A new histochemical method for the selective periodate oxidation of total tissue sialic acids

Evaluation of the intensity of the periodic acid-Schiff (PAS) staining produced following oxidation for 1 h at 4 degrees C with 0.4 mM periodic acid in approximately 1 M hydrochloric acid indicated that this reagent completely oxidized all available sialic acid residues of either the sialo- or sialosulphoglycoproteins of human and rat colon or the sialoglycoproteins of rat sublingual gland. These conditions produced no visible Schiff staining of either neutral macromolecules or vicinal diols located on hexose, 6-deoxyhexose or N-acetylhexosamine residues ('neutral sugars') of sialo- and sialosulphoglycoproteins. Furthermore, there was no extraction of epithelial glycoproteins or de-O-acylation of side chain substituted sialic acid residues. These data demonstrate that 0.4 mM periodic acid in approximately 1 M hydrochloric acid can be used as a specific reagent for the selective visualization of sialic acids in the PAS procedure. Studies of the mechanism of the oxidation of neutral sugars with 0.4 mM periodic acid in approximately 1 M hydrochloric acid indicated that their lack of PAS reactivity was not due to the production of Schiff unreactive hemiacetals or hemialdals. It is suggested that the selectivity of 0.4 mM periodic acid in approximately 1 M hydrochloric acid is a result of an increase in the rate of the oxidation of the sialic acid residues together with a decrease in the rate of oxidation of neutral sugars.