Degradation of glucagon-like peptide-1 by human plasma in vitro yields an N-terminally truncated peptide that is a major endogenous metabolite in vivo.

TLDR

GLP‑1 is rapidly cleared from circulation, yet its detailed metabolism remains poorly understood, and measurement is complicated by antisera that detect both intact and inactive N‑terminal fragments. The authors developed a high‑pressure liquid chromatography coupled with radioimmunoassay method that specifically quantifies intact GLP‑1 and its metabolites. Human plasma rapidly degrades GLP‑1‑(7‑36)amide to the N‑terminal truncated GLP‑1‑(9‑36)amide (t½ ≈ 20 min), a process blocked by DPP‑IV inhibition or low temperature, with the metabolite comprising ~54 % of intact GLP‑1 in fasting plasma and rising to ~131 % after a meal, indicating DPP‑IV as the main in‑vitro degradation pathway and a likely in‑vivo inactivation mechanism.

Abstract

The metabolism of glucagon-like peptide-1 (GLP-1) has not been studied in detail, but it is known to be rapidly cleared from the circulation. Measurement by RIA is hampered by the fact that most antisera are side-viewing or C-terminally directed, and recognize both intact GLP-1 and biologically inactive. N-terminally truncated fragments. Using high pressure liquid chromatography in combination with RIAs, methodology allowing specific determination of both intact GLP-1 and its metabolites was developed. Human plasma was shown to degrade GLP-1-(7-36)amide, forming an N-terminally truncated peptide with a t1/2 of 20.4 +/- 1.4 min at 37 C (n = 6). This was unaffected by EDTA or aprotinin. Inhibitors of dipeptidyl peptidase-IV or low temperature (4 C) completely prevented formation of the metabolite, which was confirmed to be GLP-1-(9-36)amide by mass spectrometry and sequence analysis. High pressure liquid chromatography revealed the concentration of GLP-1-(9-36)amide to be 53.5 +/- 13.7% of the concentration of endogenous intact GLP-1 in the fasted state, which increased to 130.8 +/- 10.0% (P < 0.01; n = 6) 1 h postprandially. Metabolism at the C-terminus was not observed. This study suggests that dipeptidyl peptidase-IV is the primary mechanism for GLP-1 degradation in human plasma in vitro and may have a role in inactivating the peptide in vivo.