Abstract
In humans, glucagon-like peptide-1 (GLP-1) is rapidly degraded by dipeptidyl peptidase-4 to a relatively stable metabolite, GLP-1(9-36)NH2, which allows measurement of GLP-1 secretion. However, little is known about the kinetics of the GLP-1 metabolite in mice. We hypothesised that the GLP-1 metabolite is rapidly degraded in this species by neutral endopeptidase(s) (NEP[s]). We administered glucose, mixed meal or water orally to 256 mice, and took blood samples before and 2, 6, 10, 20, 30, 60 or 90min after stimulation. To study the metabolism of the GLP-1 metabolite, i.v. GLP-1(9-36)NH2 (800fmol) or saline (154mmol/l NaCl) was administered to 160 mice, some of which had a prior injection of a selective NEP 24.11±inhibitor (candoxatril, 5mg/kg) or saline. Blood was collected before and 1, 2, 4 and 12min after GLP-1/saline injection. Plasma GLP-1 levels were analysed using a customised single-site C-terminal ELISA, two different two-site ELISAs and MS. GLP-1 secretion profiles after oral glucose administration differed markedly when assayed by C-terminal ELISA compared with sandwich ELISAs, with the former showing a far higher peak value and AUC. In mice injected with GLP-1(9-36)NH2, immunoreactive GLP-1 plasma levels peaked at approximately 75pmol/l at 1min when measured with sandwich ELISAs, returning to baseline (~20pmol/l) after 12min, but remained elevated using the C-terminal ELISA (~90pmol/l at 12min). NEP 24.11 inhibition by candoxatril significantly attenuated GLP-1(9-36)NH2 degradation in vivo and in vitro. MS identified GLP-1 fragments consistent with NEP 24.11 degradation. In mice, the GLP-1 metabolite is eliminated within a few minutes owing to endoproteolytic cleavage by NEP 24.11. Therefore, accurate measurement of GLP-1 secretion in mice requires assays for NEP 24.11 metabolites. Conventional sandwich ELISAs are inadequate because of endoproteolytic cleavage of the dipeptidyl peptidase-4-generated metabolite.
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