PRAZOSIN BINDING TO HUMAN ALPHA-1-ACID GLYCOPROTEIN (OROSOMUCOID), HUMAN-SERUM ALBUMIN, AND HUMAN-SERUM - FURTHER CHARACTERIZATION OF THE SINGLE DRUG-BINDING SITE OF OROSOMUCOID

The plasma protein binding of the .alpha.1-adrenergic blocking agent prazosin was investigated by means of circular dichroism (CD) and equilibrium dialysis (ED) measurements. The interaction of prazosin with human .alpha.1-acid glycoprotein (.alpha.1-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 1 site of the protein. Various basic drugs, and warfarin also, at 50 .mu.M displace prazosin 10 .mu.M from its binding site on .alpha.1-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 .alpha.1-AGP is saturable with an association constant of 48000 M-1 and 0.85 binding sites per protein molecule. Displacement of prazosin from .alpha.1-AGP by the same drug as used for the CD experiments at displacer/prazosin 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 .mu.M) with about 80-85% bound at prazosin concentrations from 1-100 .mu.M. Since prazosin binding to human serum is only slightly higher (80-90%) prazosin binding in serum must be largely mediated by the albumin fraction. Evidently prazosin binds to the single drug binding site of human .alpha.1-AGP, this site is slightly sterospecific for propranolol, and anionic as well as cationic drugs with high affinity to this drug binding site of .alpha.1-AGP.