Role of the carbohydrate moiety in the antigenic site(s) of human serum low-density lipoprotein.

Radioimmunoassay techniques have been used to evaluate the contribution of the carbohydrate moiety to the immunological reactivity of human serum low-density lipoprotein (LDL). Low-density lipoprotein (d = 1.024--1.045 g/mL) was isolated from normolipidemic serum by ultracentrifugal flotation. Radioimmunoassay was performed with 125I-labeled LDL and several homologous antisera, each corresponding to different periods (1--18 weeks) of immunization and thus containing various antibody populations. Unlabeled LDL and different monosaccharides characteristic to this particle, i.e., mannose, sialic acid, glucose, N-acetylglucosamine, galactose, N-acetylgalactosamine, and fucose, were used as competitors in the binding of the labeled antigen with antibody. In the reaction with antisera corresponding to the highest antibody titer, unlabeled LDL, sialic acid, and mannose inhibited the binding of labeled LDL up to 62%, 25%, and 16%, respectively; a low degree of inhibition (some 13%) was occasionally obtained with glucose. Galactose, galactosamine, glucosamine, and fucose failed to compete with labeled LDL. Studies with antisera corresponding to different periods of immunization (2, 4, and 8 weeks) indicated that antibodies reacting with mannose appeared early (maximum 31% inhibition at 2 weeks), disappearing at 6--8 weeks; in contrast, antibodies reacting with sialic acid augmented progressively (10% inhibition at 2 weeks, 20% at 4 weeks, and 35% at the end of the immunization). These data are consistent with the conclusion that sialic acid and mannose, the terminal residues of LDL glycopeptides I and II [Swaminathan, N., & Aladjem, F. (1976) Biochemistry 15, 1516--1621], are implicated in the antigenic site(s) of LDL.