An improved modification of the biuret method for the determination of protein in turbid materials with high lipid and hemoglobin content.

The quantitative determination of protein by means of the biuret method frequently yields erroneous values, especially when applied to turbid, lipid- and hemoglobin-containing materials. These errors can only partially be abolished either by the addition of detergents or by destroying the Cu-protein complex by KCN addition. It was found that most disturbances were almost completely eliminated after prior precipitation of protein by the addition of Triton X-100 to the solubilizing biuret reagent and absorbance measurements being performed at a wavelength of 572 nm before and after the addition of KCN. Values of protein determinations according to the proposed assay and protein concentrations calculated from Kjeldahl nitrogen determinations have been shown to agree fairly satisfactory. The proposed assay represents a relatively simple and versatile approach for the evaluation of protein concentrations in a variety of materials containing lipids, hemoglobin and of some other turbidities as well.