The most striking sequence difference between glucagon-like peptide-1 (GLP-1)(2) and the longer-acting GLP-1 receptor agonist, exendin-4 (Ex-4),(3) is the nine-amino acid COOH-terminal extension of Ex-4. We investigated the contribution of this extension to the survival time of Ex-4. We assessed the overall metabolism of GLP-1, Ex-4, a COOH-terminally extended GLP-1 peptide (GLP-1+Ex(31-39); GLP-Ex),(4) and a COOH-terminally truncated exendin peptide (Ex(1-30)) in anaesthetized, catheterized pigs, with focus on the extraction across the kidneys and a peripheral tissue (a hindleg, representing muscle, adipose- and connective tissue). Peptide analysis was carried out with assays against the mid-region of the peptides, whereby the role of dipeptidyl peptidase-4 (DPP-4)(5) mediated NH(2)-terminal degradation could be disregarded. The half-life of GLP-1 was significantly increased when the COOH-terminal extension of Ex-4 was added (GLP-1 4.8±3.3min; GLP-Ex 19.5±3.3min). In contrast, there was no effect of truncating Ex-4 (Ex-4 32.4±4.1min; Ex(1-30) 28.4±1.7min). Ex-4 and Ex(1-30) were cleared solely by the kidneys at rates corresponding to the glomerular filtration rate (GFR),(6) while GLP-1 and GLP-Ex were cleared by both the kidneys and peripheral tissues. Both extraction rates were, however, significantly reduced with GLP-Ex compared to GLP-1. The renal clearance rate of GLP-1 greatly exceeded GFR, while GLP-Ex was cleared at a rate resembling GFR. In conclusion, the COOH-terminal extension of Ex-4 contributes minimally to the increased survival time of Ex-4, while addition of this sequence to GLP-1 significantly reduces its clearance.
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