Enzymatic Degradation

Peptides are cleared largely by being taken apart. Exopeptidases work inward from the ends, aminopeptidases and carboxypeptidases trimming single residues and dipeptidyl peptidase-4 removing pairs from the amino terminus. Endopeptidases cut internally at sequence-specific sites: trypsin after basic residues, chymotrypsin after aromatic ones, neprilysin and insulin-degrading enzyme at their own preferred motifs. They sit in the gut lumen, on vascular endothelium, in liver and kidney and on cell surfaces, which is why an unmodified peptide seldom lasts more than minutes.

Every structural trick in peptide medicinal chemistry exists to slow one of these steps. Substituting a D-amino acid or a non-natural residue at a cleavage site removes recognition. Cyclisation and stapling lock the backbone into a conformation a protease cannot engage. Terminal acetylation and amidation block exopeptidase entry. PEGylation and fatty acid acylation shield the molecule sterically and by binding albumin. Inhibiting a peptidase can also be the drug itself, as with neprilysin inhibition in heart failure.

Which enzyme dominates decides which problem is worth solving. A peptide destroyed by pancreatic proteases has no oral future without a protective formulation but may be entirely serviceable injected. One cleaved in plasma needs sequence modification. One cleared mainly by renal filtration needs size, not stability.

The familiar overreach is the serum stability figure. A peptide that survives twenty-four hours in serum is often advertised as lasting that long in the body, but the assay contains only soluble plasma enzymes. It omits membrane-bound endothelial peptidases, tissue uptake and the kidney, which together usually dominate. Stability in a tube and half-life in an organism are different measurements, and the first routinely overstates the second by an order of magnitude.