A method for quantitating nanogram amounts of soluble protein using the principle of silver binding

A highly sensitive and quantitative assay for measuring protein in solution based on the capacity of protein to bind silver is described. In this procedure, protein samples are first treated with glutaraldehyde and then exposed to ammoniacal silver. After 10 min, the reaction is terminated by the addition of sodium thiosulfate and the optical density measured at 420 nm. The useful range of the assay for the majority of standard proteins tested lies between 15 and 2000 ng. This represents a 100-fold increase in sensitivity over the Coomassie brilliant blue dye-binding procedure. There is little or no interference from carbohydrates, nonionic detergents, or ethanol, and pretreatment of protein samples with Bio-Gel P-2 to remove salts, thiol agents, EDTA, and sodium dodecyl sulfate makes this procedure compatible with most commonly used buffers. The cost in terms of silver utilization is nominal with a typical assay involving 10 samples tested in triplicate amounting to less than $0.02 U. S.