DPP-4 Cleavage

Dipeptidyl peptidase-4 is a serine exopeptidase that clips two residues from the free amino terminus of a peptide whenever the second position is alanine or proline. It exists both anchored to endothelial cell membranes, where circulating peptides meet it immediately, and as a soluble plasma form. The reaction is fast and specific to that position, so susceptibility is predictable from sequence alone.

The incretins are its canonical substrates. Glucagon-like peptide-1 carries alanine at position two and is cut to a shortened form within a couple of minutes of secretion, which is why the native hormone can only be given as a continuous infusion. Glucose-dependent insulinotropic polypeptide is inactivated the same way. Long-acting analogues all answer this: exenatide derives from exendin-4, which carries glycine at the vulnerable position, while liraglutide and semaglutide substitute aminoisobutyric acid there. Gliptin drugs take the other approach and inhibit the enzyme so that endogenous incretin survives longer.

Knowing the mechanism tells you what a structural modification is for and what it is not for. Blocking cleavage removes one clearance route; it does not address glomerular filtration, which still disposes of small peptides quickly. Exenatide is fully DPP-4 resistant and still has a half-life of a few hours, whereas semaglutide reaches a week only because albumin binding is added on top of the substitution.

The error worth naming is reading DPP-4 resistance as a claim of long duration; it is one requirement among several and rarely the limiting one. The second is assuming the cleaved product is simply gone. The shortened peptides remain in circulation, have their own reported activities, and are counted by assays that do not distinguish intact from cleaved forms, so total immunoreactive measurements overstate active hormone.