Characteristics of postnatally induced alterations in thyroxine 5'- and 5-monodeiodinating activities in several pig tissues.

Changes in the in vitro hepatic, kidney, skeletal muscle, heart and brain 5'- and 5-monodeiodinase activities were studied in piglets aged 4-12 h and 3, 7, 14, 21, and 42 days. As compared to the adult pig, the hepatic conversion of thyroxine (T4) to 3,5,3'-triiodothyronine (T3) and 3,3'5'-triiodothyronine (rT3) was on average 3-5 times higher in the liver, and equal or lower in the kidney. At birth, the liver generated about 66% of T3 and 75% of rT3, the other tissues the rest. Increasing contribution of skeletal muscles in the postnatal process of triiodothyronines generation was found. The apparent Km and Vmax values suggested higher capacity of T4 converting systems in newborns than in adults, and an increase in the enzyme affinities. Shortly after birth serum rT3 but not T3 corresponded to the level of hepatic and kidney T4 conversion.