Mass spectrometric analysis of the Aβs generated in cell-free γ-secretase assays, in the presence of DMSO or fipronils 18, 20, or 21. A) The Aβs generated by highly purified γ-secretase in the presence of 100 μM of fipronil (18), fipronil sulfone (20) or fipronil desulfinyl (21) or DMSO (vehicle) were pooled from triplicates of the activity assays, immunoprecipitated overnight with the anti-Aβ antibody 4G8 and protein G and analyzed by MALDI-TOF in a reflectron mode. The Aβs generated from the recombinant APP-C99 contain an N-terminal methionine, which results in a mass shift of +149m/z when compared to endogenous Aβ peptides. B) Note the increased Aβ42/Aβ40 ratio for fipronils 18, 20, and 21, and the increased Aβ43/Aβ40 ratio for fipronil sulfone (20).