In vitro adenylate cyclase activity assay of EF, EF(H351A), or EF(H351R). (a) CHO cells were incubated for 2 h with forskolin (50 μg/mL and 5 μg/mL), SQ22536(50 μg/mL and 5 μg/mL) or DMSO control. Intracellular cAMP was measured using an EIA kit; (b) CHO cells were incubated for 2 h with PA (1 μg/mL) and EF, EF(H351A), or EF(H351R) at the indicated concentrations. Intracellular cAMP was measured using an EIA kit. An average of 13 pmol of cAMP/107 CHO cells was detected in control cells (untreated cells or cells treated with only one toxin component). Results are expressed as mean ± standard error. Statistical significance: (**
p < 0.01, *
p < 0.05).