AJP - Endocrinology and Metabolism, Vol 262, Issue 5 E712-E720, Copyright © 1992 by American Physiological Society
Binding of thyroxine and 3,5,3'-triiodothyronine to trout plasma lipoproteins
P. J. Babin
Laboratoire de Physiologie Cellulaire et Metabolique des Poissons, Unite de Recherche Associee 1134 du Centre National de la Recherche Scientifique, Universite Paris-Sud, Orsay, France.
The plasma vectors of thyroid hormones (TH) in trout have been
characterized. Plasma components were separated by density gradient
ultracentrifugation after first labeling binding sites with trace levels of
radioactive hormones, both in vivo and in vitro. Lipoproteins play only a
minor role in humans but are major carriers of thyroxine (T4) and
3,5,3'-triiodothyronine (T3) in trout plasma. More than 67% of T4 and 89%
of T3 were bound to lipoproteins (density less than 1.210 g/ml),
predominantly to high-density lipoproteins (HDL), regardless of the
nutritional status of the animals. The percentage of hormone bound to
very-low-density lipoproteins, on the other hand, was proportional to their
concentration and thus to nutritional status. T3 and T4 could also bind to
vitellogenin, a very-high-density lipoprotein, which could transfer TH to
the yolk of oocytes. Homologous ligand displacement indicated that T3 could
bind to at least two classes of saturable sites in the plasma. In addition,
plasma HDL were the major binding sites with low affinity (1.7 +/- 0.4 x
10(5) M-1) but with high capacity (3.1 +/- 0.3 x 10(-5) M). In conclusion,
these results show that lipoproteins are the principal binding sites of TH
in trout plasma.
