Release of reactive oxygen species (ROS), measured as the sum of hydrogen peroxide (H2O2) and superoxide anion radical (O-2(center dot-)), from respiring rat heart and skeletal muscle mitochondria was significantly decreased by millimolar concentrations of GTP or GDP. Attempts to differentiate between the two forms of ROS showed that the release of O-2(center dot-) rather than that of H2O2 was affected. Meanwhile, intramitochondrial ROS accumulation, measured by inactivation of aconitase, increased. These results suggest that guanine nucleotides inhibit the release of O-2(center dot-) from mitochondria. As these nucleotides are known inhibitors of uncoupling proteins (UCPs), it is proposed that UCPs may function as carriers of O-2(center dot-), thus enabling its removal from the matrix compartment. (C) 2011 Elsevier Inc. All rights reserved.