The effect of long-chain alkyl betainates on the arylesterase-like activity of bovine serum albumin

The effect of a hydrolyzable cationic surfactant, tetradecyl betainate (tetradecyloxycarbonyl-N,N,N-trimethylmethanaminium chloride), on the arylesterase-like activity of bovine serum albumin (BSA) was investigated. The rate of hydrolysis of p-nitrophenyl hexanoate in the presence of BSA and varying concentrations of the surfactant was followed. The rate was found to be dependent on the concentration of the cationic surfactant and a maximum was found in the curve at ca. 3 m. The Michaelis-Menten constants (K_m/n, where n is the number of active sites) and the “catalytic” rate constants (k₂) were determined for the reactions, and were found to be 11 and 40 times larger, respectively, in the presence of the surfactant. The hydrolysis of radiolabeled tetradecyl betainate in the presence and absence of BSA was also followed. This study, together with a binding study based on gel permeation chromatography, showed that the surfactant binds to the protein, but that no hydrolysis of the betaine ester takes place while bound to BSA. It was thus concluded that the increased value of k₂ in the presence of a cationic surfactant was not, as has been previously suggested, due to an increased local hydroxide concentration resulting from the formation of a new pseudophase.