Differences in the effects of solution additives on heat- and refolding-induced aggregation.

Although a number of low-molecular-weight additives have been developed to suppress protein aggregation, it is unclear whether these aggregation suppressors affect various aggregation processes in the same manner. In this study, we evaluated the differences in the effect of solution additives on heat- and refolding-induced aggregation in the presence of guanidine (Gdn), arginine (Arg), and spermidine (Spd), and the comparable analysis showed the following differences: (i) Gdn did not suppress thermal aggregation but increased the yield of oxidative refolding. (ii) Spd showed the highest effect for heat-induced aggregation suppression among tested compounds, although it promoted aggregation in oxidative refolding. (iii) Arg was effective for both aggregation processes. Lysozyme solubility assay and thermal unfolding experiment showed that Spd was preferentially excluded from native lysozyme and Arg and Gdn solubilized the model state of intermediates during oxidative refolding. This preference of additives to protein surfaces is the cause of the different effect on aggregation suppression.