Dipeptidyl peptidase-4 (revision 7)

Dipeptidyl peptidase-4 (DPP-4), also known as CD26, is a serine exopeptidase that removes the N-terminal two residues from peptides presenting proline or alanine in the second position. Both incretin hormones — Glucagon-like peptide-1 and Glucose-dependent insulinotropic polypeptide — carry alanine at position 2 and are therefore inactivated within minutes of secretion.^[1]

The enzyme exists as a type II membrane protein on endothelial cells, epithelial cells and lymphocytes, and as a catalytically active soluble form in plasma. Cleavage of GLP-1 begins in the capillaries of the intestinal lamina propria, before the hormone reaches the portal vein, so that only a minority of secreted GLP-1 arrives at the systemic circulation intact.^[2]

DPP-4 defines the design problem for the entire GLP-1 receptor agonist class, and also constitutes a drug target in its own right. Inhibitors of the enzyme raise endogenous incretin concentrations two- to three-fold and lower glycated haemoglobin by roughly 0.5–0.8 percentage points, an effect substantially smaller than that of receptor agonists because the ceiling is set by physiological secretion.^[1]

DPP-4 cleaves the dipeptide His-Ala from the N-terminus of GLP-1(7–36)amide to yield GLP-1(9–36)amide. The truncated product binds the GLP-1 receptor with roughly two orders of magnitude lower affinity and behaves as a weak antagonist at pharmacological concentrations; whether it has independent cardiovascular activity remains an open question in the literature.^[2]

The substrate range is wide. Substance P, neuropeptide Y, several chemokines and the incretins are all cleaved, and the enzyme also has non-catalytic functions as a co-stimulatory molecule on T cells and as a binding partner for adenosine deaminase. The breadth of this range is the reason inhibitor development required careful selectivity against the related enzymes DPP-8 and DPP-9.^[1]

Kinetically the enzyme is efficient rather than abundant: plasma DPP-4 activity is sufficient to halve circulating intact GLP-1 in about one minute, which is why the intact fraction must be measured with an assay that distinguishes GLP-1(7–36) from GLP-1(9–36) if the measurement is to mean anything.