1.
In vitro effects of thyroxine upon the succinic oxidase system as a whole together upon significant components of the system in the whole liver homogenate of the mouse were examined with wide range of concentration of the hormone.
2. It was learned that thyroxine exerted inhibitory effect upon the succinic oxidase system as a whole under the presence of cytochrome c. On the contrary, succinic dehydrogenase activity, which was measured by means of Warburg manometer in the presence of KCN and methylen blue, was augmented by the hormone.
3. Cytochrome oxidase activity when observed manometrically on ascorbate employed as substrate in the presence of cytochrome c was activated by the hormone.
4. Effect of thyroxine is deduced to be inhibitory upon the succinic cyto chrome c reductase activity, though the hormone was observed to activate the succinic dehydrogenase activity.
5. On the basis of the present study, inhibitory effect of the hormone observed with the over-all reaction of succinate oxidation in mouse whole liver homogenate is deduced to be due to hormonic inhibition of succinic cytochrome c reductase activity.
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