Apparent failure of 2-hydroxy-3-naphthoic acid hydrazide to block the periodic acid-Schiff reactivity of sialic acids. Implications for the PANFOPAS method

Histochemical studies of the mechanism of staining of the periodic acid/2,-hydroxy-3-naphthoic acid hydrazide/Fast Black B/saponification/periodic acid-Schiff (PANFOPAS) method indicate that while 2-hydroxy-3-naphthoic acid hydrazide blocks the periodate engendered Schiff reactivity of vicinal diols unassociated with anionic groups, it fails to block such reactivity in sialic acid residues. It is suggested, therefore, that the positive Schiff staining observed following application of the PANFOPAS method to colonic epithelial mucins may be due to sialic acids without side chain substituents (or which are substituted at C7 or C9) in addition to sialic acids substituted at C8 (or which are di- or trisubstituted). Consequently the PANFOPAS method cannot be used to study the side chain substitution pattern of the sialic acids of epithelial mucins.