Sialic acid in colonic mucin: an evaluation of modified PAS reactions in single and combination histochemical procedures.

Two new histochemical procedures for detecting sulphated and non-sulphated sialomucin in colonic mucosa were assessed: the saponification-Alcian Blue pH 1-periodic acid-phenylhydrazine-Schiff method (KOH-AB pH 1-PAPS) and the mild periodic acid modification of this (KOH-AB pH 1-mPAS). Using normal colonic mucosa obtained from 11 non-cancer patients, the mPAS and PAPS techniques were tested for specificity and reproducibility for staining sialic acid, either alone or in combination with Alcian Blue. A spectrophotometric method was devised to quantify the uptake of both Schiff and Alcian Blue stain by sections. At low temperature and pH 5.5, the mPAS procedure had improved specificity over the PAPS procedure, and after saponification it could be used to stain O-acetyl-substituted sialic acid. When used in combination with Alcian Blue at pH 1, however, underestimation of the sialic acid content occurred owing to interference between Alcian Blue and Schiff dyes. Interference was even greater with KOH-AB pH 1-PAPS procedure for both sialic acid and sulphate components. We conclude that caution must be exercised in interpretation of the staining results obtained with these new combination methods and that more accurate information on the sialic acid and sulphate content of colonic mucin is obtained by staining serial sections with the mPAS technique and Alcian Blue pH 1 alone.