L-arginine hydrochloride increases the solubility of folded and unfolded recombinant plasminogen activator rPA

L-arginine hydrochloride (L-ArgHCl) was found to be an effective enhancer for in vitro protein refolding more than two decades ago. A detailed understanding of the mechanism of action, by which L-ArgHCl as co-solvent is capable to effectively suppress protein aggregation, while protein stability is preserved, has remained elusive. Concepts for the effects of co-solvents, which have been established over the last decades, were found to be insufficient to completely explain the effects of L-ArgHCl on protein refolding. In this article, we present data, which clearly establish that L-ArgHCl acts on the equilibrium solubility of the native model protein recombinant plasminogen activator (rPA), while for S-carboxymethylated rPA (IAA-rPA) that served as a model protein for denatured protein states, equilibrium solubilities could not be obtained. Solid to solute free transfer energies for native rPA were lowered by up to 14 kJ mol(-1) under the tested conditions. This finding is in marked contrast to a previously proposed model in which L-ArgHCl acts as a neutral crowder which exclusively has an influence on the stability of the transition state of aggregation. The effects on the apparent solubility of IAA-rPA, as well as on the aggregation kinetics of all studied protein species, that were observed in the present work could tentatively be explained within the framework of a nucleation-aggregation scheme, in which L-ArgHCl exerts a strong effect on the pre-equilibria leading to formation of the aggregation seed.