Protein assay sensitive at nanogram levels.

A simple, fast protein assay which utilizes the affinity of colloidal gold for proteins is described. This assay is sensitive at the 20-ng level when a visible light spectrophotometer is used to measure absorbance. Few chemicals interfere with the assay. Interfering reagents include those that are strongly alkaline, contain high levels of salt, or contain sodium dodecyl sulfate. The problem of alkaline interference can be overcome by acidifying the protein solution before performing the assay. Purified proteins have different capacities to interact with the colloidal gold but this variability is not greater than that seen with the Bradford protein assay.