Problems associated with determining protein concentration: a comparison of techniques for protein estimations

Although a range of methods are available for determining protein concentration, many scientists encounter problems when quantifying proteins in the laboratory. The most commonly used methods for determining protein concentration in a modern biochemistry laboratory would probably be the Lowry and/or the Bradford protein assays. Other techniques, including direct spectrophotometric analysis and densitometry of stained protein gels, are applied, but perhaps to a lesser extent. However, the reliability of all of the above techniques is questionable and dependent to some extent on the protein to be assayed. In this paper we describe problems we encountered when using some of the foregoing techniques to quantify the concentration of poly(adenosine diphosphate-ribose) polymerase-1 (PARP-1), a nuclear enzyme found in most eukaryotes. We also describe how, by using a fluorescence-based assay and amino acid analysis, we overcame the problems we encountered.