The BCA Micro BCA assay is a 3-component protocol which uses concentrated stocks of the Biuret reaction, BCA, and copper(II) reagents. It allows for an improved sensitivity of ~2 - 40 μg/mL vs 20 - 2000 µg/mL of the original BCA assay. However, it has a different, and generally speaking more sensitive, interference from non-protein components. Kits for the Micro BCA assay are available from at least two commercial vendors. Notably, the composition and use of a "Micro BCA Reagent and Protocol" was described in the original manuscript by Smith, and modern kits likely consist of an exact or highly similar formulation. The protocol consists of mixing Micro-Reagent B and the Copper Solution 25:1 to form Micro-Reagent C (MC), which is not shelf stable and should be freshly prepared, and then mixing MC 1:1 with Micro-Reagent A to produce the final (also unstable) assay working solution. Micro-Reagent A, Micro-Reagent B, and Copper Solution are stable indefinitely at room temperature.
Micro-Reagent A (MA)
* 8% w/v NaCO·HO (CAS: 5968-11-6)
* 1.6% w/v NaOH (CAS: 1310-73-2)
* 1.6% w/v Na tartrate (CAS: 868-18-8) (10x concentration as Reagent A in Original BCA Assay above)
* Sufficient NaHCO (CAS: 144-55-8) to adjust pH to 11.25
Micro-Reagent B (MB)
* 4% w/v BCA-Na (CAS: 979-88-4) (4x concentration as Reagent A in Original BCA Assay above)
Copper Solution
* 4% w/v CuSO·5HO (CAS: 7758-99-8) (Same concentration as Reagent B in Original BCA Assay above)