Glucose-6-phosphate dehydrogenase partitioning in two-phase aqueous mixed (nonionic/cationic) micellar systems

The enzyme glucose-6-phosphate dehydrogenase (G6PD) plays an important role in maintaining the level of NADPH and in producing pentose phosphates for nucleotide biosynthesis. It is also of great value as an analytical reagent, being used in various quantitative assays. In searching for new strategies to purify this enzyme, the partitioning of G6PD in two-phase aqueous mixed (nonionic/cationic) micellar systems was investigated both experimentally and theoretically. Our results indicate that the use of a two-phase aqueous mixed micellar system composed of the nonionic surfactant C(10)E(4) (n-decyl tetra(ethylene oxide)) and the cationic surfactant C(n)TAB (alkyltrimethylammonium bromide, n = 8, 10, or 12) can improve significantly the partitioning behavior of G6PD relative to that obtained in the two-phase aqueous C(10)E(4) micellar system. This improvement can be attributed to electrostatic attractions between the positively charged mixed (nonionic/cationic) micelles and the net negatively charged enzyme G6PD, resulting in the preferential partitioning of G6PD to the top, mixed micelle-rich phase of the two-phase aqueous mixed micellar systems. The effect of varying the cationic surfactant tail length (n = 8, 10, and 12) on the denaturation and partitioning behavior of G6PD in the C(10)E(4) /C(n)TAB/buffer system was investigated. It was found that C(8)TAB is the least denaturing to G6PD, followed by C(10)TAB and C(12)TAB. However, the C(10)E(4)/C(12)TAB/buffer system generated stronger electrostatic attractions with the net negatively charged enzyme G6PD than the C(10)E(4)/C(10)TAB/buffer and the C(10)E(4)/C(8)TAB/buffer systems, when using the same amount of cationic surfactant. Overall, the two-phase aqueous mixed (C(10)E(4)/C(10)TAB) micellar system yielded the highest G6PD partition coefficient of 7.7, with a G6PD yield in the top phase of 71%, providing the optimal balance between the denaturing effect and the electrostatic attractions for the three cationic surfactants examined. A recently developed theoretical framework to predict protein partition coefficients in two-phase aqueous mixed (nonionic/ionic) micellar systems was implemented, and the theoretically predicted G6PD partition coefficients were found to be in reasonable quantitative agreement with the experimentally measured ones.