Secretion of the incretin hormone glucagon-like-peptide 1 (GLP-1) from the intestinal L-cells play a significant role in improving glycaemic control following a meal(1) but it is rapidly inactivated by dipeptidylpeptidase-4 (DPP-4)(2). Orally active DPP-4 inhibitor drugs are used to improve glycaemic control in people with diabetes. Several peptide component peptides from the edible seaweed Palmaria palmata (Dulse) have been identified as having DPP-4 inhibitory actions(3). Here we examined the efficacy of three short synthetic peptides derived from Palmaria palmata to stabilise glucagon-like peptide-1(7-36)amide (GLP-1) using an in vitro HPLC assay and to affect insulin secretion and glycaemic control in mice challenged with an intraperitoneal glucose tolerance test (ipGTT).

        The actions of these peptides, Leu-Leu-Ala-Pro (LLAP), Met-Ala-Gly-Val-Asp-His-Ile (MAGVDHI) and Ile-Leu-Ala-Pro (ILAP) at preserving the stability of the incretin hormone GLP-1 were examined using a HPLC assay. GLP-1 stability was assessed at 0, 2, 8 and 24 h in the presence of porcine DPP-4 (5 mU) at 37ºC in triethanolamine buffer (50 mM, pH 7.8) with a fixed concentration (10-6 M) of each peptide.  In addition, synthetic peptides (10-12 to 10-6 M) were tested for their ability to promote acute (20 min) insulin secretion from cultured pancreatic BRIN-BD11 cells at 5.6 mM glucose. Finally, peptides were co-administered (25 nmol/kg) by intraperitoneal injection along with glucose (18 mmol, ipGGT) to healthy male NIH Swiss mice and tail blood samples collected at intervals from 0-120 min. 
 
        All three peptides LLAP, MAGVDHI and ILAP demonstrated efficacy as DPP-4 inhibitors. GLP-1 control exposed to DPP-4 had a half-life of 1.5 h, but the above synthetic peptides reduced the action of DPP-4 and prolonged the half-life of GLP-1 to 8, 10 and 13 h, respectively, as assessed using an in vitro HPLC assay.  LLAP and ILAP (but not MAGVDHI) produced a dose-dependent (10-11 and 10-6 M) increase in insulin secretion (1.4- to 2.0-fold) from cultured BRIN-BD11 cells at 5.6 mM glucose versus controls (Student t-test P<0.01 to P<0.001).  When tested in vivo in mice LLAP and ILAP produced a 43-52% reduction (P<0.05) in the blood glucose area under the curve (AUC0-120 min) which was accompanied by a 2.9 to 4.4-fold rise in plasma insulin (AUC0-120 min, P<0.01 to p<0.001), compared to the glucose control.

        Overall these three synthetic peptides derived from Palmaria palmata, helped stabilise GLP-1 against DPP-4 degradation in vitro. Furthermore LLAP and ILAP stimulated acute insulin secretion in cultured pancreatic cells, as well as demonstrating potent anti-hyperglycaemic and insulinotropic actions in mice.


1.	Baggio LL, Drucker DJ (2007) Gastroenterology 132, 2131-2157.
2.	Omar B, Ahrén B (2014) Diabetes 63, 2196-2202.
3.	Harnedy PA, O’Keeffe MB, FitzGerald RJ (2015) Food Chem 72, 400–406.

09. Synthetic peptides identified from Palmaria palmata enhance glucagon-like peptide-1 stability in vitro and show acute anti-hyperglycaemic and insulinotropic actions in mice.