pH curve. FRET peptide, Arg-Lys(DABCYL)-Thr-Ser-Gly-Pro-Asn-Gln-Glu-Gln-Glu(EDANS)-Arg was incubated with purified APCE or rFAP at the indicated pH. Two buffer systems were used: 0.1 M sodium phosphate for the pH range 5.8-7.5 and 0.1 M Tris-HCl for the pH range 7.9-9.0; each value represents the average of duplicate incubations. One unit of enzyme activity is defined as picomoles of Asn-Gln-Glu-Gln-Glu(EDANS)-Arg released per minute at 22°C.