Influence of pH on the thyroxine effect upon succinoxidase system.

There have been many conflicting results concerning the in vitro effect of thyroxine upon respiratory enzyme systems (Barker, 1951). With regard to the succinoxidase system, Gemmill (1952) and Radsma et al.(1954) showed that thyroxine augmented the enzyme activity in vitro, while, in contrast to such results, McShan et al.(1947) reported inhibitory effect of the hormone, and furthermore, Smith and Williams-Ashman (1951) found no effect. On the other hand, in this laboratory, Yoshihiro (1954) found the diphasic action of thyroxine owing to the hormone concentrations with use of dog thyroid slices employing glucose as a substrate, and Katsura (1954) reported that effect of the hormone upon succinoxidase of heart muscle and adrenal cortex homogenate was also diphasic, namely, in relatively low concentrations the hormone was effective in the sense of activation, while in higher concentrations it was inhibitory.Sugisawa (1955) showed similar results using succinic dehydrogenase of mouse liver by means of an improved Thunberg technique.