Stimulatory Effects of Thyroxine Administration on Reducing‐Equivalent Transfer from Substrate to Oxygen during Hepatic Metabolism of Sorbitol and Glycerol

Abstract

I n order to identify rate-limiting processes of hepatic sorbitol and glycerol metabolism in the hyperthyroid state, suspensions of isolated liver cells were prepared from thyroxine-treated rats. Rates of utilization of sorbitol and glycerol by these cells were compared with the data obtained in previous studies with euthyroid animals. At saturating substrate concentrations hydrogen flux from sorbitol or glycerol to 0, was increased 50-60°/, over control values to 2.7-2.8 pmol x g-1 x min-l following thyroxine treatment. When cells were exposed to two sources of reducing-equivalents by incubation with sorbitol and glycerol in combination, the rate of hydrogen flux to 0, was increased about 160°/, above control levels to 4.7 p r n o l~g -~x m i n -~. However, this value was only 85O/, of the sum of the flux rates in cells incubated with sorbitol and glycerol separately, since each member of the substrate mixture partially inhibited the uptake of the other. Pyruvate, added as a cytoplasmic hydrogen acceptor, did not enhance sorbitol and glycerol uptake by liver cells from thyroxine-treated rats except when the cells were incubated with these substrates in combination. In this circumstance pyruvate addition overcame the inhibition of sorbitol uptake by glycerol. It is inferred from these findings that in the hyperthyroid state the transfer of reducing-equivalents from substrate to 0, is not rate-limiting for the hepatic metabolism of sorbitol or glycerol individually, as it is in control animals, but it remains the rate-limiting process when liver cells from thyroxine-treated rats are incubated with mixtures of these substrates.Glucose was the major end-product of sorbitol and glycerol metabolism in cells from both euthyroid and hyperthyroid rats. As anticipated, therefore, rates of gluconeogenesis in cells from thyroxine-treated rats were increased in a manner corresponding to the enhanced capacity for hydrogen translocation. Maximal rates of glucose formation (3.39 pmol x g-l x min-l) were observed when cells were incubated with a mixture of sorbitol, glycerol and pyruvate. It was found that up to 7501, of the hydrogen flux to 0, during sorbitol and glycerol metabolism could utilize an antimycin-sensitive, rotenone-insensitive pathway, most likely involving mitochrondrial glycerolphosphate dehydrogenase. Less than 10 of the reducing-equivalent flux in these experiments was mediated by NAD-linked shuttles involving malate dehydrogenase. Significant stimulation of hydrogen translocation was observed a t 12 h after thyroxine administration and reached a plateau about 4 days after the initial thyroxine injection. This timecourse is consistent with the hypothesis that the effects of thyroxine are mediated through new protein synthesis and may reflect the time-course for the induction of mitochondria1 glycerolphosphate dehydrogenase. It is suggested that induction of this enzyme may represent an example of a general action of thyroxine to increase the capacity of the carrier systems which mediate intercompartmental hyd...

Thyroid thermogenesis. Relationships between Na+-dependent respiration and Na+ + K+-adenosine triphosphatase activity in rat skeletal muscle.

“…Unpublished observations. equivalents from glycerol to 02 on treatment with thyroid hormone has been identified as an indication of direct modulation of energy metabolism (41). At the mitochondrial level, two effects of thyroid hormone have been described: (a) some degree of uncoupling or loosening of the coupling ratio between oxidation and phosphorylation (42,43), and (b) increased carrier-mediated ADP transport into mitochondria (44).…”

The function of energy‐dependent redox reactions in cell metabolism

“…It has been shown that in the hyperthyroid state the 02-consumption by liver cells, in excess of that expected on the basis of the energy demand of gluconeogenesis, is much greater than under control conditions and reducing-equivalent transfer between cell compartments is greatly enhanced [37,74]. A stimulation of redox cycle activity by thyroxine would be in keeping with the hormone's calorigenic effect, since in view of their dissipative nature, redox cycles involving reversed electron transfer (tables 4 and 5) are likely to have a major function in heat production within the animal.…”

13 C NMR studies of gluconeogenesis in rat liver cells: Utilization of labeled glycerol by cells from euthyroid and hyperthyroid rats

“…This process has been intensively studied in perfused liver (5,6), freeze-clamped liver (7), and isolated hepatocytes (8,9). The early measurements by Freedland and Krebs (5) of the rate of glucose production from glycerol in perfused rat liver showed that the rate was appreciably increased in thyroxine-treated rats.…”