Effects of additives on irreversible inactivation of lysozyme at neutral pH and 100 degrees C.

ABSTRACT

The mechanism of irreversible inactivation of lysozyme at neutral pH at 100 degrees C, and effects of additives on the inactivation were investigated. The thermoinactivation of lysozyme at neutral pH was caused by intra- and intermolecular disulfide exchange and the production of irreversibly denatured lysozyme, which was destabilized by multiple chemical reactions other than disulfide exchange. In addition, independently, deamidation slightly affected the inactivation by causing a decrease of electrostatic interaction between positive charges of lysozyme and negative charges of the bacterial cell wall. As for the effects of additives on the inactivation, a small amount of copper ion suppressed intra- and intermolecular disulfide exchange by catalyzing air oxidation of heat-induced trace amounts of free thiols, and organic reagents (acetamide, ethanol, and glycerol) changed the mechanism of the inactivation to that under acidic conditions by shifting the pKa values of dissociable residues and also suppressed intermolecular disulfide exchange by decreasing hydrophobic interactions.