Effect of mammalian kidney osmolytes on the folding pathway of sheep serum albumin

- In the absence of kidney osmolytes SSA undergoes reversible, two-step denaturation induced by urea and GdmCl.
- SSA in the presence of kidney osmolytes undergoes reversible single-step unfolding induced by urea and GdmCl.
- All kidney osmolytes protect SSA against denaturation by urea and GdmCl
- The order of stabilization of SSA by kidney osmolytes follows: Glycine betaine > Myo-inositol > Sorbitol.

Recently, we had published that urea-induced denaturation curves of optical properties of sheep serum albumin (SSA) are biphasic with a stable intermediate that has characteristics of molten globule (MG) state. In this study, we have extended the work by carrying out urea- and guanidinium chloride (GdmCl)-induced denaturations of SSA in the presence of naturally occurring mammalian kidney osmolytes, namely, sorbitol, myo-inositol and glycine betaine. We have observed that all these osmolytes (i) transform this biphasic transition into a co-operative, two-state transition and (ii) increase the stability of the protein in terms of midpoint of denaturation (Cm) and Gibbs free energy change in the absence of both denaturants (ΔGD0). The relative effectiveness of different osmolytes on the stability of SSA follows the order: glycine betaine > myo-inositol > sorbitol. In this paper, we also report that kidney osmolytes destabilize MG state by shifting the equilibrium, native state ↔ MG state toward the left. This study will be helpful in understanding the existence of osmolytes in kidney and their role in folding of kidney proteins soaked with urea.