Effect of DMSO on Denaturation of Hen Egg-white Lysozyme in Aqueous Solution

Abstract

A study on thermal denaturation of hen egg-white lysozyme(HEWL)was carried out by differential scanning calorimetry(DSC)and modulated temperature differential scanning calorimetry(MDSC).It was found that DSC profiles of HEWL were dependent on heating rates and concentrations of enzyme as well as cosolvent.The enthalpy of denaturation(ΔHm)increased with increasing enzyme concentration in aqueous solution.In aqueous solution containing dimethylsolfoxide(DMSO),denaturation temperature(Tm)shifted toward lower temperature,and the transition peak became shallower and broader with increasing volume proportion of DMSO.When the content of DMSO was more than 70%,only a declining smooth line could be observed instead of a single endothermic transition curve.Interestingly,from MDSC curves of HEWL in aqueous solution without DMSO,a small and symmetrical peak in front of the main transition(I)was observed marginally,which was regarded as a pre-transition(II).Specially,this small and marginal peak could not be found in aqueous solution containing DMSO.In addition,with increasing amount of protein,Tm(II)shifted toward lower temperature,and ΔHm(II)decreased along with the temperature gap between two transitions lengthened.The results were considered to be due to the reversible dissociation of HEWL dimers in aqueous solution.