RADIOIMMUNOLOGICAL STUDY OF THE SURFACE PROTEIN OF THE HUMAN-SERUM LOW-DENSITY LIPOPROTEIN - COMPARISON OF THE NATIVE PARTICLE AND THE PRODUCTS OBTAINED BY TRYPTIC TREATMENT

The present report describes radioimmunological studies of the native low-density lipoprotein from human serum, and of the products obtained by limited tryptic treatment, i.e.a protein-depleted particle lacking some 20% of the original protein moiety and a peptide fraction of low molecular weight (<5000). The liberated peptides were highly immunogenic and elicited antibodies which reacted with both the native and protein-deficient lipoprotein particles. Moreover these peptides exhibited competitive reactivity with [125I]-labelled low-density lipoprotein in binding with homologous antisera, and with antisera to the native and trypsin-treated lipoproteins. These findings suggest that the peptides liberated from low-density lipoprotein by tryptic digestion contain the major antigenic site(s) of the molecule. Consideration of the nature of the competitive displacement of radiolabelled low-density lipoprotein from antisera to low-density lipoprotein, to the trypsinised lipoprotein and to the peptide fraction indicate that a marked repetition of the antigenic site(s) occurs in the structure of the protein moiety, a possibility consistent with the recurrence of similar subunits in the apoprotein of low-density lipoprotein. © 1979.