Protein measurement of particulate and solubilized ovine liver membranes.

Protein concentration measurements are critical in biochemical work with cellular membranes, including the determination of cell surface receptor concentration in human malignant tissues obtained at surgery or after biopsy. In this study we compared the results of protein concentration measurements in ovine liver cellular membranes using either particulate preparations or membranes solubilized with four different detergents. In all cases protein was determined by two different indirect methods (Lowry's Folin phenol method and Bradford's Coomassie Brilliant Blue dye binding method) and compared to the direct biuret method. Our results indicate that the direct biuret method gives the highest protein concentrations followed by the method of Lowry. Maximal concentrations (approaching those obtained by the direct biuret method) were obtained after membrane solubilization with Triton X-100 (3-5%). It is suggested that either the direct biuret method (whenever protein concentrations permit it) or the method of Lowry after solubilization of membranes with Triton X-100 (3-5%) should be used preferentially for the determination of membrane protein samples.