NH2-terminally modified gastric inhibitory polypeptide exhibits amino-peptidase resistance and enhanced antihyperglycemic activity.

Gastric inhibitory polypeptide (GIP) is an important insulin-releasing hormone of the enteroinsular axis that, like glucagon-like peptide 1(7-36) amide (tGLP-1), has a functional profile of possible therapeutic value for type 2 diabetes. Both incretin hormones are rapidly inactivated in plasma by the exopeptidase dipeptidyl peptidase (DPP) IV. The present study examined the ability of NH2-terminal modification of human GIP to protect from plasma degradation and enhance insulin-releasing and antihyperglycemic activity. Degradation of GIP by incubation at 37 degrees C with purified DPP IV was clearly evident after 4 h (54% intact). After 12 h, >60% of GIP was converted to GIP(3-42), whereas >99% of NH2-terminally modified Tyr1-glucitol GIP remained intact. Tyr1-glucitol GIP was similarly resistant to serum degradation. The formation of GIP(3-42) was almost completely abolished by inhibition of plasma DPP IV with diprotin A. Effects of GIP and Tyr1-glucitol GIP were examined in Wistar rats after intraperitoneal injection of either peptide (10 nmol/kg) together with glucose (18 mmol/kg). Plasma glucose concentrations were significantly lower and insulin concentrations higher after both peptides compared with glucose alone. More importantly, individual glucose values at 15 and 30 min together with the areas under the curve (AUCs) for glucose were significantly lower after administration of Tyr1-glucitol GIP compared with GIP (AUC 255 +/- 33 vs. 368 +/- 8 mmol x l(-1) x min(-1), respectively; P < 0.01). This was associated with a significantly greater and more protracted insulin response after Tyr1-glucitol GIP than GIP (AUC 773 +/- 41 vs. 639 +/- 39 ng x ml(-1) x min(-1); P < 0.05). These data demonstrate that Tyr1-glucitol GIP displays resistance to plasma DPP IV degradation and exhibits enhanced antihyperglycemic activity and insulin-releasing action in vivo.