A huge amount of information is known about peptidases,22 but much remains to be understood regarding their interactions with xenobiotic substrates. This is rendered all the more imperative by an ever increasing interest in therapeutic peptides and peptidomimetics. A number of such peptidases (EC 3.4) are listed in Table 2, but this list is based on indirect evidence and hypotheses much more than on solid data. As a reminder, we show in Figure 4 the classification of peptidases based on the bonds they cleave along the polypeptide chain. Two examples have been selected for presentation here: one (oxytocin) to illustrate the multiplicity of enzymes acting on clinically used natural peptides, and the other (cetrorelix) to hint at the impossibility of predicting the metabolites of a synthetic peptide containing nonproteinogenic acids.
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