Thyroxine-induced changes in rat liver mitochondrial cytochromes☆

Abstract The effects of 10 days of thyroxine injection (15 μg/100 g body weight) on rat liver mitochondrial cytochrome concentration and on the percent reduction of the individual cytochromes during succinatedriven state III and IV respiration was spectrophotometrically determined at cytochrome-specific wavelength pairs. The concentrations of cytochromes b , c , total c ( c + c 1 ) and a a 3 increased in hyperthyroid rats. The concentration of cytochrome C 3 remained constant in euthyroid and hyperthyroid rats. Changes in the concentration of the membrane-bound cytochromes were also determined by difference spectra in cytochrome c-depleted mitochondrial membranes. Cytochromes b and a a 3 showed increased concentrations in hyperthyroid rats while the concentration of cytochrome C 1 remained unchanged. Hyperthyroid mitochondria showed increased reduction of cytochromes b , c 1 , c and total c during state III respiration and cytochromes c 1 , c , and total c during state IV respiration. The percent reduction of cytochrome b decreased during state IV respiration in the hyperthyroid mitochondria. These results suggest that the increase in respiration observed in the hyperthyroid state may be related to changes both in the mitochondrial cytochrome concentration and in the cytochrome reduction level.