The Effects of Bisphenol A and Di-Isononyl Phthalate on Conformational Stability and Activity of Lysozyme

Abstract

The purpose of this study was to investigate the effect of Bisphenol A (BPA) and Di-Isononyl Phthalate (DINP) on conformational stability and biological activity of a model protein lysozyme. Lysozyme was incubated to various concentrations (2-100 μM) of bisphenol A and phthalate for 6 weeks. The conformational stability of lysozyme was figured out by using Differential Scanning Calorimeter (DSC) and Fourier Transform Infrared (FTIR) spectrophotometer. Biological activity of the samples was determined by an enzyme activity assay using Micrococcus lysodeikticus (M. luteus) as a specific substrate to lysozyme. A rate of change of 0.001 in absorbance at 450 nm (A450 nm) was used to define 1 unit of biologically active lysozyme. The DSC data showed significant (p<0.05) decrease in Tm and H values for lysozyme samples treated with BPA or DINP with increase in exposure time. The H value of BPA treated 7 days’ sample was negative which may indicate complete reversal of conformational integrity. In 7 days old DINP treated sample, no measurable peak was observed which may be due to complete loss of conformational structure. DSC data were further corroborated by FTIR and biological activity data. FTIR spectra of all the treated lysozyme samples showed the splitting of β sheet secondary structure which was completely lost in 7 days DINP treated samples which also showed the minimal biological activity. BPA and DINP (constituents of plastic bottles) may disrupt the conformational stability of protein which is expected to be more severe in human where they can remain longer than the 7 days’ exposure period used in this study.