The Bicinchoninic Acid (BCA) Assay for Protein Quantitation

The bicinchoninic acid (BCA) assay, first described by Smith et al. (1) is similar to the Lowry assay, since it also depends on the conversion of Cu2+ to Cu+ under alkaline conditions (see Chapter 2 ). The Cu+ is then detected by reaction with BCA. The two assays are of similar sensitivity, but since BCA is stable under alkali conditions, this assay has the advantage that it can be carried out as a one-step process compared to the two steps needed in the Lowry assay. The reaction results in the development of an intense purple color with an absorbance maximum at 562 nm. Since the production of Cu+ in this assay is a function of protein concentration and incubation time, the protein content of unknown samples may be determined spectrophotometrically by comparison with known protein standards. A further advantage of the BCA assay is that it is generally more tolerant to the presence of compounds that interfere with the Lowry assay. In particular it is not affected by a range of detergents and denaturing agents such as urea and guanidinium chloride, although it is more sensitive to the presence of reducing sugars. Both a standard assay (0.1–1.0 mg protein/mL) and a microassay (0.5–10 μg protein/mL) are described.