Protein Assays Compared: Bradford, BCA, Biuret, Lowry
Accurate protein quantification underpins virtually every biochemistry workflow, from normalizing western blot loading to verifying antibody concentration before an ELISA. Four colorimetric assays dominate the field: Bradford, BCA, Biuret, and Lowry. Each exploits a different chemical interaction between the protein and a chromogenic reagent, and each comes with its own set of advantages, interferences, and working ranges. Choosing the right method for a given sample can save significant time and reduce costly errors.
Bradford Assay
The Bradford assay (1976) is the fastest of the four. Coomassie Brilliant Blue G-250 dye binds to basic and hydrophobic residues on proteins, shifting its absorption maximum from 465 nm (free dye, reddish-brown) to 595 nm (bound dye, blue). The color change is proportional to protein concentration. The assay reaches a stable endpoint within about 10 minutes and is relatively insensitive to most reducing agents and chelating agents. Its principal interference is detergent: even small amounts of SDS or Triton X-100 displace the dye. Working range is approximately 1-1500 ug/mL depending on the micro or standard format used.
BCA Assay (Bicinchoninic Acid)
BCA (Smith et al., 1985) is a two-step reaction. First, the peptide bonds and certain amino acid residues (cysteine, tryptophan, tyrosine) reduce Cu2+ to Cu1+ under alkaline conditions (the biuret reaction). Second, two molecules of bicinchoninic acid chelate each Cu1+ ion, forming a water-soluble purple complex absorbing at 562 nm. BCA tolerates detergents far better than Bradford and works well with membrane protein preparations. Its main limitations are sensitivity to reducing agents (DTT, beta-mercaptoethanol) and the requirement for a 30-60 minute incubation at 37 degrees C or 60 degrees C for higher sensitivity. Working range is typically 20-2000 ug/mL (standard) or 0.5-10 ug/mL (enhanced micro BCA).
Biuret Assay
The biuret assay is the oldest and simplest of the four. In alkaline solution, Cu2+ forms a violet complex with the peptide bonds of proteins (absorbing at 540-560 nm). Because the signal depends purely on peptide bonds rather than specific side chains, the biuret assay is the least susceptible to variation in protein composition. It is also unaffected by most detergents and many common reagents. The trade-off is sensitivity: the working range of approximately 1-10 mg/mL requires relatively concentrated samples, making it less suitable for typical molecular biology extracts but well suited for industrial or food-protein applications where large amounts of protein are expected.
Lowry Assay
The Lowry assay (1951) combines the biuret Cu2+-peptide reaction with the Folin-Ciocalteu phenol reagent, which reacts with the reduced copper complex and with tyrosine/tryptophan residues to produce a blue color (750 nm). This two-stage amplification makes Lowry approximately 10-20 times more sensitive than biuret, with a working range of about 10-1000 ug/mL. However, Lowry is notoriously susceptible to interferences: reducing sugars, EDTA, Tris, SDS, and many other common buffer components can all produce spurious signals. The multi-step protocol (approximately 30-60 minutes total) and the need for fresh Folin reagent have caused many labs to replace Lowry with BCA for routine work.
Choosing the Right Assay
- Speed and simplicity with clean aqueous samples: Bradford
- Detergent-containing lysates or membrane proteins: BCA
- High-concentration industrial or food samples with minimal interference concern: Biuret
- Maximum sensitivity with well-controlled buffer chemistry: Lowry
All four assays require a standard curve prepared with bovine serum albumin (BSA) or a protein standard matching the sample composition. Results should be read on the same plate or cuvette run to avoid inter-run variation. The K LAB NanoQ supports all four assay protocols in direct microvolume mode (1-2 uL of the final colored reaction product), while the MRX A2000 microplate reader enables high-throughput screening in 96- or 384-well plate format with a read time of approximately 8 seconds per plate — making it straightforward to process dozens of samples at once for proteomics or quality-control workflows.