Polyacrylamide films as a tool for investigating qualitative and quanitative aspects of the staining of glycosaminoglycans with basic dyes.
With the introduction of model films of polyacrylamide gel into which purified glycosaminogly cans (GAGs) have been 'incorporated', the direct recording of metachromatic spectra with virtually no interference of the corresponding orthochromatic peaks has become possible. Because this model system yields situations comparable to those of stained sections under the microscope, it is well suited for investigating qualitative and quantitative aspects of histochemical staining procedures. Previous model experiments have shown that under aqueous conditions only minor differences can be observed between the metachromatic peaks of different GAGs complexed with a suitable dye (e.g. Toluidine Blue O, Thionin, Safranin O, Cresyl Violet, Cystal Violet). In non-aqueous media, such as glycerol and ethylene glycol, the complexes with Toluidine Blue O revealed a special pattern for heparin, having a metachromatic peak (517 nm) about 30 nm lower than that of all other GAGs. This observation has formed the basis of a method for the qualitative microspectrophotometric detection of heparin in situ which was worked out by combining model film experiments with microspectrophotometric data obtained from rat mast cells. Since only a limited number of cells in necessary for obtaining reliable data with this method, the presence of heparin in the cytoplasmic granules of normal human mast cells and basophilic granulocytes could thus be proved directly. Alcian Blue 8GX, another basic dye frequently use in GAG histochemistry, has also been investigated with polyacrylamide films. In contrast to the metachromatic dyes, the rate of staining with Alcian Blue depends to a large extent on the rate of penetration of the dye into the model films. The rate of penetration is also a phenomenon of great importance for dye binding in situ, where complex basic protein molecules may form a barrier for the Alcian Blue molecules. The model film studies performed so far have yielded conditions that provide maximal staining (up to an optimal level) and a linear relationship betweeen the concentration of GAG and the AB binding. The presence of basic protein, electrostatically bound to the GAG, was not found to influence either the rate of staining or the maximal amount of dye binding.