Interaction of diiodothyronines with isolated cytochrome c oxidase

Diiodothyronines (3,3'-T2 and 3,5-T2) stimulate the activity of isolated cytochrome c oxidase (COX) from bovine heart mitochondria. Maximal stimulation of activity (about 50%) is obtained with 3,3'-T2 at pH 6.4 and with 3,5-T2 at pH 7.4. In contrast, 3,5,3'-triiodothyronine (T3) exhibited no or little stimulation of COX activity. Binding of the hormones to COX leads to conformational changes as shown by modified visible spectra of the oxidized enzyme. It is suggested that 'short-term' effects of thyroid hormones on mitochondrial respiration are at least partly due to the allosteric interaction of diiodothyronines with the COX complex.