Thyroid hormone-induced changes in cytoplasmic and mitochondrial proteins of a teleost

The effect of triiodothyronine (T-3) on the cytoplasmic and mitochondrial protein contents were studied in the liver and skeletal muscle of a freshwater teleost. The fish exposed to thiouracil for 28 days showed 1.5-2 times reduction in the total protein contents of cytoplasmic and mitochondrial fractions. A single injection of Tg to thiouracil exposed fish caused the earliest induction in the liver and skeletal muscle mitochondrial protein and the skeletal muscle cytoplasmic protein at 12 hr of lapses. However, the initial induction in the cytoplasmic protein of the liver was observed at 3 hr after T-3 treatment. The maximum inductions (1.5-3.2 fold) in the cytoplasmic and mitochondrial proteins of the liver and skeletal muscle were obtained at 18-24 hr following hormonal administration. Thereafter, the cytoplasmic and mitochondrial protein contents of both the tissues declined to their control levels within 36-48 hr of T-3 injection which reflected the half-life and turnover period of the induced proteins. These T-3 dependent inductions in the cytoplasmic and mitochondrial proteins of the liver (1.4-3.2 fold) and skeletal muscle (1.8-2.7 fold) were inhibited by actinomycin D and cycloheximide indicating T-3-induced de novo synthesis of the proteins. The induction in the cytoplasmic protein (3 fold) was almost double to that of the mitochondrial protein (1.6 fold) suggesting more synthesis of protein molecules in the cytoplasm for cellular and subcellular activities.