Mitochondrial metabolism in different thyroid states.
The protonmotive force, as well as the mitochondrial and cytosolic concentrations of malate, 2-oxoglutarate, glutamate and aspartate, were determined in livers from hypo-, eu- and hyper-thyroid rats, by density-gradient centrifugation of freeze-clamped livers in non-aqueous solvents [Soboll, Akerboom, Schwenke, Haase & Sies (1980) Biochem. J. 192, 951-954]. The mitochondrial/cytosolic pH difference and the membrane potential were significantly enhanced in hyperthyroid livers compared with the hypothyroid state, resulting in an increased protonmotive force in the presence of thyroid hormones [Soboll & Sies (1989) Methods Enzymol. 174, 118-130]. The mitochondrial concentrations of 2-oxoglutarate, glutamate and aspartate were significantly higher in the euthyroid than in the hypothyroid state, but only slightly higher in the hyperthyroid state. Mitochondrial malate, on the other hand, increased significantly from the hypothyroid to the hyperthyroid state. The mitochondrial/cytosolic concentration gradients were significantly increased in the presence of thyroid hormones only for malate. The changes in steady-state metabolite concentrations reflect a higher substrate supply and a stimulation of mitochondrial metabolism. However, a clear relationship between the increased protonmotive force, as the driving force for mitochondrial metabolite transport, and the subcellular metabolite concentrations is not observable in different thyroid states.