Contrary to DPP I, reducing agents such as cysteine, DTT, and mercaptoethanol (4 mM) cause a strong inhibition of DPP III activity (Figure 7.3); thus, their use must be avoided in reaction buffers. This enzyme is also inhibited by chelating agents such as o-phenanthroline (0.8 mM), EDTA and EGTA (4 mM); 3/-chloromercuribenzoate and DTNB (0.05 mM); 4-DCI (0.05 mM), Hg2+ (0.05 mM), and Cd2+ (0.2 mM), and the dipeptide Tyr-Tyr (0.4 mM). DPP III is activated by Co2+ (0.05 mM). The enzyme is sensitive to temperature and is inactivated in just 30 min when incubated at 55°C.
Assay techniques, based on color, fluorescence, or peptide determination, are the same as for DPP I, except pH 8, the presence of 0.05 mM Co2+, and the absence of any reducing agent in the reaction buffer (Table 7.3).
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