The Three Thyroxine-Binding Proteins in Rat Serum: Binding Capacities and Effects of Binding Inhibitors

Polyacrylamide gel electrophoresis shows radiothyroxine added to rat serum is distributed among 3 proteins: a slow-migrating prealbumin (carrying 55% of tracer T4), albumin (15%) and a postalbumin (18%). This tracer pattern is qualitatively similar to that in human serum. Quantitatively, however, the prealbumin is the principal carrier in rat serum, whereas a postalbumin (thyroxine-binding globulin, TBG) is the major carrier in human serum. Rat postalbumin (R-TBG) T4-binding capacity is about 2 5g/100 ml, measured at a T4 concentration in serum of 25 5g/100 ml. Rat prealbumin (RTBPA)- binding capacity determined at a T4 level of 200 5g/100 ml is about 140 5g/100 ml. Effects of competitive inhibitors of T4 binding on the prealbumin, as well as on albumin and RTBG, indicate that the T4 sites on the 3 proteins are qualitatively distinct in a pattern similar to that of primates. Prealbumin has not been generally recognized as the major plasma carrier of T4 in the rat, presumably because the electrophoretic mobility of R-TBPA is not as great as that of primate TBPA. The present observations and method for quantitation of the T4 carriers suggest that the rat may prove to be a useful model for a variety of studies including those of factors controlling free hormone levels in plasma. (Endocrinology87: 978, 1970)

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