Protein purification for the Lowry assay: acid precipitation of proteins in the presence of sodium dodecyl sulfate and other biological detergents.

The Lowry protein assay is widely used to quantify proteins in the laboratory. As the assay is susceptible various interfering substances, the protein test sample is frequently first purified by precipitating the proteins using trichloroacetic acid (TCA). However, TCA precipitates proteins poorly in the presence of sodium dodecyl sulfate (SDS), a detergent very commonly used in biological research. The inhibitory effect of up to 1% SDS on protein precipitation is effectively overcome by a combination of 5% TCA and 1% phosphotungstic acid (PTA). The TCA/PTA mixture also precipitates proteins readily in the presence of other detergents such as 0.1% Tween 20, Triton X-100 or Nonidet P-40. TCA/PTA has the capacity to precipitate a wider cross-section of proteins than TCA alone, the combination of acids remaining effective in the presence of SDS and other detergents.