Assays for total protein.

This unit describes three copper-based assays to quantitate total protein: the biuret method, a variation of the Lowry method (Hartree-Lowry method), and the bicinchoninic acid (BCA) assay. Acid hydrolysis of a protein is coupled with ninhydrin detection to quantitate amino acid content of a sample. Ultraviolet spectrophotometry is used to measure total protein and evaluate samples for the presence of contaminants. The Coomassie dye binding, or Bradford, assay is a quite simple assay and frequently is quite sensitive, although it sometimes gives a variable response depending on how well or how poorly the protein binds the dye in acid pH. Finally, dry weight measurement is used to quantitate pure protein. Support protocols describe heat sealing glass tubes for acid hydrolysis, sample dialysis in polyacrylamide gel wells to remove low-molecular-weight contaminants, and TCA precipitation to precipitate and concentrate proteins and remove low-molecular-weight contaminants.