A comment on estimating binding constants at variable protein concentrations

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 one site of the protein. Various basic drugs, and warfarin also, at 50 microM displace prazosin 10 microM 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 48 000 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 microM) with about 80-85% bound at prazosin concentrations from 1-100 microM. 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.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of physostigmine to rat and human plasma and crystalline serum albumins

The binding of 3H-physostigmine (3H-Ph) to human and rat plasma proteins and crystalline serum albumin was studied by ultrafiltration technique. This study showed that the percentage of 3H-Ph bound to rat plasma slightly decreased from 49% to 41% whereas human plasma showed an increase in binding from 29% to 43% over a 50-fold increase in drug concentration. Human plasma samples which were collected in a bag coated with citrate phosphate dextrose adenine-1 solution bound 50% less 3H-Ph than samples collected with EDTA indicating a drug-drug interaction between 3H-Ph and anticoagulants. No significant change in binding was observed if the samples were frozen prior to use. Scatchard plots for binding of 3H-Ph resulted in a positive slope for human plasma and a negative slope for rat plasma; whereas curvilinear Scatchard plots with negative slopes were obtained for binding to human and rat crystalline serum albumin.

Ultrafiltration in serum protein binding determinations

the ultrafiltration technique was evaluated theoretically and experimentally for use in clinical serum binding determinations. It is apparent from free energy considerations that the ultrafiltrate concentration approaches the true free concentration only as the pressure gradient causing flow reduces to zero. The theory presented accounts for the previously unexplained lower ultrafiltrate concentration observed at higher filtration pressures. Mathematical simulations of the molecular separation show that the ultrafiltrate concentration remains constant during filtration, and, thus, binding equilibria are not disturbed by this procedure, suggesting that an arbitrary restriction on the volume filtered is unnecessary. This finding greatly extends the value of the ultrafiltration technique in clinical binding determinations, especially for strongly bound, potent drugs where assays may be insufficiently sensitive to detect the extremely small free fraction reliably. These theoretical findings were verified experimentally by ultrafiltration of salicylate, ibuprofen, and carprofen in buffer, purified proteins, and whole serum.

Inverse dependence of binding constants upon albumin concentration. Results for L-tryptophan and three anionic dyes

The interactions of methyl orange, bromocresol green, 2-(4'-hydroxybenzeneneazo)benzoic acid (HABA) and L-tryptophan with human albumin at pH 7.4 were investigated by equilibrium dialysis at 37 degrees C. Binding characteristics of each of the three dyes were studied by two approaches: (1) variation of total ligand concentration with a single albumin concentration and (2) variation of albumin concentration with a single total ligand concentration. Both approaches gave typical Scatchard plots with negative slope for methyl orange and bromocresol green, with good agreement between the two sets of data for each dye. In contrast, approach (2) gave Scatchard plots with a positive slope for HABA and L-tryptophan, indicating a decrease in the number of binding sites (n) and/or association constant (k) as the albumin concentration increased. This inverse dependence of nk upon albumin concentration for 2-(4'-hydroxybenzeneazo)benzoic acid was mainly due to changes in n which were still observed in the presence of inhibitory chloride ions at pH 5.75. Reasons for this type of binding behaviour are discussed together with general implications for binding studies. The results show 2-(4'-hydroxybenzeneazo)benzoic acid to be a useful ligand for investigation of this problem.