**5. Mitochondrial Metabolism, ROS, and OS in Kidney Diseases**

The kidneys remove waste from the blood, reabsorb nutrients, regulate electrolyte balance, maintain acid-base homeostasis and regulate blood pressure [10]. These processes require high amounts of energy, which come from OXPHOS or anaerobic glycolysis, depending on the region of the kidneys. For example, the renal cortex uses OXPHOS and low amounts of glucose, while the renal medulla uses glycolysis and lactate. Therefore, the medulla necessarily uses anaerobic glycolysis due to low oxygen levels. In contrast, the renal cortex uses OXPHOS, fed mainly by FA oxidation [10,101].

A significant number of mitochondria in renal cells is located in PT, the most metabolically active [10]. OXPHOS is the principal mechanism to produce ATP in renal proximal tubular cells (RPTCs). Ninety percentage of ATP is required to reabsorption of glucose, ions, and nutrients through the sodium-potassium ATP pump (Na+/K+ ATPase) [102]. PT uses FA, such as palmitate, through FA β-oxidation to produce high ATP levels (Figure 3) [103]. Therefore, RPTCs have high levels of carnitine *O*-palmitoyl transferase I (CPT I) isoforms A (CPT IA) and B (CPT IB), and carnitine *O*-palmitoyl transferase II (CPT II) [104].
