Monoclonal antibody to rat apoC: multiple binding to apoC-I on lipoproteins increases its affinity constant

Using solid phase systems, the kinetics of binding of monoclonal antibody (LRB 45, IgG2b,kappa) to apoC-I and apoC-I on lipoproteins were investigated. At 25 degrees C, the association constant of LRB 45 antibody to apoC-I (3.56 X 10(6) M-1 X sec-1) was almost three times slower than the association constant LRB 45 antibody to lipoproteins (10.4 X 10(6) M-1 X sec-1). However, the dissociation constant of apoC-I from LRB 45 antibody (0.865 X 10(-4) sec-1) was also slower than the dissociation constant of lipoprotein from antibody (1.5 X 10(-4) sec-1). Thus, the calculated affinity constant (association constant/dissociation constant) of LRB 45 antibody for apoC-I was approximately half of that for lipoproteins (4.12 X 10(10) M-1 vs. 6.92 X 10(10) M-1). The intrinsic affinity constants for antibody binding to apoC-I and apoC-I on lipoproteins were determined by Scatchard analysis. The intrinsic affinity constant of antibody bound to apoC-I was estimated to be 5.49 X 10(10) M-1 whereas that of antibody binding to lipoproteins was 30 to 200 times less. Furthermore, ascites fluid from LRB 45 cell lines could immunoprecipitate serum lipoproteins. Thus, it is concluded that there is multiple binding of antibody to apoC-I on lipoproteins. This binding appears to increase the calculated affinity constant (avidity) for antibody-antigen interaction.