FIG. 7.

¹²⁵I-ETA binding to 3T3 fibroblasts in the presence of immune rabbit serum. ¹²⁵I-ETA was preincubated with a 1:10 dilution of serum from group (Gr) IV rabbits (immunized with ETA) or group IIIb rabbits (immunized with peptide 11) and preimmune serum. The mixture was preincubated for 1 h at 37°C and added to monolayers of 3T3 fibroblasts, which were then incubated for 6 h at 37°C. The level of ¹²⁵I-ETA remaining after washing was measured with a gamma scintillation counter (A). (B) ¹²⁵I-ETA was preincubated with affinity-purified antibodies at a concentration 4× the amount necessary to provide 3T3 fibroblasts with 50% protection against inhibition of protein synthesis caused by the same amount of ETA (30 ng/ml). Data were expressed as the percentage of ¹²⁵I-ETA binding in the absence of antibody. Results represent the mean from two rabbits ± standard error. All experiments were performed in duplicate. An asterisk indicates statistical significance compared to toxin mixed with preimmune serum (A) or PBS (B) (P < 0.05). pep, peptide.