Studies on the immunochemistry of human low density lipoproteins utilizing an hemagglutination technique

Hemagglutination is a specific and sensitive technique for investigating the purity of lipoproteins and the immunologic relationships between low density lipoprotein fractions. The S_f 10–400 and S_f 3–9 lipoprotein fractions, isolated from human serum by dextran sulfate-density gradient centrifugation procedure and repurified by centrifugation appeared to contain only lipoprotein antigens since these fractions did not stimulate the production of antibodies against other serum proteins. Cross-absorption experiments with lipoproteins carried on "tanned" cells demonstrated that the S_f 3–9 lipoprotein fraction contains all the antigenic components of the S_f 10–400 lipoprotein fraction together with additional antigenic components not found in the S_f 10–400 lipoprotein fraction. Thus S_f 3–9 and S_f 10–400 lipoprotein fractions are immunologically similar but not identical. Low density lipoproteins contain no antigens in common with the high density lipoproteins. An S_f 3–9 antiserum can be used to detect both S_f 3–9 and S_f 10–400 antigens. The S_f 3–9 lipoprotein fraction used as an antigen will detect antibodies against both S_f 3–9 and S_f 10–400 lipoprotein fractions. The S_f 3–9 and S_f 10–400 antisera did not contain immune antibodies against erythrocytes of the different blood groups or against sheep, guinea pig, dog, calf, pig, horse, and chicken erythrocytes. Normal subjects and subjects with recent myocardial infarctions had no circulating autoantibodies against the S_f 3–9 and S_f 10–400 lipoprotein fractions.

Studies on the antigenicity of beta- and alpha-lipoproteins of human serum

The techniques of agar immunoelectrophoresis and agar double diffusion were applied to the study of the antigenicity of β- and α₁-lipoproteins separated by ultracentrifugation from normal human sera. The effects of delipidation were also investigated. It was shown that β- and α₁-lipoproteins are antigenically distinct. For each class of lipoprotein studied, a single antigenic component was demonstrated. In some, but not all, preparations of α₁-lipoprotein a second, small antigenic component was detected, and identified as albumin. Absorption with lipoprotein-free serum or albumin removed this component without changing the lipoprotein band. Delipidation did not affect the antigenicity of either β- or α₁-lipoproteins. Immunoelectrophoresis, because of its high sensitivity and specificity, provides an additional criterion of purity for antigenic proteins in addition to the data that can be obtained from ultracentrifugal and free electrophoretic analysis.