Effect of Thyroid Hormones and Their Metabolic Products Upon Succinic Dehydrogenase Activity of Mouse Heart Muscle in Vitro

Abstract

As to the effect of thyroxine upon succinic dehydrogenase activity, some observations have been made in this laboratory employing an improved Thunberg technique. Thyroxine was learned to be effective in a diphasic way in the sense that the hormone in its low concentration range activated succinic dehydrogenase, while inhibitory effect resulted at its relatively high concentrations, when homogenates of mouse and dog tissues were used as the enzyme sources (6, 7). Further detailed work was done on the mode of action of thyroxine upon the same enzyme of rat liver and heart muscle under various experimental conditions, in which concentrations of homogenate and substrate, succinate, were changed (11). Following up these observations, experiments were extended in order to test whether triiodothyronine, and the acetic and propionic acid analogues of thyroxine and triiodothyronine show a similar effect upon succinic dehydrogenase of heart muscle homogenate of mouse . MATERIALS AND METHODSAdult male mice of ddN strain, weighing approximately 20g. each, were employed.Animals were sacrificed by decapitation and heart muscle was excised, weighed on a torsion balance, homogenized with 500 volumes of distilled water using the whole-glass homogenizer, and the homogenate obtained was used for enzyme source.All these procedures were carried out at room temperature.The activity of succinic dehydrogenase was measured employing Thunberg tubes after Yamamoto and Sugisawa (11). The composition of the reaction mixture was as follows:In the main tube:1) 0.1M K2HPO4-KH2PO4 buffer (PH 7.4) 1.3 ml., 2) 0.2% homogenate 0.6ml., 3) hormone solutions of various Received for publication October 24, 1958.