Reverse-flow electrophoretic and raclioautographic studies of fetal sheep serum show T4 and T3 binding to a serum alpha globulin different from TBG. This anodal T4-binding alpha globulin peak corresponds to the fetuin band. Purified calf fetuin also binds T4 in vitro. The optimal pH for this binding in 0.02M glycine acetate buffer is 8.0−8.4. Calf fetuin appears to contain one primary thyroxine-binding site per molecule with an association constant estimated to be 1.2 ì 109 L/M. Analogue binding studies in vitro suggest that optimal binding to the primary binding site requires the levorotatory configuration, the diphenyl ether structure, the phenyl hydroxyl radical, and 4 iodine atoms. (Endocrinology94: 49, 1974)
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Fisher et al. (1974) studied this question.