D-Thyroxine monosodium salt

Analysis for Binding of Thyroxine Enantiomers to Human Serum Albumin

Tomoko Kimura, et al. Pharmaceutical research, 2005, 22, 667-675.

Thyroxine (T4) is a thyroid hormone that binds strongly to plasma proteins such as albumin, thyroxine-binding globulin, and transthyretin. High-performance frontal analysis (HPFA) methods can be used to qualitatively and enantioselectively characterize the binding properties between thyroxine and human serum albumin (HSA). An HPFA-based online HPLC system was developed to determine the free concentrations of thyroxine enantiomers (L-thyroxine and D-thyroxine).
Both enantiomers exhibited binding to human serum albumin at two high-affinity sites, displaying comparable affinities. According to Scatchard plot analysis, the binding constants (K) and the number of binding sites (n) for l-thyroxine were determined to be K = 1.01 × 10^6 m^-1 and n = 1.90, while for d-thyroxine, K = 9.71 × 10^5 m^-1 and n = 1.97.
The specific binding sites were characterized using phenylbutazone and diazepam as probes for sites I and II, respectively. It was found that each enantiomer can bind to both sites. Additionally, the integration of a chiral HPLC column into the on-line system allowed for the analysis of enantiomer-enantiomer interactions, indicating that they competitively bind to the same sites without exhibiting notable allosteric effects.