Rapid stimulation in vitro of rat liver cytochrome oxidase activity by 3,5-diiodo-L-thyronine and by 3,3'-diiodo-L-thyronine.

The effect of the iodothyronines (thyroxine (T4), 3,5,3'-triiodo-L-thyronine (L-T3), 3,5-diiodo-L-thyronine (3,5-T2), 3,3'-diiodo-L-thyronine (3,3'-T2), 3',5'-diiodo-L-thyronine (3',5'-T2), 3'-monoiodo-L-thyronine (3'-T1), 3-monoiodo-L-thyronine (3-T1) and thyronine (T0)) on rat liver cytochrome oxidase (COX) activity after their addition to rat liver homogenate and isolated mitochondria from normal and hypothyroid rats has been investigated. The addition of 3,3'-T2 and 3,5-T2 (T2s) to the liver homogenate from hypothyroid rats, but not from normal rats, significantly enhanced COX activity. The addition of T3 had a remarkably lower effect that was almost completely abolished when the propylthiouracil (PTU), an inhibitor of the type I deiodinase activity, was also added to the incubation mixture. After the addition of T2s the maximum effect was obtained at a concentration of about 10(-6) M for both 3,3'-T2 and 3,5-T2, while a 50% increase was obtained at a concentration of about 10(-9) M in both cases. The effects of T2s were rapid and already evident after 5 min of incubation (+40-50%). The maximal effect was reached after only 30 min of incubation. The above effects were not observed after the addition of T2s to the isolated mitochondria. The results clearly demonstrate that both 3,3'-T2 and 3,5-T2 directly stimulate mitochondrial COX activity which is possibly achieved through a cytoplasmic factor. The addition of the other iodothyronines (T4, 3',5'-T2, 3'-T1, 3-T1 and T0).