The apparent failure of sodium borohydride reduction to block further PAS reactivity in rat epithelial mucins.

Sections of rat small intestine were oxidized with 1% periodic acid for periods of 1, 2, 5, 10 and 30 min and were subsequently either (a) stained with Schiff reagent, or (b) reduced with sodium borohydride and then treated with either Schiff reagent alone or by the standard PAS procedure. It was found that whereas sodium borohydride reduction abolished all Schiff staining, initial periods of oxidation in excess of 10 min were necessary to abolish any subsequent PAS reactivity. The theoretical and practical significance of these data is discussed in relation to the recent publication of Bayliss & Adams (1976).