The role of net charge on the renaturation of reduced lysozyme by the sulfhydryl-disulfide interchange reaction.

Abstract

Reduced and acetylated lysozymes are basic proteins. When their amino groups were variously acetylated and then renatured by sulfhydryl-disulfide (SH-SS) interchange reaction at pH 8.0, the final folding yield decreased as the number of positive charges decreased. The final folding yield of native and Ac1 lysozyme, with one positive charge eliminated, was less sensitive to increasing protein concentration than that of Ac2 lysozyme, where two positive charges had been eliminated. The final folding yield of reduced Ac2 lysozyme increased in the presence of 1 M urea, which reduced the aggregation of unfolded lysozyme. Thus, the aggregation of unfolded lysozymes, which leads to a decrease in the final folding yield, was found to be heavily dependent on their net charges. Moreover, the final folding yield of reduced lysozyme was shown to be increased by use of cystamine as an oxidizing reagent in comparison with 2-hydroxyethyl disulfide or dithiodiglycolic acid. This may support the idea that the final folding yield is influenced by electrostatic interaction between unfolded lysozymes in the early stage of renaturation. In contrast, the concentration dependency of the final folding yield of Ac1 lysozyme was different from those of carboxymethylated His15 and Asp106 lysozymes whose positive net charges were similar to that of Ac1 lysozyme. On the basis of the observations, it is suggested that the formation of the aggregates in the renaturation process might also be affected by the structure of the unfolded state of lysozyme in solution.