1.Introduction
The expression of recombinant products is an essential method for producing target proteins and one of the most important unit processes for production of therapeutic protein and structural study [1–6]. There are different expression systems for producing recombinant protein in biotechnology industry. Expression in mammalian cells produces active recombinant proteins that have posttranslation modification but expression by this system is time consuming and expensive. Although bacteria still represent a convenient production system, most of recombinant polypeptides produced in prokaryotic hosts undergo misfolding or incomplete folding processes that usually result in their accumulation as insoluble aggregates, which called inclusion bodies [7–9]. All these studies of recombinant protein expression in Escherichia coli showed that inclusion body formation is the rule rather than the exception. In vitro refolding is required for obtaining active protein from inclusion body aggregates which consist of four main steps: inclusion body isolation, solubilization, refolding, and purification. Because of high density of IBs, they separated easily by centrifuge. After isolation of the IBs, the proteins contained in the inclusion bodies need to be solubilized, usually by high concentration of denaturing agents such as
urea or guanidinium chloride (Gn-HCl). Then, solubilized IBs are subjected for refolding in specified conditions. Refolding of proteins from inclusion bodies is influenced by different factors including solubilization method, removal of the denaturant, and assistance of refolding by co-solute or additives. The refolding condition is critical in order to obtain acceptable amounts of active protein. It has been known that additives, especially low molecular weight compounds, may significantly enhance the yield of the refolding process. In many cases, inclusion body protein can only be successfully refolded by making use of these effects.