AJP - Endocrinology and Metabolism, Vol 261, Issue 2 E257-E261, Copyright © 1991 by American Physiological Society
Postnatal changes in lambs of two pathways for thyroxine 5'-monodeiodination in brown adipose tissue
S. Y. Wu, J. K. Kim, I. J. Chopra, Y. Murata and D. A. Fisher
Nuclear Medicine and Medical Services, Veteran Administration Medical Center, Long Beach 90822.
We have recently shown that ovine fetal brown adipose tissue (BAT) contains
two distinct iodothyronine 5'-monodeiodinase (5'MDI) activities, one with a
high Km (type I) and another with a low Km (type II). Both activities
increased to maximum levels near term (150 days gestation). BAT plays a
major role in neonatal temperature regulation in lambs, and available
evidence suggests that BAT 5'MDI activity is closely linked to thermogenic
capacity. To better characterize the changes in 5'MDI after birth, we
studied both type I and type II 5'MDI in lamb BAT from the time of birth to
30 days of postnatal age. Type I 5'MDI activity [pmol
3,5,3'-triiodothyronine (T3).mg protein-1.h-1] showed no significant
changes during the first 11 days after birth [newborn (NB), 95 +/- 16; 1
day, 83 +/- 20; 3-4 days, 80 +/- 11; 10-11 days, 92 +/- 28]. Activity
decreased significantly at 30 days (24 +/- 8.9, P less than 0.05). On the
other hand, the type II 5'MDI activity (fmol I- released.mg protein-1.h-1)
increased significantly (P less than 0.01) during the first 4 days, (NB,
348 +/- 23; 1 day, 679 +/- 37; 3-4 days, 785 +/- 199), decreased toward NB
values (401 +/- 87) at 10-11 days of age, and fell to 66 +/- 31 at 30 days
(P less than 0.05 vs. NB). Kinetic analysis of BAT type II thyroxine 5'MDI
revealed a rise in maximum velocity from NB to 1 and 3-4 days of age
without a change in the enzymatic activity Km.(ABSTRACT TRUNCATED AT 250
WORDS)
