The Lowry Method for Protein Quantitation
1. Introduction
The most accurate method of determining protein concentration is probably acid
hydrolysis followed by amino acid analysis. Most other methods are sensitive to the
amino acid composition of the protein, and absolute concentrations cannot be obtained
(1). The procedure of Lowry et al. (2) is no exception, but its sensitivity is moderately
constant from protein to protein, and it has been so widely used that Lowry protein
estimations are a completely acceptable alternative to a rigorous absolute determination
in almost all circumstances in which protein mixtures or crude extracts are
involved.
The method is based on both the Biuret reaction, in which the peptide bonds of
proteins react with copper under alkaline conditions to produce Cu+, which reacts with
the Folin reagent, and the Folin–Ciocalteau reaction, which is poorly understood but in
essence phosphomolybdotungstate is reduced to heteropolymolybdenum blue by the
copper-catalyzed oxidation of aromatic amino acids. The reactions result in a strong
blue color, which depends partly on the tyrosine and tryptophan content. The method is
sensitive down to about 0.01 mg of protein/mL, and is best used on solutions with
concentrations in the range 0.01–1.0 mg/mL of protein.
