EFFECTS OF SHORT-TERM 17-BETA-ESTRADIOL TREATMENT ON THE PROPERTIES OF T4-BINDING PROTEINS IN THE PLASMA OF IMMATURE RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

To determine the effects of 17-beta-estradiol (E2) on the properties of the plasma proteins that bind L-thyroxine (T4), immature rainbow trout, Oncorhynchus mykiss, were injected intraperitoneally on days 0 and 3 with 0.5 mg E2-3-benzoate/100 g body weight, and plasma was sampled on days 4, 7, or 12. Control trout received peanut oil alone. E2 caused a small but significant decrease in the free T4 index. Saturation analysis on miniature G-25 Sephadex columns revealed at least two major T4-binding sites. Filtration on agarose Bio-gel A 1.5 also indicated two major T4-binding protein fractions with molecular weights of 150 and 55 kDa with a small proportion of T4 binding to a 1,500-kDa site presumed to be lipoprotein. Addition of unlabeled T4 displaced [I-125]T4 from the 55-kDa site and unmasked an adjacent site of higher molecular weight. E2 increased the proportion of T4 bound to the low-affinity (150 kDa) site relative to that bound to the high-affinity (55 kDa) site, increased the level of protein associated with the 1,500-kDa site and its T4 binding, and also initiated the production of presumed vitellogenin (VTG), which bound a small amount of T4. It is concluded that the E2-induced depression in FT4 is caused by a shift in T4 binding between high-affinity and low-affinity sites, and also by binding of small amounts of T4 to presumed lipoprotein and VTG.