A colorimetric determination for glycosidic and bile salt-based detergents: applications in membrane protein research

Detergents are crucial to the isolation of integral membrane proteins. During membrane protein purification, it is useful to accurately quantify detergent, especially if concentration steps have been used. Previously, this has been difficult and time-consuming. We present a simple, rapid, and sensitive procedure for the quantification of glycosidic and bile salt-based detergents such as dodecylmaltoside, octylglucoside, and CHAPS. The method directly quantifies sugar or cholate moieties via colorimetric reactions with phenol and sulfuric acid. A number of detergents have been screened, and the assay has been validated in the presence of commonly used reagents. In addition to determining the overall detergent concentration in solution, the procedure allows accurate quantification of specific binding of glycosidic or bile salt-based detergents to purified membrane proteins. Both the colorimetric method and the radiometric 14C method were used to determine detergent binding to two integral membrane proteins: the cytochrome cbb3 oxidase from Pseudomonas stutzeri and the turkey β-adrenergic receptor. Both methods gave similar results. After separating monomeric glycosidic detergent from micellar solutions by ultrafiltration, we used the colorimetric method to determine the concentration of monomeric detergent present. We observed that values obtained are in close agreement with previously determined critical micelle concentrations.