Handbook of Vitamin C Research

10Kelsey H. Fisher-Wellman and Richard J. BloomerFormation of RONS: Exercise ConditionsThe two primary radicals produced during exercising conditions are the superoxideradical and nitric oxide (NO·) [47]. Multiple potential sites exist for such formation andinclude both primary sources in which RONS are generated in direct response to a givencondition, as well as secondary sources in which RONS production may occur in response tomuscle damage induced through other mechanisms (e.g., high force eccentric muscleactions).Primary SourcesSimilar to resting conditions, it is commonly believed that RONS produced duringexercise is partially due to an increased leakage of electrons and subsequent production ofsuperoxide from the mitochondrial electron transport chain [53]. However, controversyexists as to whether this mechanism is relevant during exercising conditions, as themitochondria would be expected to be undergoing state 3 respiration [active phosphorylationof adenosine diphosphate (ADP)] [63]. Mitochondrial superoxide production appearscontingent upon the reducing potential present within the inner mitochondrial membrane[assessed via the ratio of reduced to oxidized nicotinamide adenine dinucleotide(NADH/NAD) or flavin adenine dinucleotide (FADH 2 /FAD)], as well as the ratio ofATP/ADP [64,65]. Because both the NADH/NAD and ATP/ADP ratios would be expectedto be decreased during exercise due to accelerated electron transfer and rapid ATPregeneration, electron leakage from the electron transport chain would be expected to beminimal [63]. However, mitochondrial superoxide production has in fact been shown tooccur during conditions of state 3 respiration (albeit to a lesser extent than what wasoriginally hypothesized) and appears to result from electron ―leakage‖ at complex I of theelectron transport chain [66]. Moreover, mitochondrial superoxide production has also beenshown to occur as a function of increasing temperature (i.e., heat stress) during exercise [67].This heat stress is believed to promote the instability of ubisemiquinone species bound to Qbinding proteins within the electron transport chain, ultimately promoting mitochondrialuncoupling and accelerated electron ―leakage‖ [53]. In addition to superoxide, mitochondriaalso have been shown to produce NO·, which in the presence of superoxide, rapidly leads tothe formation of the harmful radical peroxynitrite [68].Aside from direct production by the mitochondria, superoxide is also generated by wayof certain radical generating enzymes, including xanthine oxidase and NADPH oxidase, withthe latter being involved in both primary and secondary generating conditions. Periods ofintense exercise can result in a transient ischemic state within certain regions of the body,which then triggers the production of hypoxanthine from ATP and the conversion of xanthinedehydrogenase to xanthine oxidase by cysteine residue modification and/or partialproteolosysis [69]. Xanthine oxidase is capable of the direct production of both superoxideand hydrogen peroxide [53]. Superoxide generation also can occur via NADPH oxidaseassociated with the plasma membrane [70], sarcoplasmic reticulum [71], or triads andtransverse tubules [72] of skeletal muscle.