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264 biuret reagents for total protein determination in serum containing dextran were investigated. The usefulness of the method is dependent on proper ratio of the components of the reagent: concentrations of NaOH and copper sulphate, and the ratio of sodium-potassium tartrate to copper sulphate.
We describe a detection method relying both on the copper displacement from a Sephadex gel by peptides and proteins, and on the subsequent colorimetric determination of the complexed copper. The system described is fully automated and it permits a continuous analysis of column effluents. The choice of cuprizone as a detecting reagent for copper, enables one to bring the detection limit down to 200 ng for albumin and 60 ng for alanylglycylglycin. The specificity of the method is the same as the biuret reaction. Some examples of the possible applications are given.
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A review is given of the different composition of biuret test solutions and experiments with the estimation of protein content of milk, cheese and meat by means of the biuret method are reported. The pure protein and the casein content of skimmed milk and the protein content of whole milk could be determined with biuret test solutions containing potassium hydroxide and a detergent. The use of the biuret method is advantageous in the case of cheese and meat, which are dissolved in potash lye or alkaline detergent solutions. Disturbances of the biuret method caused by lactose, fat or turbidity were eliminated after addition of hydrogen peroxide, by means of extraction or of additional measurements with a copper-free, zinc-containing biuret reagent. Deviations of the color intensity of biuret complexes were compensated by inclusion of standards in the measurement series.
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The Lowry and biuret reactions have been adapted for the selective detection of chromatographically resolved proteins, specifically proteins separated by high-performance liquid chromatography. The protein reagents are continuously added to the column effluent and produce the characteristic chromophores with both proteins and peptides. The reaction chemistries are compatible with ion-exchange, steric exclusion, and reverse-phase chromatography. Detection limits for proteins resolved by ion-exchange are about 5 to 10 micrograms with the Lowry reaction. Peptides containing tyrosine can be detected at the 100-ng level when chromatographed on reverse-phase columns. The biuret reaction is about 8 times less sensitive for proteins and not very effective for peptides. Reaction detection can be combined with direct absorbance detection in the uv to distinguish proteinaceous peaks from other peaks containing uv-absorbing compounds.