Fig. 1

Mitochondrial energy transduction and patho-physiology of oxidative stress upon heat stress. Mitochondrial electron transport chain (ETC.; C I, complex I; C II, complex II; Q, coenzyme Q; C III, complex III; cyt c, cytochrome c; C IV, complex IV), ATP synthase (coupling through oxidative phosphorilation) and uncoupling mechanisms (non-protein or protein, by avian uncoupling protein, avUCP or A nucleotide translocator, ANT, catalyzed proton leak) are shown. In the initial phase of acute HS the mitochondrial substrate oxidation (tricarboxylic acid cycle and/or β-oxidation) and ETC. activity are increased resulting in more reduced state of the electron carriers of the ETC. and an increase of ΔѰ resulting in elevated superoxide production (O₂°⁻), while during the later stage of acute HS, downregulation of avUCP worsens the oxidative stress. Chronic HS, however, leads to downsizing the mitochondrial metabolic oxidative capacity, upregulation of avUCP and a clear alteration in the pattern of antioxidant enzyme activities. Superoxide is readily dismutated by superoxide dismutase, SOD (CuZnSOD in intermembrane space and MnSOD in matrix) to give hydrogen peroxide (H₂O₂). H₂O₂ functions as the common ROS messenger in cell signaling due to its constant production, relative stability and diffusion properties. The primary targets of ROS for cell signaling are cysteine residues and protein bound metals, including heme iron. In cell signaling, downstream cascades will effect the activity of transcription factors of which AP-1, NF-kB and Nrf2 have been shown to be affected under HS conditions in poultry. Upon oxidative stress, an overflow of H₂O₂ can either be controlled by catalase (CAT; low affinity, high reactivity) and/or the glutathione-peroxidase/glutathione system (GSH-Px/GSH; high affinity, low reactivity) or undergo further reduction to yield the extremely reactive and dangerous hydroxyl radical (OH°) (Fenton reaction), possibly causing major damage to cellular biomolecules