DESTRUCTION OF ALDEHYDE 83 



chain fatty aldehydes and aldehydes which may be present in sugars, etc. 

 Extraction with acetone is a convenient way of distinguishing between 

 these two classes of substances. It is necessary, however, to carry out the 

 extractions on tissue sections, for with thicker pieces of tissue very pro- 

 longed treatment is necessary to secure complete removal of lipoidal 

 aldehyde. 



2. Another substance which reacts with aldehydes in a fairly specific 

 manner is azobenzene phenylhydrazine sulfonic acid. This substance re- 

 acts with aldehydes to give a purple colour. The purple colour is fully de- 

 veloped only in rather strong acid; consequently permanent mounts can- 

 not be made. But the colour reaction is very useful for checking results 

 obtained by the reduced-fuchsin method. This reagent also reacts with 

 ketones, but not to give a purple colour. 



3. Aldehydes readily reduce alkaline silver solutions to give a black 

 precipitate of metallic silver. This reaction, however, is not very sensitive 

 and can only be expected to be found at sites containing a high concentra- 

 tion of aldehyde. Ability to reduce alkaline silver solutions is not, of 

 course, restricted to aldehydes: many substances will carry out this re- 

 duction. On the other hand, any substance which is an aldehyde must 

 reduce alkaline silver. 



4. A very useful group of reagents introduced by Bennett (1940) are the 

 hydrazines. Phenylhydrazine itself does not give a deeply coloured com- 

 pound with aldehyde groups, and indeed it is possible that it may be a 

 useful compound for blocking free aldehyde groups. But 2:4 dinitro 

 phenylhydrazine gives a fairly deep yellow colour with aldehydes. This re- 

 agent will also react with keto groups and is, therefore, not to be regarded 

 as specific for aldehydes. Camber (1949) has introduced another hydra- 

 zine which can be used for obtaining more intense colouration, using tech- 

 niques similar to those described in Chapter 5. 



5. A very useful procedure is to block the aldehyde group by prior 

 treatment with hydroxylamine or dimedone. When treated with one of 

 these reagents before treatment with the reduced fuchsin, the aldehyde 

 group is blocked up and no purple colour develops in the reduced fuchsin. 

 It is probable that for many purposes dimedone is the better substance 

 to use as a blocking agent, for L. G. Bell has shown that during the re- 

 action with hydroxylamine the reagent solution may remove considerable 

 amounts of the protein from the tissue section. Dimedone can be used 

 in alcoholic solution, in which proteins do not dissolve readily. 



Details of the reactions given above are provided elsewhere 

 (Danielli, 1949a). 



Destruction of Aldehyde 



It is always desirable to be sure that the aldehyde which is 

 being studied in an experiment is not being destroyed by a par- 

 ticular step in the experimental procedure. This can be done 

 simply by increasing the duration of each step in the procedure 



