Investigation of Iodothyronine Binding to Plasma Proteins in Brook Trout, Salvelinus fontinalis, Using Precipitation, Dialysis, and Electrophoretic Methods

Over 95% of 3,5,3′-triiodo-L-thyronine (T3) or L-thyroxine (T4) up to added hormone levels of at least 5 μg/ml, were precipitated by trichloroacetic acid with plasma proteins of brook trout using a semimicro method. Hormone recovery in the precipitate was higher than with precipitation methods previously used on fish plasma.Equilibrium dialysis showed over 99% of T4 or T3 bound to plasma proteins of trout up to added hormone levels of at least 5 μg/ml.Acrylamide gel was superior to paper and particularly cellulose polyacetate as a medium for electrophoretic separation of proteins responsible for binding T3 or T4. In vitro studies at high hormone levels showed that T3 and T4 bound mainly to prealbumin-like proteins. In vivo studies at more physiological levels showed T4 bound to fast prealbumin-like, albuminlike, and β-globulin-like proteins, while T3 bound to slow prealbumin-like and probably the same albumin-like and β-globulin-like proteins as T4.