**2.7 Dehydropeptidase I and IV activity assays**

RPTECs were incubated overnight with Gly-Phe-*p*-nitroanilide (DHP-I substrate; Sigma-Aldrich) 1mM in PBS for DHP-I activity determination or with Gly-Pro-*p*-nitroanilide (DHP-IV substrate; Sigma-Aldrich) 1 mM for DHP-IV activity determination. Both activities were measured in the presence or absence of cilastatin (200 µg/ml). *P*-Nitroanilide was quantified in aliquots from supernatants by measuring at 410 nm absorbance.
