Adaptation of the Bradford Protein Assay to Membrane-bound Proteins by Solubilizing in Glucopyranoside Detergents

A procedure was developed for the quantitation of solubilized proteins using the Bradford assay in the presence of glucopyranoside detergents. These detergents solubilized membrane-bound proteins with minimal background absorbance at 595 nm. Absorbance at 650 nm was also low, indicating that these detergents do not significantly stabilize the neutral species of Coomassie brilliant blue G-250 that produces interference in the presence of other detergents. Hexyl-beta-D-glucopyranoside produced less absorbance than did larger glucopyranosides, and the increase in its absorbance at 595 nm in the presence of dye reagent was related linearly to its concentration from 0 to 2%. Absorbance produced by membrane-bound protein was increased by the presence of up to 0.2% hexyl-beta-D-glucopyranoside (final concentration in dye reagent) and then remained stable up to 1%, indicating that these concentrations of this detergent allowed membrane-bound proteins to react completely with the dye reagent. Standard curves of several proteins were similar in the absence or presence of 0.1-0.5% hexyl-beta-D-glucopyranoside. The quantitation of both soluble and membrane-bound proteins by the Bradford assay was similar in the presence of 0.2% hexyl-, heptyl-, and octyl-beta-D-glucopyranoside. Estimates of membrane-bound protein by this assay agreed with estimates obtained with the Lowry assay and with quantitative amino acid analysis. This procedure requires no extra steps; thus, it is as rapid and convenient as the original Bradford protein assay.