Time course of the cleavage of the MKRPPGFSPFRSSR peptide using Sap9. MKRPPGFSPFRSSR peptide (5 μM) was cleaved using 0.1 μM Sap9 in 50 mM citrate buffer (pH 5.0) at 37°C for the specified time. The reaction was stopped using HCl, and the sample was analyzed using HPLC as specified in Figure 1. The results from a representative kinetic experiment are shown. The areas under the peaks of separated peptides are expressed relative to the substrate at the beginning of the reaction.