One to three per cent of inspired molecular oxygen is converted to O2-,
which is the most common of the ROS and a powerful precursor of H2O2.5 Although cellular H2O2 is stable, it has the potential to interact with a variety of substrates to cause damage, especially in the presence of the reduced metal ion C59 wnt Fe2+. This leads to H2O2 to break down and form the most reactive and damaging of the ROS, OH-. In healthy cells, the production of the potentially harmful H2O2 is countered by the catalysing actions of mitochondrial or cytosolic catalase (CAT) or thiol peroxidases into H2O and O2. Figure 1 demonstrates pathways to, and natural anti-oxidant neutralization of, common ROS. Given that ROS are likely to be highly damaging molecules to cells, why have the mitochondria not evolved more efficient systems that limit mitochondrial oxidants? One possible answer is that ROS have an essential
role in oxidant metabolism where they are involved in highly conserved basic physiological processes as effectors of downstream pathways. Thus, to some, oxidative stress theories of disease pathogenesis must be intrinsically flawed.6 Nonetheless, ROS are damaging molecules. Even when they are produced during normal respiration, they could cause cumulative damage that would eventually lead to loss of cell and tissue function and, ultimately, disease. Their production is known to increase, over natural anti-oxidant levels, CT99021 during progressive disease and during ageing.4 The kidney is highly energetic and therefore relies heavily on aerobic metabolism for the production of ATP by oxidative phosphorylation. The reduction of molecular O2 along the electron transport chain (ETC) within mitochondria is vital for renal cellular function, yet potentially devastating long-term. The ETC consists of five multi-enzyme complexes responsible for maintaining mitochondrial membrane potential Phosphatidylinositol diacylglycerol-lyase and ATP generation.
Each of these complexes presents a site of ROS generation; however, complexes I and III have been identified as primary sites of O2- generation.7 Complex I, also known as nicotinamide adenine dinucleotide (NADH) dehydrogenase, or NADH-CoenzymeQ (NADH-CoQ) reductase, facilitates the transfer of electrons between NADH and CoQ10 (sometimes known as ubiquinone). Defects in oxidative phosphorylation may be due to the use of substrates in the respiratory chain, such as the reduced NADH and NADH oxidase, and not due to alterations in the proteins of the respiratory complexes. Thus, it is likely that altered respiratory complexes and substrates lead to an inefficiency of electron transport, and subsequent increased ROS, decreased ATP and a loss of the mitochondrial membrane potential. Oxidatively damaged proteins of the mitochondrial complexes increase with age in mice.8 In CKD patients (stages 2–3) and haemodialysis patients, impaired mitochondrial respiration was recorded.