GLYCATION OF INSULIN RESULTS IN REDUCED BIOLOGICAL-ACTIVITY IN MICE

Bovine insulin was glycated by in vitro incubation with 20-220 mM D-gl ucose for 1-48 h. The percentage of glycation was dependent on time, g lucose concentration, temperature and pH, attaining values up to 28%. Glucose-lowering activities of glycated and control (nonglycated) insu lin preparations were assessed in mice by intraperitoneal injection in a 39% (w/v) glucose solution (2 g/kg body weight) at doses of 0.05 an d 0.25 units/kg body weight. Injection of glucose alone significantly (P < 0.001) increased plasma glucose concentrations at 30 min. Simulta neous administration of non-glycated insulin with glucose significantl y decreased the 30-min glycaemic excursion (P < 0.001) in a dose-depen dent manner. Glycated insulin exhibited a significant reduction (P < 0 .001) in glucose-lowering activity under these conditions. The relatio nship between the extent of insulin glycation and glucose-lowering act ivity at 0.25 units/kg was assessed using five different insulin prepa rations glycated between 6%-28%. The insulin-induced decrease in plasm a glucose at 30 min was inversely related to the extent of glycation ( r=0.99). Glycated insulin (10(-8) and 10(-6) M) also exhibited a signi ficantly reduced (P < 0.05) ability to stimulate glucose oxidation in isolated mouse diaphragm muscle compared with non-glycated insulin. Th ese data indicate that glycated insulin exhibits impaired biological a ctivity which may contribute to glucose intolerance in diabetes. Furth er studies are required to determine if glycation of insulin occurs in man and if this process contributes to the pathogenesis of diabetes.