DPP-4 (dipeptidyl peptidase-4) is an enzyme that rapidly degrades the body's own GLP-1. Native GLP-1 has a half-life of only about one to two minutes, largely because DPP-4 breaks it down so quickly.
GLP-1 receptor agonist medications are engineered to resist this enzyme, which is what lets them stay active in the body far longer than natural GLP-1 does.