Simultaneous preparation and quantitation of proteoglycans by precipitation with alcian blue

Conditions for specific interaction between Alcian blue and proteoglycans were optimized by comparing the differential spectra of Alcian blue obtained with purified chondroitin sulfate dissolved in water with the spectra obtained with nasal cartilage proteoglycans dissolved in synovial fluid. A method was then designed that provides specific precipitation of proteoglycans or glycosaminoglycans in 4 M guanidine - HCl in the presence of protein, hyaluronic acid, or nucleic acids. The specificity is achieved by using a low pH in combination with detergent and high salt concentration. Stepwise addition of reagents is necessary to avoid binding of Alcian blue to proteins and nucleic acids. All polyanions, except polysulfates, are first neutralized by lowering the pH to 1.5. By including detergent in this step, the hydrophobic protein regions are blocked and not accessible for binding with the dye. These regions could otherwise bind Alcian blue by hydrophobic interaction. When the Alcian blue reagent is added after, only the polysulfated molecules will remain charged and free to interact with Alcian blue. At least 0.4 M guanidine-HCl is required to abolish the negative interference by proteins. All sulfated glycosaminoglycans are precipitated at 0.4 M guanidine-HCl. With increasing guanidine-HCl concentrations, the different glycosaminoglycans are precipitated in accordance with the critical electrolyte concentration of the respective glycosaminoglycan. The Alcian blue precipitation can be performed at different concentrations of guanidine-HCl in order to separate different classes of proteoglycans. Excess dye and contaminating proteins are removed by a wash in a DMSO-MgCl2 solution and the precipitate is dissolved in a mixture of guanidine-HCl and propanol. For quantitation, the absorbance is recorded in a microplate reader with the 600-nm filter, the assay being linear between 0.5 and 20 micrograms proteoglycan. Since no digestion of samples with protease is needed, the proteoglycans are recovered in native form. The proteoglycan-Alcian blue complexes dissociate in the guanidine-HCl/propanol mixture and the proteoglycans can be selectively precipitated with propanol. The dye is used for quantitation and the proteoglycans can be utilized for further analysis.