ARGENTAFFIN AND SCHIFF REACTIONS AFTER PERIODIC ACID OXIDATION AND ALDEHYDE BLOCKING REACTIONS

selective bistochemical localization of aldehydes. This belief undoubtedly rests on the statements of the organic chemists regarding the reactions of aldehydes. Karrer (10) states : “Ammoniacal silver solutions are reduced to the metal on warming with aldehydes”. Fieser and Fieser (5) state that aldehydes can be distinguished from ketones by their ability to reduce a solution of silver nitrate in an excess of ammonia to the metaffic state. But Feigi (4) does not include the reaction among his “spot tests” as specific for identification of aldehydes. Among the users of the reaction for histochemical purposes Pearse (14) states: “The reaction with ammoniacal silver solutions, in particular, is regarded as specific for aldehydes.” And Gomori (8) writes: “Another good analytical reagent for aldehydes is an alkaline solution of silver nitrate.” but points out that ascorbic acid, polyphenols, uric acid, etc. also exhibit the argentaffin reaction. Lison (13), discussing the argentaffin reaction of polyphenols, interpolates: “par exemple, les aldehydes la donnent aussi.” aldehydes give it also. Chu (3) utilized Gomori’s methenamine silver (pH 8.4) for the histochemical demonstration of aldehydes produced from a-amino acids by hypochlorite, chloramine T or ninhydrin. In the discussion following his paper, he insisted that this silver reaction was a true argentaffin reaction with aldehydes, while Seligman (15) asked whether other aldehyde reagents had been used to confirm the silver reaction, and Gomori (9) warned that methenamine silver ultimately blackens everything. No one questioned the validity of the methenamine silver method as an aldehyde reaction. G. C. Zorzoli (16) also refers to partial blackening of nerve cell (lipofuscin) pigment by the Masson argentaffin technic as due to a weak aldehydic function of the pigment, and (17) quotes Lison that aldehydes are also argentaffin. Glick (7), however, does not include the argentaffin reaction even as an alternative method for aldehyde localization and states, in relation to its use for localization of polyphenols: “The argentaffin test is quite unspecific, since many reducing substances can likewise give a positive reaction.” it is universally recognized that acid formaldehyde solutions rapidly reduce the silver in sections previously impregnated with ammoniacal silver solution. But it is also obvious that in combination with ammonia as methenamine, formaldehyde no longer reduces alkaline silver solutions. The conditions of the argentaffin reaction are such that “aldehyde” must stifi be capable of reacting in the presence of ammine complexes or even free ammonia. In the course of study of the effects of various oxidants on the argentaffin reaction of enterochromaffin, I observed that this reaction was largely or com-