GLYCATION OF GLUCAGON-LIKE PEPTIDE-1(7-36)AMIDE - CHARACTERIZATION AND IMPAIRED ACTION ON RAT INSULIN-SECRETING CELLS

Glucagon-like peptide-1 (7-36) amide (truncated GLP-1, tGLP-1) is a po tent insulin releasing hormone of the enteroinsular axis. This study h as examined glycation of tGLP-1 and effects of such structural modific ation on insulin secretion. Monoglycated tGLP-1 (M-r 3463.8, determine d by plasma desorption mass spectrometry) was prepared by incubation w ith glucose under reducing conditions and purified by reversed-phase h igh performance liquid chromatography. Automated Edman degradation ind icated that tGLP-1 was specifically glycated at the amino terminal His (7) site. In extracts from mouse small intestine, glycated tGLP-1 repr esented approximately 14 % of the total tGLP-1 content. Effects of gly cated and non-glycated tGLP-1 on insulin secretion were examined using glucose-responsive clonal BRIN-BD11 cells. In acute (20 min) incubati ons, 10(-9) mol/l tGLP-1 enhanced insulin release by 2.2-fold and 1.5- fold at 5.6 and 11.1 mmol/l glucose, respectively. In contrast, 10(-9) mol/l glycated tGLP-1 failed to stimulate secretion and insulin outpu t was decreased by 34-73 % following glycation. At 5.6 mmol/l glucose, non-glycated tGLP-1 (3 x 10(-10) mol/l-10(-8) mol/l) exerted a 2.3-fo ld to 3.2-fold increase in insulin secretion compared with controls. T he effect of glycated tGLP-1 at 10(-9) mol/l and 3 x 10(-9) mol/l was reduced by 51-55 % compared with non-glycated peptide, and its insulin otropic action was effectively abolished. These data indicate that whe n tGLP-1 is glycated at the amino terminal His7, this modification sub stantially reduces the glucose-dependent insulinotropic action of the peptide.