of the D lipoprotein rabbit-anti-D lipoprotein system. Cross-reactions between antibody to A and D lipoproteins and the above lipoproteins have been demonstrated by quantitative precipitation, quanitative complement fixation, and single and double diffusion in agar. The antigenic similarities appear to be associated with the protein portions of the molecule. The antisera produced did not differentiate the low density lipoprotein classes.