A Thyroxine-Binding Protein in Human Breast Milk Similar to Serum Thyroxine-Binding Globulin*

Abstract

A thyroid hormone-binding substance (TBG_W) similar to serum T₄-binding globulin (TBG) has been identified in the whey fraction of human breast milk. TBG_W coeluted with serum TBG, as determined by Bio-Gel P-100 chromatography, and has an isoelectric point of 4.2–4.8, similar to that of serum TBG. The affinity constant of TBGW for T₄ was similar to that of serum TBG(K_a, 1.54 ± 0.38 × 10⁹ M^–1). Marked inhibition of TBG_W binding of [¹²⁵I]T₄ was achieved by the addition of 1.5 × 10^–3 M 8-anilino-l-naphthalene-sulfonic acid. An albumin-like low affinity site (K_a, >10⁷ M^–1) was also found. RIA of whey concentrates serially diluted in TBG-depleted serum indicated nearly identical binding curves for TBGW and TBG, with slopes of 2.33 (n = 15; r = 0.947) and 2.54 (n = 7; r = 0.996), respectively. Using a specific TBG RIA, a TBG_W concentration of 0.29 ± 0.08 μg/ml (mean – SD) in breast milk (n = 26) was determined. Paired serum and whey specimens from individuals between 6 and 20 weeks of lactation were analyzed by TBG RIA; in these individuals, mean serum TBG concentrations were 25.0 ± 3.4 μg/ml (n = 7); corresponding TBG_W levels were 0.26 – 0.08 μg/ml (n = 7), i.e. approximately 1% of serum levels. Nonetheless, linear regression analysis of the data revealed no significant correlation between serum and whey TBG concentrations in these individuals.