Salt accumulation and mode of action of auxin 



lAA it could hardly be said that there was markedly less of this crystalline 

 material and it is therefore very doubtful if these crystals really are the 

 copper-thiocholine compound. It seems rather uncertain whether a 

 choline-esterase is present. It should be noted that the apparent activity is 

 very low in comparison with the esterase of neuro-muscular junctions, some 

 10 hours or more is needed to observe the apparent acetylcholine hydrolysis, 

 and the pH changes are unspectacular compared with animal sovuxes of 

 choline-esterases. 



A SUGGESTED MECHANISM OF ION UPTAKE 



No clear mechanisms involving choline-esterases have been advanced though 

 it seems highly probable that a role is played by these enzymes. Before 

 enunciating a very tentative working hypothesis it should be mentioned that 

 two other enzyme systems are known to be widespread. 'Choline-acetylase' 

 is a mixture of enzymes requiring as co-factors, at least CoA and ATP. In 

 presence of these factors and the enzyme-complex, acetylcholine is built up 

 from acetate and choline. 



This enzyme system exists in red blood cells and other tissues. It has not 

 been reported and probably has not been looked for in plant tissues as yet. 



Lecithinase-D is, however, widespread in particulate preparations from 

 plants and from animal tissues. It causes hydrolysis of lecithin to phosphatidic 

 acid and choline. 



Finally, one must recall an important property of the phosphatides. 

 Lecithins or phosphatidyl-cholines form complexes with salts which are 

 soluble in non-polar solvents like benzene. The lecithin-CdClg complex is 

 probably the best known as its high solubility in benzene is used in preparing 

 and purifying lecithin. Other salts also form complexes possibly as a result 

 of the peculiar structure of the lecithin zwitterion. 



The mechanism of ion uptake suggested is therefore as shown in the 

 following diagram: 



R— O— CHo 



R'— O— CH 



K+ 



o- 



ci- 



CH— O— P— OCHoCH2N(CH3)3 



I 

 O 

 lecithin 



Lecilhinase D 



Choline-esterase 



R— O— CH., 



R'— O— CH 



I 

 I 



OH 



Acetylcholine^ 



CH2— O— P— OH + Choline 



O 



Phosphatidic acid 



Choline-acetylase 



289 



