FIG. 8.

Inhibition of ADP-ribosyltransferase activity with immune rabbit serum. ETA (20 ng per reaction mixture) was preincubated for 1 h at 37°C with a 1:5 dilution of preimmune serum or serum from rabbits from group (Gr) I (immunized with peptides [pep] 1, 2, 7, and 10), group II (immunized with peptides 3, 8, and 9), group IIIa (immunized with peptides 4, 5, and 6), group IIIb (immunized with peptide 11), or group IV (immunized with ETA). ETA was then activated, and the reaction mixture was assayed for ADP-ribosylation in the presence of ¹⁴C-labeled NAD and eEF-2. The reaction was stopped after 15 min at 25°C with 10% TCA. Samples were then counted with a beta counter (A). (B) ETA (20 ng/ml) was preincubated with 40 μg of affinity-purified peptide-specific antibodies. Inhibition of activity was expressed as the percentage of activity measured in the absence of antibody. Results represent the mean for each group of rabbits ± standard error. Spontaneous ADP-ribosylation of eEF-2 in the absence of ETA was deducted from values obtained in all experiments. Experiments were repeated twice, and each was performed in duplicate. Asterisks indicate statistical significance compared to toxin mixed with preimmune serum (A) or PBS (B) (P < 0.05).