The activity of hepatic α-glycerophosphate dehydrogenase (α-GPD) and malic enzyme (ME) was measured in rats bearing the Walker 256 carcinoma to assess the relationship between the reduction in the nuclear L-T3 (T3) receptor concentration which occurs in tumor-bearing (T) rats and the biological effects of thyroid hormones. T3 receptor concentration was uniformly decreased when tumors exceeded 9.5% of body weight. No change in basal α-GPD activity was observed, but ME activity was decreased to between 29–61% of control. The decrease in ME activity did not appear to be related to decreased nuclear T;) receptor concentration since ME was also decreased in T rats with tumors that were too small to effect a decrease in the concentration of nuclear T3 receptors or plasma T4 and T3. The response of both ME and α-GPD to T3 administration in T rats was comparable to that of control despite reduced nuclear T3 receptor and plasma hormone concentrations. When non-T rats were subjected to food restriction to simulate the attenuated growth curves of T rats, no changes were observed in nuclear T.t receptor concentration, plasma T4 and T3 concentration, or basal activity of a-GPD or ME. The induction of two thyroid hormonesensitive hepatic enzymes in rats with reduced nuclear T3 receptor and plasma T3 concentrations suggests that nuclear receptors are lost from sites which are not involved with the regulation of α-GPD or ME. Thus, unchanged receptor levels at sites which control α-GPD and ME may lead to normal biological responses of these enzymes despite reduced total receptor concentration.
ASJC Scopus subject areas