Binding of Glycyrrhetinic Acid to Rat Plasma, Rat Serum Albumin, Human Serum, and Human Serum Albumin

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The binding of glycyrrhetinic acid (GLA) to rat plasma, rat serum albumin (RSA), human serum, and human serum albumin (HSA) was examined by an ultrafiltration technique. Specific and nonspecific bindings were observed in all cases. The assosiation constant (K) for the specific binding was 1.34 × 10⁵ M^-1 in rat plasma and 1.56 × 10⁵ M^-1 in RSA. The K value for human serum (14.91 × 10⁵ M^-1) and that for HSA (15.09 × 10⁵ M^-1) were approximately 10-fold larger than those for rat plasma and RSA, suggesting a species difference in the specific binding affinity for GLA. The number of specific binding sites (n) was 2.49 and 1.18 for RSA and HSA, respectively. The linear binding coefficient (ψ) for the nonspecific binding was 7.11 × 10³ M^-1 in RSA and 6.57 × 10³ M^-1 in HSA.When the rat plasma and human serum protein concentrations are respectively assumed to be 3.5% and 4.2% (equal to the measured plasma and serum albumin concentrations), n was 1.83 in rat plasma and 1.31 in human serum, which corresponded fairly well to those for RSA and HSA. Further, for ψ in rat plasma and human serum, when rat plasma and human serum protein are the measured albumin concentrations, respectively, ψ was 10.97 × 10³ M^-1 for rat plasma and 14.83 × 10³ M^-1 for human serum, which corresponded well to those for RSA and HSA.It was concluded that the GLA-binding sites in rat plasma and human serum exist in albumin and GLA binds to specific and nonspecific binding sites at lower and higher concentrations than approximately 2 mM, respectively.