Regulation of glucose-6-phosphate dehydrogenase by diet and thyroid hormone.

The regulation of hepatic glucose-6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) RNA by thyroid hormone and high carbohydrate (sucrose) diet was studied. Previous studies from several laboratories have demonstrated that thyroid hormone modulates G6PDH activity. However, the point at which thyroid hormone exerts this regulation has not been adequately addressed. In order to assess the role of thyroid hormone in this regulation, levels of G6PDH mRNA were determined in hypothyroid rats maintained on normal or high carbohydrate diets with or without thyroid hormone (triiodothyronine; T3) supplementation. A dot-blot hybridization procedure with nick-translated cDNA probes was used to directly assess the relative concentrations of G6PDH mRNA. Enzyme activity increased when the animals were treated with T3 and/or placed on a high carbohydrate diet. However, there was no effect of T3 and diet, alone or in combination, on G6PDH mRNA levels in hypothyroid rats. The data suggests that thyroid hormone and high carbohydrate diet are acting at a translational level to increase G6PDH enzyme activity in these animals.