Thyroidal regulation of substrate kinetics properties of cytochrome oxidase in rat liver mitochondria.

Effects of thyroidectomy (T(x)) and subsequent treatment with 3,5,3'-triiodothyronine (T(3)), and combined treatment (T(R)) with T(3) + thyroxine (T(4)) on substrate kinetics properties of cytochrome oxidase of rat liver mitochondria were examined. T(x) resulted in lowering of cytochromes content with decrease in the enzyme activity, and K(m) and V(max). T(3) and T(R) regimens restored the cytochromes contents and the V(max) values to normal. In control, T(3) and T(R) groups the enzyme activity resolved in two kinetic components; in T(x) group three kinetic components were evident. The K(m) values for all components decreased significantly in the experimental groups with concomitant increase in catalytic efficiency, K(cat)/K(m). Significant alterations in the contents of total phospholipid and of cholesterol were noted while the changes in the phospholipids composition were only of restricted nature. Regression analysis revealed that total phospholipid, cholesterol and phosphatidylcholine, phosphatidylethanolamine play significant role in fine tuning the enzyme activity.