Interaction between carbohydrate residues of α1-acid glycoprotein (orosomucoid) and progesterone. A fluorescence study Original Research Article

Interaction between progesterone and the carbohydrate residues of α1-acid glycoprotein was followed by fluorescence studies using calcofluor white. The fluorophore interacts with polysaccharides and is commonly used in clinical studies. Binding of progesterone to the protein induces a decrease in the fluorescence intensity of calcofluor white, accompanied by a shift to the short wavelengths of its emission maximum. The dissociation constant of the complex was found equal to 8.62 μM. Interaction between progesterone and free calcofluor in solution induces a low decrease in the fluorescence intensity of the fluorophore without any shift of the emission maximum. These results show that in α1-acid glycoprotein, the binding site of progesterone is very close to the carbohydrate residues. Fluorescence intensity quenching of free calcofluor in solution with cesium ion gives a bimolecular diffusion constant (kq) of 2.23×109 M−1 s−1. This value decreases to 0.19×109 M−1 s−1 when calcofluor white is bound to α1-acid glycoprotein. Binding of progesterone does not modify the value of kq of the cesium. Previous studies have shown that the terminal sialic acid residue is mobile, while the other glycannes are rigid [Albani, J. R.; Sillen, A.; Coddeville, B.; Plancke, Y. D.; Engelborghs, Y. Carbohydr. Res. 1999, 322, 87–94]. Red-edge excitation spectra and Perrin plot experiments performed on sialylated and asialylated α1-acid glycoprotein show that binding of progesterone to α1-acid glycoprotein does not modify the local dynamics of the carbohydrate residues of the protein.