Chronic exposure of rat pancreatic islets and INS-1 insulinoma cells to glucosamine (GlcN) produced a reduction of glucose-induced (22.2 mM) insulin release that was associated with a reduction of ATP levels and ATP/ADP ratio compared with control groups. To further evaluate mitochondrial function and ATP metabolism, we then studied uncoupling protein-2 (UCP2), F1-F0-ATP-synthase, and mitochondrial membrane potential, a marker of F1-F0-ATP-synthase activity. UCP2 protein levels were unchanged after chronic exposure to GlcN on both pancreatic islets and INS-1 beta-cells. Due to the high number of cells required to measure mitochondrial F1-F0-ATP-synthase protein levels and mitochondrial membrane potential, we used INS-1 cells, and we found that chronic culture with GlcN increased F1-F0-ATP-synthase protein levels but decreased glucose-stimulated changes of mitochondrial membrane potential. Moreover, F1-F0-ATP-synthase was highly glycosylated, as demonstrated by experiments with N-glycosidase F and glycoprotein staining. Tunicamycin (an inhibitor of protein N-glycosylation), when added with GlcN in the culture medium, was able to partially prevent all these negative effects on insulin secretion, adenine nucleotide content, mitochondrial membrane potential, and protein glycosylation. Thus we suggest that GlcN-induced pancreatic beta-cell toxicity might be mediated by reduced cell energy production. An excessive protein N-glycosylation of mitochondrial F1-F0-ATP-synthase might lead to cell damage and secretory alterations in pancreatic beta-cells.