Thyroxine binding properties of glycosylated human serum albumin as measured by fluorescence.

Thyroid hormone, thyroxine (T4) binding to glycosylated human serum albumin (G-HSA), and native human serum albumin (HSA) were studied as a function of pH using the fluorescence method. T4 binding affinity for G-HSA was remarkably reduced in an alkaline pH as compared with the native HSA. The thermodynamic parameters for binding are estimated at pH 7.5: (a) for G-HSA, delta G = -8.50 +/- 0.04 kcal mol-1 (30 degrees C), delta H = -5.2 kcal mol-1, delta S = +11 e.u.; (b) for HSA, delta G = -8.89 +/- 0.04 kcal mol-1 (30 degrees C), delta H = -3.5 kcal mol-1, delta S = +18 e.u. These results suggest that the glycosylation of HSA causes a variation in the electrostatic interaction between T4 and HSA.