STUDIES ON THE APPEARANCE AND ACTIVITY OF TISSUE THYROXINE 5'-MONODEIODINASE AND 5-MONODEIODINASE DURING ONTOGENY IN THE FETAL PIG

The ontogeny of the fetal monodeiodinase systems, during the period be fore the onset of thyroid activity until birth, has been described onl y in the rat, rabbit and chick. We have studied 5'- and 5-monodeiodina se activities in the liver, kidney and placenta of pig fetuses, from d ay 32 of gestation up to birth, and on days 1, 3, 7 and 14 after birth . Fetuses (123) from 15 litters, 32 newborn piglets, 15 sows (mothers) and 10 non-pregnant pigs were used. The relationships between monodei odinase activities, thyroxine (T-4), tri-iodothyronine (T-3) and rever se tri-iodothyronine (rT(3)) blood serum concentrations and sulphydryl groups (total (T-SH) and non-protein (NP-SH), were measured. 5'-Monod eiodinase activity (5'-MD type I; converting T-4 to T-3) and 5-monodei odinase (5-MD; converting T-4 to rT(3)) were detectable in the liver a t the onset of thyroid hormone biosynthesis, and before that time (day 32 of gestation) in the placenta. Liver 5'-MD activity was very low b etween days 32 and 84 of gestation, and then increased rapidly at abou t day 93, reaching a maximal value 2 days before term. Activity in the liver (but not in the kidney) was higher than that in maternal, newbo rn and non-pregnant adult tissue. The increase in NP-SH groups in the liver and kidney and T-SH groups in the placenta during the last 5 day s of gestation coincided with the increase in 5'-MD activity. The 5-MD activity of the liver was generally five to ten times lower than that of 5'-MD, being highest on days 65-76 and 65-84 in the kidney and pla centa respectively. Serum T-4 concentrations paralleled the steady inc rease in T-4- to T-3-converting activity in the kidney and liver, but not in the placenta. Two weeks before birth, fetal T-3 concentrations exceeded those of rT(3) by a factor of 1.5 or more, indicating that T- 4 metabolism in the pig is dominated by the activity of 5'-MD. This pa ttern is different from that in prenatal humans, rats or sheep in whic h rT(3) is the primary circulating T-4 metabolite.