Comparison of inactivation and conformational changes of aminoacylase during denaturation in lithium dodecylsulphate solutions

The denaturation of aminoacylase in LDS solutions of different concentrations has been studied by following the changes in the ultraviolet absorbance, circular dichroism and intrinsic fluorescence. The results obtained show that the denaturation of the enzyme results in negative peaks at 287 and 295 nm in the denatured minus native enzyme difference spectrum. The fluorescence emission intensity of the enzyme decreases with no red shift of emission maximum in LDS solutions of increasing concentrations. In the LDS concentration regions employed in present study, no marked changes of secondary structure of the enzyme have been observed by following the changes in far ultraviolet CD spectra. The inactivation of this enzyme has been followed and compared with the unfolding observed during denaturation in LDS solutions. A marked inactivation is already evident at low LDS concentrations before signification conformational changes can be detected by ultraviolet absorbance and fluorescence changes. The inactivation rate constants of free enzyme and substrate-enzyme complex were determined by the kinetics method of the substrate reaction in the presence of inactivator previously described by Tsou. At the same LDS concentrations, the inactivation rate constants of the enzyme are a order of magnitude faster than the rate constants of conformational changes at least. The above results show that the active sites of metal enzyme containing Zn2+ are also situated in a limited and flexible region of the enzyme molecule that is more fragile to denaturants than the protein as a whole. (C) 1997 Elsevier Science B.V.