Interference by Cyclodextrins in Protein Determination by the Bradford Method

Cyclic glucose oligomers known as cyclodextrins were found to interfere in the quantitation of micro amounts of protein by the Bradford method. Formation of an inclusion complex between cyclodextrin and reagent dye led to enhanced absorbance at the measuring wavelength. However, protein values were inhibited in samples containing cyclodextrin. The extent of interference was difficult to predict since it was dependent on the amount of protein as well as the concentration and type of cyclodextrin present in the sample. All three commonly used cyclodextrins (α-, hydroxypropyl-β-, and γ-cyclodextrin) caused significant interference in the microassay procedure. The use of cyclodextrin blanks were not helpful in correcting for this interference, which was probably due to a three-way interaction between dye, protein, and cyclodextrin molecules. Preliminary results showed a similar interference by cyclodextrins in the bicinchoninic acid and Lowry protein assays.