362 ENZYMES 



gentisic acid (C 8 H 8 O 4 ),* a substance which is also produced 

 in a similar fashion in the human body.f 



OH 



<^ \CH 2 CHNH 2 COOH + 3 O = <f \ 



NH 



HO 



Tyrosine Homogentisic acid 



Homogentisic acid may be prepared from root-tips by 

 grinding them in 96 per cent alcohol, filtering off the solids, 

 and evaporating over a water bath. The residue when dis- 

 solved in water forms a brown solution which is free from 

 sugar, and gives a faintly acid reaction. In the air it turns 

 dark. 



The following are characteristic reactions. 



1. Treated with alkali it turns reddish-brown. 



2. It reduces an ammoniacal solution of silver nitrate on 

 warming. 



3. Fehling's solution is feebly reduced by it on warming. 



4. Ferric chloride gives i a green coloration. 



5. Ferric sulphate gives a violet-blue coloration. 



6. Millon's reagent gives a yellow colour. 



7. Homogentisic acid is precipitated by lead acetate. 



8. It also gives a reddish colour with hydrogen peroxide. 

 Czapek further found that if ground-up root-tips were kept 



for some time under the influence of chloroform, the mass 

 gradually loses its power of reducing silver, and even small 

 quantities of homogentisic acid intentionally added to the 

 preparation gradually disappear ; this is due to the action of 

 an oxidase. 



It has been seen that root-tips stimulated by gravity give 

 a strong reduction of silver, but not so the unstimulated roots. 

 To explain this there are two alternative hypotheses ; either 

 the action of the oxidase is inhibited during geotropic stimulus, 

 so that the homogentisinic acid, which is otherwise acted upon, 

 disappears more slowly than under ordinary circumstances and 

 so accumulates, or, there is a diminution in the production of 

 oxidase by the root-tip. 



* The occurrence of this acid in plants is denied by Schultze and Castorcr. 

 " Z. physiol. Chem.," 1906, 48, 387, 396. See also Bertel : " Ber. deut. hot. 

 Gesells.," 1902, 20, 454. 



fWolkow and Baumann : id., 1891, 15, 260; Huppert: " Z. physiol. 

 Chem.," 1897, 23, 412; Garrod and Hele: id., 1905, 33, 198. 



