Prazosin binding to human α1-acid glycoprotein (orosomucoid), human serum albumin, and human serum. Further characterization of the 'single drug binding site' of orosomucoid

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Abstract
The plasma protein binding of the α1-adrenergic blocking agent prazosin was investigated by means of circular dichroism (CD) and equilibrium dialysis (ED) measurements. The interaction of prazosin with human α1-acid glycoprotein (α1-AGP) results in pronounced negative extrinsic Cotton effects at 255 nm and a smaller negative band at 285 nm which are associated with the binding of prazosin to only one site of the protein. Various basic drugs, and warfarin also, at 50μM displace prazosin 10 μM from its binding site on α1-AGP and reduce the CD-spectra at 255 nm by 26% (disopyramide), 52% (mepivacaine), about 70% (verapamil, biperiden), and 90–100% (trihexyphenidyl, warfarin). (±)-Propranolol reduces the CD-spectra by 76%, its (-)-isomer by 89%, and the (+)-isomer by 65%. ED experiments indicated that the binding of prazosin to α1-AGP is saturable with an association constant of 48 000 M−1 and 0.85 binding sites per protein molecule. Displacement of prazosin from α1-AGP by the same drug as used for the CD experiments at displacer/porazosin ratios of 5 resulted in comparable reductions of the fraction bound as obtained by the CD experiments. Prazosin was also highly bound to human serum albumin (600 μM) with about 80–85% bound at prazosin concentrations from 1–100 μM. Since prazosin binding to human serum is only slightly higher (80–90%) it is concluded that prazosin binding in serum is largely mediated by the albumin fraction. The results indicate that prazosin binds to the single drug binding site of human α1-AGP, that this site is slightly stereospecific for propranolol, and that anionic as well as cationic drugs bind with high affinity to this drug binding site of α1-AGP.