Pancreatic beta-cells exposed to hyperglycemia produce reactive oxygen species (ROS). Because beta-cells are sensitive to oxidative stress, excessive ROS may cause dysfunction of beta-cells. Here we demonstrate that mitochondrial ROS suppress glucose-induced insulin secretion (GIIS) from beta-cells. Intracellular ROS increased 15 min after exposure to high glucose and this effect was blunted by inhibitors of the mitochondrial function. GIIS was also suppressed by H2O2. a chemical substitute for ROS. Interestingly, the first-phase of GIIS could be suppressed by 50 muM H2O2. H2O2 or high glucose Suppressed the activity of glyceraldehyde 3-phosphate dehydrogenase (GAPDH), a glycolytic enzyme, and inhibitors of the mitochondrial function abolished the latter effects. Our data suggested that high glucose induced mitochondrial ROS, which suppressed first-phase of GIIS. at least in part. through the suppression of GAPDH activity. We propose that mitochondrial overwork is a potential mechanism causing impaired first-phase of GIIS in the early stages of diabetes mellitus. (C) 2002 Elsevier Science (USA). All rights reserved.