Presence of iodothyronines in the yolk of the hen's egg.

Thyroxine (T4), 3,5,3'-triiodothyronine (T3) and 3,3',5-triiodothyronine (rT3) were determined in the yolk and white of the hen's egg and in the oocytes at various stages of development. For this purpose we have utilized the property of 0.08 N NaOH both to dilute the yolk or white and to extract and bind iodothyronines by strong alkaline Sephadex G-25. No iodothyronines were detected in the egg white. The total levels of T4 and T3/per 100 mg of yolk increased gradually with increased weight of oocytes weighing less than 6 g. Above this weight, levels of both iodothyronines were stable. The concentrations of T4 and T3 ranged from 0.6 to 1.0 ng and 150 to 230 pg/100 mg of yolk, respectively. The concentration of rT3 per 100 mg of yolk was independent of the weight of the oocytes and ranged between 10 and 100 pg/100 mg. The serum:yolk ratio oscillated between 2.62 and 1.15; 1.16 and 0.48; 0.11 and 0.15 in smallest and largest oocytes for T4, T3 and rT3, respectively. The perfusion experiment of the ovary indicates that all three iodothyronines can enter the ovary. The accumulation of iodothyronines by the ovary after 3 hr of incubation was as follows (in percentage of total radioactivity): T4, 35.85 +/- 2.94; T3, 26.73 +/- 2.97; rT3, 27.02 +/- 4.38. The highest accumulation of 125I-labelled iodothyronines per unit mass was seen in the oocytes of lowest size. The 3' or 5' deiodination of iodothyronines by the ovary, measured in the medium after 3 hr of perfusion, was negligible.