A protocol for protein estimation that gives a nearly constant color yield with simple proteins and nullifies the effects of four known interfering agents: microestimation of peptide groups.

A modified version of the Lowry protocol for protein measurement is being introduced in which the increment in color due to copper-protein interactions can be directly equated to the concentration of peptide groups (the CO-NH moieties) present in the sample. The method provides an accurate estimation of protein concentrations in the range of 5-40 micrograms, with a sensitivity similar to that of the original assay procedure. It yields hardly any protein-to-protein variation with simple proteins and eliminates the interference by four compounds that commonly affect the Lowry assay. A numerical factor has been calculated that allows conversion of micromoles of peptide groups into actual micrograms of protein present in the sample.