Abstract
Thyroxine (T4) 5'-monodeiodinating activity in rat pituitary was studied, using paper chromatography. Rat anterior pituitary homogenates were incubated with 125I-T4 , dithiothreitol and unlabeled L-T4 at 37°C. The incubation mixture was extracted by ethanol and the extracts were subjected to descending paper chromatography. The conversion ratio of T4 to 3, 5, 3'L-triodothyronine (T3) was calculated from the radioactivity of T3spot to total radioactivity on the paper strip. The T3amount generated was estimated from the T4 concentration in the incubation mixture and the conversion ratio.
The T3generating activity from T4 increased as the amount of tissue increased. It was temperature-and pH-dependent, and thiol sensitive. These results suggest the enzymatic nature of T4 5'-monodeiodinating activity of the anterior pituitary. A kinetic study revealed low Km for T4 (7.9±1.6nM, Mean±SE), with Vmax of 68.0±12.7 fmoles T3/mg Protein/min.
T4 monodeiodinating activity was consistently, though minimally, detected in fetal rat pituitaries and increased after birth, reaching the maximum at 22days. It declined thereafter to the young adult level. Serum TSH levels were markedly elevated in fetus. They decreased after birth, reaching the nadir at 22days, and then increased to young adult levels. Serum T4 and T3levels were markedly diminished in fetus and gradually increased after birth, reaching the young adult levels at 17 days. Thus, a reciprocal relationship was observed between intrapituitary T3generating activity from T4 and serum TSH levels in developing rats. It suggests that the conversion of T4 to T3in the pituitary plays a role in regulating TSH secretion.
