Table 2. Comparison of recombinant human QPP and purified human DPPII regarding K_m and IC₅₀ values observed with different substrates and inhibitors.

Each assay was carried out in 0.05 M cacodylic acid buffer, pH 5.5, and 0.05 M Hepes buffer, pH 7.0, at 37 °C with the substrates mentioned in the Table. The measurements with DPPII and QPP were carried out simultaneously using the same reagent dilutions for both enzymes. Values for K_m and IC₅₀ compound 1 are in μM. Values for IC₅₀ compound 2 are in nM. Errors represent S.E.M. on the fit. IC₅₀ values were obtained using substrate concentrations near the K_m value. n.d., not determined.

	pH 5.5		pH 7.0
	DPPII	QPP	DPPII	QPP
Km – substrate analysis
Lys-Ala-pNA	570±40	560±30	40±5	35±3
Ala-Pro-pNA	40±4	30±3	9±2	7±3
Gly-Pro-pNA	210±30	180±30	70±6	60±5
Nle-Nle-pNA	20±3	15±1	n.d.	n.d.
IC50 compound 1*
Lys-Ala-pNA	1.72±0.06	1.19±0.03	0.052±0.002	0.043±0.002
Ala-Pro-pNA	2.64±0.04	2.26±0.04	0.063±0.001	0.049±0.002
IC50 compound 2†
Lys-Ala-pNA	0.95±0.04	1.10±0.02	1.00±0.04	0.95±0.01
Ala-Pro-pNA	1.45±0.03	1.30±0.02	1.05±0.02	1.00±0.02

* Lys-Pip (Figure 1).

† N- (4-chlorobenzyl)-4-oxo-4-(1-piperidinyl)-1, 3- (S)-butane-diamine dihydrochloride (Figure 1).