The Modified Lowry Protein Assay will form precipitates in the
presence of detergents or potassium ions. The problem of precipitation
caused by the presence of potassium ions in the sample
can sometimes be overcome by centrifuging the tube and measuring
the color in the supernatant. Most surfactants will cause
precipitation of the reagent even at very low concentrations.
One exception is sodium dodecyl sulfate (SDS), which is compatible
with the reagent at concentrations up to 1% in the sample.
Chelating agents interfere by binding copper and preventing
formation of the copper peptide bond complex. Reducing agents
and free thiols also interfere by reducing the phosphotungstatephosphomolybdate
complex, immediately forming an intensely
blue colored product upon their addition to the Modified Lowry
Protein Assay Reagent.