| Abstract: | Abstract— Reversible binding of model compounds, their conjugated metabolites (sulphates and glucuronides), and also derivatives of the compounds, to human serum albumin (HSA) has been examined using an ultrafiltration method. p-Nitrophenol (P-NP), α-naphthol (α-NA) and β-naphthol (β-NA) were used as model compounds. Reversible binding of 500 μm p-NP sulphate to 4% HSA (96·6 ± 0·35%, mean ± s.d. n = 3) was significantly higher (P < 0·001), whereas reversible binding of p-NP glucuronide to 4% HSA (33·3 ± 9·82%) was much lower (P < 0·001) than that of 500 μm p-NP (90·9 ± 0·60%). Reversible binding of 500 μm p-NP glucopyranoside to 4% HSA (25·8 ±2·82%) was comparable with that of the glucuronide, with which it is structurally similar. In contrast, reversible binding of 500 μm p-NP phosphate, an anionic compound like p-NP sulphate, to 4% HSA (61·4 ± 5·28%) was significantly lower than that of p-NP (P < 0·001). Similar results were observed in reversible binding of sulphates of α-NA and β-NA. Significant differences of dissociation constants for HSA binding were observed between the parent compound (α- or β-NA) and its sulphate conjugate (P < 0·005 for α-NA and α-NA sulphate, P < 0·001 for β-NA and β-NA sulphate), but the number of binding sites was the same. These results indicated that sulphate conjugation enhances reversible binding of a parent compound to HSA by increasing the binding affinity of the parent compound to HSA. This enhancement appeared to be advantageous for preventing random distribution of this metabolite to organs in the body. |
