Morris S M
Department of Microbiology, Biochemistry, and Molecular Biology, University of Pittsburgh, Pennsylvania 15261.
Arch Biochem Biophys. 1987 Nov 15;259(1):144-8. doi: 10.1016/0003-9861(87)90479-6.
Thyroxine-induced metamorphosis of the tadpole to the frog (Rana catesbeiana) is marked by increased activities of the urea cycle enzymes in liver. Cloned cDNAs for two mammalian urea cycle enzymes--carbamyl-phosphate synthetase I and argininosuccinate synthetase--were shown to cross-hybridize with the corresponding mRNAs in tadpole liver. Thyroxine treatment produced nearly 10-fold, coordinate increases in hybridizable mRNA levels for these two enzymes in tadpole liver. This increase is sufficient to account for reported increases in enzyme levels and synthesis rates, demonstrating that thyroxine largely regulates concentrations of these enzymes at a pretranslational step(s). In contrast, levels of phosphoenolpyruvate carboxykinase mRNA in tadpole liver decreased by more than 90% following thyroxine treatment. This differs from the thyroxine-induced increases in synthesis rates of enzyme and mRNA reported for phosphoenolpyruvate carboxykinase in rat liver. However, the decreased levels of this mRNA in tadpole liver may represent a secondary response due to thyroxine-stimulated release of insulin.
甲状腺素诱导蝌蚪变态为青蛙（牛蛙）的过程中，肝脏中尿素循环酶的活性增强。已证明，两种哺乳动物尿素循环酶——氨甲酰磷酸合成酶I和精氨琥珀酸合成酶的克隆cDNA与蝌蚪肝脏中的相应mRNA发生交叉杂交。甲状腺素处理使蝌蚪肝脏中这两种酶的可杂交mRNA水平协调增加近10倍。这种增加足以解释所报道的酶水平和合成速率的增加，表明甲状腺素在转录前步骤很大程度上调节这些酶的浓度。相比之下，甲状腺素处理后，蝌蚪肝脏中磷酸烯醇丙酮酸羧激酶mRNA水平下降超过90%。这与甲状腺素诱导大鼠肝脏中磷酸烯醇丙酮酸羧激酶的酶和mRNA合成速率增加不同。然而，蝌蚪肝脏中这种mRNA水平的下降可能是由于甲状腺素刺激胰岛素释放引起的次级反应。
