Table S1.

Buffer-dependent activity of IDE at low pH

Buffer	pH	Activity, nmol/min/mg IDE
Na acetate	5.06	No activity
Na citrate	5.05	No activity
MES	5.11	11.8
Na acetate	5.48	No activity
Na citrate	5.50	8.9
MES	5.62	71.1
MES	6.08	68.9
Tris	7.45	6.9

All buffers were at 50 mM. Assays were conducted with 10 µM Abz-GGFLRKHGQ-Eddnp. pH values were determined from dummy reaction mixtures containing 20% DMF in place of substrate. In a separate experiment, the Abz substrate was completely cleaved, and the total fluorescence change measured at the different pH values. There was no effect of pH on the total fluorescence change.