Hormone binding to natural mutants of human serum albumin

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Abstract
High-affinity binding of progesterone, testosterone, prostaglandin F2α and l-thyroxine to five genetic variants of human serum albumin with defined point mutations was investigated by equilibrium dialysis (pH 7.4). Endogenous albumin A (Alb A) from each individual and commercial human serum albumin were used as controls in each case. The association constant for binding of progesterone to Alb Canterbury (Lys313 → Asn) was 1.5 times that calculated for binding to the corresponding, endogenous Alb A. In contrast, the variants Alb Niigata (Asp269 → Gly), Alb Roma (Glu321 → Lys), Alb Parklands (Asp365 → His) and Alb Verona (Glu570 → Lys) all had normal progesterone binding properties. Specificity with respect to the type of mutation was also found for the binding of testosterone and prostaglandin F2α. Testosterone binding to Alb Roma was only 0.7 of that determined for endogenous Alb A, whereas prostaglandin F2α binding to Alb Niigata was increased by a factor 2.4. In the case of l-thyroxine normal binding properties were found for all the variants. Steric effects and/or conformational changes of the protein, introduced by the amino acid substitutions, probably account for the altered hormone binding. However, in the case of the increased binding of prostaglandin F2α to Alb Niigata electrostatic effects could also be involved. The experimental findings suggest different high-affinity sites for the four hormones. Progesterone, testosterone and prostaglandin F2α are apparently bound within the middle third (domain II) of the protein molecule. The possible position of the primary l-thyroxine site is discussed.

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