Protein concentration by precipitation with pyrogallol red prior to electrophoresis

The pyrogallol red protein assay (Clinical Chemistry 1986, 32, 1551-1554) is based upon formation of a blue protein-dye complex in the presence of molybdate under acidic conditions. However, centrifugation of the assay mixture results in loss of color yield and precipitation of the protein-dye complex which can be recovered and resolubilized to achieve protein concentration prior to sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The method has been evaluated relative to trichloroacetic acid (TCA) precipitation for recovery and electrophoresis of commercial protein and peptide molecular weight markers. Precipitation with pyrogallol red-molybdate (PRM) gives better and more uniform recovery of both proteins and peptides as compared to TCA. The lower limit of PRM precipitation is similar to TCA and corresponds to 1 microgram protein per mL assay mixture. This is equivalent to 100 microL of 10 micrograms/mL protein using the standard protein assay or 1 microgram/mL protein using a modified assay incorporating a fivefold concentrate of the dye reagent. Application of the method is demonstrated by concentration of urinary proteins. The method is simple and economic and useful for conserving trace amounts of precious sample as it allows recovery of protein for electrophoresis following protein assay.