DEHYDRASE 141 



By slightly varying the conditions, the same enzyme is 

 able to produce from two molecules of salicylic aldehyde one 

 molecule each of the corresponding acid and alcohol. This 

 reaction, which is of the type known as a Cannizarro reaction, 

 is usually represented by the equation 



2C,H 4 (OH) CHO + H 2 = C 6 H 4 (OH) COOH + C fi H 4 (OH) CH 2 OH 

 and was originally attributed by Parnas * to a special enzyme 

 to which he gave the name of aldehyde mutase. According 

 to the view of Wieland, however, the reaction may be explained 

 by assuming that one of the two molecules of salicylic aldehyde 

 merely acts as the hydrogen acceptor to the hydrate of the 

 other molecule as follows : — 



/OH 

 C 6 H 4 . OH . C^- o Hi + C 6 H 4 . OH . CHO: 

 ;H 



/OH 

 = C 6 H 4 . OH . C/ + C 6 H 4 . OH . CH 2 OH. 



Salicylic acid Salicylic alcohol 



The Nitrate -reducing Enzyme of the Potato. — The existence 

 of this was first demonstrated by Bach in the following simple 

 experiment. One gram of freshly pounded potato is heated 

 in a test tube to 6o° C. with 10 c.c. of 4 per cent, aqueous 

 solution of sodium nitrate together with 3 drops of 10 per cent, 

 solution of acetic aldehyde. After two minutes the solution 

 gives a strong reaction for nitrite by the Griess Ilosvay reagent. 

 This reaction was originally thought to be due to a so-called 

 hydrolytic oxidation reduction — as expressed by the equation 



NaNO s + H 2 + CH3CHO = NaN0 2 + H,0 + CH 3 COOH 

 according to which the sodium nitrate and acetic aldehyde 

 between them shared the hydrogen and oxygen of a molecule 

 of water, thereby becoming reduced and oxidized respectively. 

 On Wieland's hypothesis, however, this is simply explained 

 by the dehydrase activating the hydrogen of the aldehyde 

 hydrate whilst the sodium nitrate acts as the hydrogen ac- 

 ceptor. f 



* Parnas : " Biochem. Zeitsch.," 1910, 28, 274. 



t The ability to oxidize acetic aldehyde and effect the simultaneous 

 reduction of nitrate to nitrite is not uncommon in vegetable and animal 



