Dipeptidyl Peptidase-4 (DPP-4)

Dipeptidyl peptidase-4 is a serine protease existing both as a membrane-anchored ectoenzyme, identical to the lymphocyte marker CD26, and as a soluble plasma form. It cuts dipeptides from the free N-terminus of substrates carrying proline or alanine at the second position, a narrow specificity that happens to catch many regulatory peptides: GLP-1, GIP, peptide YY, substance P and SDF-1. Because it is an exopeptidase, blocking the N-terminus of a peptide is enough to defeat it.

Native GLP-1 has a circulating half-life of one to two minutes, and a large share of what L-cells secrete is inactivated in the capillary bed before it reaches the portal vein. The gliptin class, beginning with sitagliptin in 2006, roughly doubles or triples active incretin concentrations, delivering an HbA1c reduction of about half a percentage point to eight tenths. Peptide engineering takes the other route: exenatide carries glycine at position two and semaglutide substitutes aminoisobutyric acid at position 8, so neither is a substrate.

The comparison between the two strategies is the useful part. Inhibiting the enzyme can only raise endogenous peptide to physiological ceilings, which is why gliptins are oral, weight-neutral and modest. A protease-resistant agonist with albumin binding reaches concentrations far above physiology and produces the weight and outcome effects the class is known for. Same axis, different exposure.

The error worth naming is treating a gliptin as a weaker version of the same drug, which patients are frequently told; the mechanisms differ in kind, not in dose. On the safety side, saxagliptin in SAVOR-TIMI 53 showed an excess of hospitalisation for heart failure, a class question that does not transfer to receptor agonists. And a research listing describing a peptide as DPP-4 resistant asserts nothing verifiable without the sequence at position two.