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

In 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 O 2 was increased 50–60% over control values to 2.7–2.8 μmol × g −1 × min −1 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 O 2 was increased about 160% above control levels to 4.7 μmol × g −1 × min −1 . However, this value was only 85% 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 O 2 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 μmol × g −1 × min −1 ) were observed when cells were incubated with a mixture of sorbitol, glycerol and pyruvate. It was found that up to 75% of the hydrogen flux to O 2 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 at 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 mitochondrial 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 hydrogen and metabolite translocation and thereby stimulate hepatic intermediary metabolism.