Pro88 is the critical amino acid residue for the determination of length of secreted Aβ. (A) Aβ42(43)/Aβ40 ratio in secreted Aβ from DKO cells expressing Single-Cys mt PS1. Conditioned media were analyzed by ELISA (n = 3, mean ± SD, *p < 0.05, **p < 0.01 (Compared with Cys-less) at Student’s t test). (B) Immunoblotting analysis of DKO cell lysate expressing PS1 mutant carrying P88C or P88L by indicated antibodies. (C) Aβ42(43)/Aβ40 ratio in secreted Aβ from DKO cells in (B) measured by ELISA (n = 3, mean ± SD). (D) Urea-SDS gel analysis for secreted Aβ from DKO cells expressing wild-type PS1, PS1 with P88L or PS1 with L166P mutation. Conditioned media of cells treated with 10 μM DAPT were also analyzed. Two types of Urea-SDS gels were used to obtain better resolution in the detection of longer Aβ peptides (i.e., Aβ45 and Aβ46).