Normally, proteins will aggregate and precipitate by direct folding processes. In this study, we report that quasi-static processes can restore both the structure and bio-function of two kinds of fish recombinant growth hormones (Plecoglossus altivelis and Epinephelus awoara). The conformational changes and the particle-size-distribution (PSD) of each refolding intermediate can be monitored by circular dichroism spectroscopy (CD) and dynamic light scattering (DLS), respectively. Conformation analysis of the CD spectra of the refolding intermediates indicated that the secondary structures were restored in the initial refolding intermediate. However, the tertiary interactions of the proteins were restored during the last two refolding stages, as elucidated by thermal stability tests. This is consistent with a sequential model. DLS analysis suggested that the average hydrodynamic radii of the refolding intermediates shrank to their native-like sizes after the first refolding stage. This is consistent with a collapse model. After comparison with the data on the direct folding process, it is concluded that the denaturant-containing protein folding reaction is a first-order-like state transition process.

Key words: Quasi-static, Stepwise, Re-nature, Denaturant, Aggregation, First order phase transition, Growth hormone.