Prevention of Mitochondrial Oxidative Damage as a Therapeutic Strategy in Diabetes Katherine Green, Martin D. Brand, and Michael P. Murphy Hyperglycemia causes many of the pathological conse- quences of both type 1 and type 2 diabetes. Much of this damage is suggested to be a consequence of elevated production of reactive oxygen species by the mitochon- drial respiratory chain during hyperglycemia. Mitochon- drial radical production associated with hyperglycemia will also disrupt glucose-stimulated insulin secretion by pancreatic -cells, because pancreatic -cells are particularly susceptible to oxidative damage. There- fore, mitochondrial radical production in response to hyperglycemia contributes to both the progression and pathological complications of diabetes. Consequently, strategies to decrease mitochondrial radical production and oxidative damage may have therapeutic potential. This could be achieved by the use of antioxidants or by decreasing the mitochondrial membrane potential. Here, we outline the background to these strategies and discuss how antioxidants targeted to mitochondria, or selective mitochondrial uncoupling, may be potential therapies for diabetes. Diabetes 53 (Suppl. 1):S110 –S118, 2004 O xidative damage due to hyperglycemia contrib- utes to the microvascular pathology of diabetes that occurs particularly in the retina, renal glomerulus, and peripheral nerves, causing blindness, renal failure, and peripheral neuropathy (1– 4). Although the death of -cells that underlies type 1 diabetes is probably due to an autoimmune response, the particular susceptibility of -cells to oxidative damage from reactive oxygen species (ROS) produced during inflammation may be a predisposing factor (5,6). Supporting this, the strep- tozotocin and alloxan models of diabetes in rodents use ROS production to kill -cells, and oxidative damage to -cells during hyperglycemia may contribute to the pro- gression of the disorder (7,8). The association between hyperglycemia and oxidative damage has been noted for some time with various sources proposed for the under- lying ROS (1,2). Recently, it has been suggested that increased mitochondrial ROS production during hypergly- cemia may be central to much of the pathology of diabetes (3,9). Furthermore, because -cell mitochondria play a central role in glucose-stimulated insulin secretion (GSIS), damage to -cell mitochondria will attenuate this response (7). Therefore, mitochondrial ROS production and oxida- tive damage may contribute to the onset, progression, and pathological consequences of both type 1 and type 2 diabetes. Here, we outline how mitochondrial oxidative damage occurs, consider the mechanisms by which it may contribute to the pathophysiology of diabetes, and discuss potential therapeutic strategies to prevent it. MITOCHONDRIAL OXIDATIVE DAMAGE Metabolism strips electrons from fatty acids, sugars, and amino acids and accumulates them on the soluble electron carrier NADH and on protein-bound FADH 2 (Fig. 1). The electrons are then passed down the mitochondrial respi- ratory chain to drive ATP synthesis by oxidative phosphor- ylation. As the electrons move down the potential energy gradient from NADH/FADH 2 to oxygen, the redox energy is conserved by pumping protons across the inner mem- brane to build up a proton electrochemical potential gradient ( H+ ). This gradient, composed of a substantial membrane potential and a smaller pH gradient, is used by the ATP synthase to make ATP, which is then mostly exported to the cytoplasm to carry out work. Protons can also reenter the mitochondrial matrix through nonspecific leak pathways and via proteins such as uncoupling pro- teins (UCPs), which may catalyze an inducible proton transport activity in the inner membrane. In both cases, redox energy is dissipated as heat rather than being used to make ATP. Mitochondrial ROS production. The mitochondrial re- spiratory chain is the major site of ROS production within the cell. Superoxide is thought to be produced continually as a byproduct of normal respiration through the one- electron reduction of molecular oxygen (Fig. 1) (10,11). Superoxide itself damages iron sulfur center– containing enzymes such as aconitase (12) and can also react with nitric oxide to form the damaging oxidant peroxynitrite, which is more reactive than either precursor (13). Nitric oxide diffuses easily into mitochondria and may also be produced there (14). The mitochondrial enzyme manga- nese superoxide dismutase (MnSOD) converts superoxide to hydrogen peroxide, which, in the presence of ferrous or cuprous ions, forms the highly reactive hydroxyl radical, which damages all classes of biomolecules. The availabil- ity of free iron and copper within mitochondria is uncer- From the Medical Research Council Dunn Human Nutrition Unit, Cambridge, U.K. Address correspondence and reprint requests to Dr. Michael P. Murphy, MRC Dunn Human Nutrition Unit, Wellcome Trust/MRC Bldg., Hills Rd., Cambridge CB2 2XY, U.K. E-mail: mpm@mrc-dunn.cam.ac.uk. Received for publication 20 March 2003 and accepted 30 May 2003. M.P.M. is a paid consultant for Antipodean Biotechnology. This article is based on a presentation at a symposium. The symposium and the publication of this article were made possible by an unrestricted educa- tional grant from Les Laboratoires Servier.  H+ , proton electrochemical potential gradient; AGE, advanced glycation end product; DCF, dichlorofluorescein; DNP, 2,4-dinitrophenol; GSIS, glucose- stimulated insulin secretion; MitoVit E, mitochondria-targeted derivative of -tocopherol; MnSOD, manganese superoxide dismutase; ROS, reactive oxy- gen species; UCP, uncoupling protein. © 2004 by the American Diabetes Association. S110 DIABETES, VOL. 53, SUPPLEMENT 1, FEBRUARY 2004