INTERACTION OF DIIODOTHYRONINES WITH ISOLATED CYTOCHROME-C-OXIDASE

Diiodothyronines (3,3'-T-2 and 3,5-T-2) stimulate the activity of isol ated cytochrome c oxidase (COX) from bovine heart mitochondria. Maxima l stimulation of activity (about 50%) is obtained with 3,3'-T-2 at pH 6.4 and with 3,5-T-2 at pH 7.4. In contrast, 3,5,3'-triiodothyronine ( T-3) exhibited no or little stimulation of COX activity. Binding of th e hormones to COX leads to conformational changes as shown by modified visible spectra of the oxidized enzyme. It is suggested that 'short-t erm' effects of thyroid hormones on mitochondrial respiration are at l east partly due to the allosteric interaction of diiodothyronines with the COX complex.