Urea Stress: Myo-inositol's efficacy to counteract destabilization of TIM-β-globin complex by urea is as good as that of the methylamine.

Do kidney osmolytes counteract the deleterious effects of urea on kidney proteins? To answer this question, we measured guanidinium chloride (GdmCl)-induced denaturation of triose phosphate isomerase-β-globin subunit complex (TIM-β-globin) from sheep kidney in the presence of various concentrations of urea and each of kidney osmolytes (glycine betaine, myo-inositol and sorbitol) alone and in combination at pH 7.5 and 25 °C. Analysis of GdmCl-induced denaturation curve at a given osmolyte (or urea) concentration gave ΔG (Gibbs free energy change in the absence of GdmCl). For a co-solute (CO), a linear plot of ΔΔG (difference between values of ΔG in the presence and absence of CO) against [CO] (molar concentration of CO) yielded m-value (=δΔΔG/δ[CO]). m-value for urea (m) and that for the osmolyte (m) were used to determine [urea]:[osmolyte], the molar ratio for the perfect counteraction of the denaturing effect of urea on the protein stability by an osmolyte. For each osmolyte, we tested this ratio for the perfect counteraction. It was observed that glycine betaine and myo-inositol provide perfect counteraction at the predicted molar ratio whereas sorbitol fails to do so. Furthermore, urea and GdmCl induced a two-state denaturation, and both TIM and β-globin have identical thermodynamic stability.