Nature of respiratory stimulation in hyperthyroidism: the redox behaviour of cytochrome c

Hyperthyroid mitochondria show an increased Km and Vmax in the high affinity phase of cytochrome oxidase kinetics. During inhibitor titrations, cytochrome c shows a different redox behaviour in hyperthyroid with respect to protonophore-treated euthyroid mitochondria. The observations are discussed in terms of a different regulation of electron input and output into the respiratory chain during slip and leak types of uncoupling. In hyperthyroid mitochondria during inhibitor titrations, the pattern of the relationship between uncoupler-induced extra-respiration and membrane potential is highly non-linear. The complex nature of the respiratory stimulation in hyperthyroid mitochondria is discussed.