Differences in the serum protein binding of prazosin in man and rat

doi:10.1016/0006-2952(84)90339-3

Biochemical Pharmacology

Volume 33, Issue 11, 1 June 1984, Pages 1719-1724

https://doi.org/10.1016/0006-2952(84)90339-3 Get rights and content

Abstract

The serum protein binding of prazosin in man and rat has been studied in vitro by equilibrium dialysis. Prazosin was more extensively bound in human serum than in rat serum with binding ratios (B/F) of 14.3 ± 3.4 and 4.4 ± 0.2 (corresponding to 93.4 and 81.4% bound), respectively. This difference in binding between the species was partly due to qualitative differences between human and rat serum albumin, but also to the lower concentration of albumin in rat serum. Rat serum albumin (RSA) apparently showed two different classes of binding sites for prazosin, one with high (K_D = 5.78 × 10⁻⁶M) and one with low (K_D = 1.1 x 10⁻⁴M) affinity; the former is suggested as representing α₁-acid glycoprotein (α₁-AGP) with one binding site for prazosin per molecule, the latter as representing RSA with 0.28 binding sites per molecule. Human serum albumin (HSA) and human α₁-AGP both showed one class of binding sites with K_D values of 2.7 × 10⁻⁵ and 1.95 × 10⁻⁶M, respectively. HSA possessed 0.5 and human α₁-AGP 1 binding site for prazosin per molecule. The binding parameters obtained for the isolated serum proteins overestimated to some degree the total serum protein binding of prazosin in man. This was explained by a specific deviation from the law of mass action. HSA was the major binding protein in human serum at therapeutic concentrations, with ca. 60% of the total binding, the remaining 40% being bound to α₁-AGP. Anticipating that the high affinity binding site on the RSA preparation represents the binding of prazosin to α₁-AGP, then this protein accounts for 70% of the binding in rat serum, while rat serum albumin accounts for approximately 23%. The binding of prazosin to lipoproteins was insignificant in both species. The observed differences between man and rat in the serum protein binding of prazosin implicate differences in the two species with respect to prazosin pharmacokinetics and the pharmacological effect.

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Differences in the serum protein binding of prazosin in man and rat

Abstract

Eur. J. Pharmac.

Biochem. Pharmac.

Analyt. Biochem.

J. biol. Chem.

Biochem. Pharmac.

Biochim. biophys. Acta.

Biochem. Pharmac.

Biochem. Pharmac.

Biochem. Pharmac.

Biochem. Pharmac.

Biochem. Pharmac.

J. pharm. Sci.

Biochem. Pharmac.

J. Pharmac. exp. Ther.

J. Pharmac. exp. Ther.