Stimulation of hepatic mitochondrial α-glycerophosphate dehydrogenase and malic enzyme by L-triiodothyronine. Characteristics of the response with specific nuclear thyroid hormone binding sites fully saturated

Experiments were designed to analyze the relationship of a single i.v. dose of triiodothyronine (T₃), the level of plasma and hepatic nuclear T₃ attained, and the tissue response as reflected in increased activity of hepatic mitochondrial α glycerophosphate dehydrogenase (α GPD) and cytosol 'malic enzyme' (ME). These studies were carried out in euthyroid rats by varying the dose of T₃ injected and the time at which the animals were killed and the enzyme levels measured. The plasma T₃ concentration was determined and the fraction of nuclear sites occupied at any time t was calculated from the known plasma:nuclear relationship. As a first step, the analysis was confined to the limiting situation in which all nuclear sites were effectively saturated. The following additional information was required and obtained: A proportional relationship between the half neutralizing volume of a specific antiserum to malic enzyme and the activity of malic enzyme was established, thus confirming previous reports that the increase in enzyme activity induced by T₃ is due to increased enzyme mass. The absolute refractory period immediately after i.v. injection of T₃, during which no enzyme response could be detected, was determined. This was shown to be 13.4 h for α GPD and 8.2 h for ME. Lastly, the t(1/2) of the enzyme decay after pulse injection of T₃ was measured. This was similar for both enzymes, 2.8±0.6 (SD) days for α GPD and 2.7±0.6 (SD) days for ME. The results of these studies indicated that the extent of hepatic response appears limited by full occupancy of a set of intracellular receptor sites by T₃ which is in rapid equilibrium with the plasma hormone pool. The kinetic properties of the receptors, as functionally defined in these studies, resemble those associated with the recently described specific nuclear T₃ sites. These data per se are thus compatible with but do not prove a nuclear site of initiation of hormone effect. They do allow the development of an internally consistent mathematical model which permits prediction of enzyme response when the receptor sites are fully occupied for a given length of time after the i.v. injection of hormone. A separate series of studies was carried out in thyroidectomized rats. The response characteristics of α GPD were similar to those observed in euthryroid animals. In contrast, however, the early response of ME to pulse injections of T₃ was very much reduced in hypothyroid animals as compared to euthyroid animals in which nuclear sites were saturated for comparable periods. These findings raise the possibility that a factor required for the induction of malic enzyme but not α GPD is deficient in the hypothyroid state.