Polymer-induced phase separation in aqueous micellar solutions of octyl-beta-D-thioglucoside for extraction of membrane proteins

A water-soluble polymer such as polyethylene glycol (PEG), Dextran T-500 (Dx), or diethylaminoethyl-Dextran (DEAE-Dx) induced aqueous micellar solutions of octyl-beta-D-thioglucoside (OTG) to phase separation at 0 degrees C. One of the two phases thus formed is a surfactant-depleted aqueous solution (aqueous phase) of a water-soluble polymer and the other a concentrated OTG solution (surfactant-rich phase). In a combination of OTG with PEG or Dx, cytochrome P450 (P450) and cytochrome b(5) (b(5)) were well extracted into the surfactant-rich phase. The extraction yield of P450 was slightly greater than that of b(5). In contrast to PEG and Dx, DEAE-Dx markedly reduced the extraction of b(5), while that of P450 remained almost unchanged. DEAE-Dx served the dual functions of inducing the phase separation and preventing the extraction of b(5) into the surfactant-rich phase. This depressed extraction of b(5) was reversed by the addition of potassium phosphate. DEAE-Dx and potassium phosphate proved effective in controlling the extractability of b(5). The polymer-induced phase separation provides a new basis for highly efficient extraction of membrane proteins under mild conditions that should be acceptable for thermolabile membrane proteins under physiological conditions. (C) 1997 John Wiley & Sons, Inc.