Individuals with higher metabolic rates have lower levels of reactive oxygen species in vivo

There is increasing interest in the effect of energy metabolism on oxidative stress, but much ambiguity over the relationship between the rate of oxygen consumption and the generation of reactive oxygen species (ROS). Production of ROS (such as hydrogen peroxide, H 2 O 2 ) in the mitochondria is primarily inferred indirectly from measurements in vitro , which may not reflect actual ROS production in living animals. Here, we measured in vivo H 2 O 2 content using the recently developed MitoB probe that becomes concentrated in the mitochondria of living organisms, where it is converted by H 2 O 2 into an alternative form termed MitoP; the ratio of MitoP/MitoB indicates the level of mitochondrial H 2 O 2 in vivo . Using the brown trout Salmo trutta , we tested whether this measurement of in vivo H 2 O 2 content over a 24 h-period was related to interindividual variation in standard metabolic rate (SMR) . We showed that the H 2 O 2 content varied up to 26-fold among fish of the same age and under identical environmental conditions and nutritional states. Interindividual variation in H 2 O 2 content was unrelated to mitochondrial density but was significantly associated with SMR: fish with a higher mass-independent SMR had a lower level of H 2 O 2 . The mechanism underlying this observed relationship between SMR and in vivo H 2 O 2 content requires further investigation, but may implicate mitochondrial uncoupling which can simultaneously increase SMR but reduce ROS production. To our knowledge, this is the first study in living organisms to show that individuals with higher oxygen consumption rates can actually have lower levels of H 2 O 2 .