Kinetic analysis of the covalent binding of captopril to human serum albumin.

A simple and direct method using an N-methylpyridinium polymer-based (4VP-Me) column for the detection of the human serum albumin (HSA)-captopril (Cp) conjugate was developed. By this method, a new peak corresponding to an HSA-Cp conjugate was detected in the serum from a patient receiving Cp. The new peak was composed of a 1:1 molar ratio of Cp and HSA. Time courses of reversible and irreversible binding of Cp to HSA were quite different. The reversible binding decreased rapidly, whereas covalent binding increased gradually. A mechanism is proposed for the formation of the HSA-Cp conjugate and, based on this mechanism, apparent first-order rate constants were calculated. Interestingly, the reactivity in serum was approximately 10-fold higher than that obtained for HSA solutions. The differences in this reaction between serum and HSA solution might be due to the fluctuations in pH as well as the presence of endogenous thiol compounds, oxygen, and metal ions in the solutions.