Nuclear binding of thyroid hormones and activity of malic enzyme and ornithine decarboxylase in rat liver during postnatal development.

The binding of 3,5,3'-L-triiodothyronine (T3) and thyroxine (T4) on liver nuclear receptors and the activity of malic enzyme and ornithine decarboxylase was examined in infantile rats aged in 1, 3, 7, 23, 29 days and in adult rats. No changes in the affinity constants (Ka) of nuclear receptors were observed for T3 or T4. The maximum binding capacity (MBC) estimated with the use of Scatchard plot analysis was unchanged for T3, the highest MBC for 125I-T4 being noted in rats aged 7 days. Malic enzyme activity in rat liver during the first three neonatal weeks was almost undetectable, but markedly increased on the 29th day. Ornithine decarboxylase activity was found to be significantly higher on the first day after birth as compared with that of the remaining age groups. The findings indicate that the thyroid hormone-nuclear receptor complex in rat liver does not seem to be sufficient for the induction of these enzymes in postnatal period of life.