Is there any protein-to-protein variation?
All proteins are unique and their varying structures can give variable responses. Each of the commonly used assay methods exhibits some degree of varying color response toward different proteins. These differences relate to dissimilarity among proteins due to amino acid sequence, pI, structure and the presence of certain side chains or prosthetic groups that can dramatically alter the protein’s color response. Most protein assay methods utilize bovine serum albumin (BSA) or immunoglobulin (IgG) as the standard against which the concentration of protein in the sample is determined. Using either of these proteins as the standard works well in most assay methods. However, if great accuracy is required, the standard curve must be prepared from a pure sample of the target protein to be measured. If a pure sample of the target protein is not available, select the standard protein from those proteins that generate a color response that is close to the color response of the target protein.
