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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 bicinchinonic 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 a technique for heat sealing glass tubes for acid hydrolysis, sample dialysis in polyacrylamide gel wells to remove low‐molecular‐weight contaminants, and TCA precipitation to …