The Periodate Oxidation of Carbohydrates in Relation to the PAS Reaction

The periodic acid-Schiff (PAS) procedure, in which aldehydes produced by periodate oxidation are visualized with Schiff reagent, is extensively employed in carbohydrate histochemistry, In this paper those aspects of the chemistry of periodate oxidation relevant to the histochemical use of the PAS procedure are reviewed and discussed. It is apparent that Schiff staining after periodate oxidation dues not necessarily indicate the presence of carbohydrate residues and, conversely, that the absence of such staining does not necessarily mean that carbohydrate residues are absent. It is concluded that the actual intensity of staining in the routine PAS is due to a combination of factors, including (1) the rate of oxidation of the available vicinal diols; (2) the reactivity of the Schiff reagent with the oxidation product; (3) the structure of the polymer oxidized; and (4) the exact reaction conditions used in the procedure.