KENNETH V. THIMANN 35 



2) The inhibitors of succinic and oi lactic dehydrogenase show marked 

 specificity. Quastel and Wooldridge (7) showed that hydroxy acids, 

 such as hydroxymalonic, which. inhibit lactic dehydrogenase, are quite 

 inactive on succinic dehydrogenase. This parallels the marked effect of 

 a side-chain hydroxyl in abolishing auxin activity. The auxin analogue of 

 hydroxymalonic acid is, of course, mandelic acid. 



3) The action of malonic acid itself is most suggestive in this connec- 

 tion. In low concentrations (lo'^'M) it protects against the inhibiting 

 action of iodoacetate on growth. In high concentrations (lO'^M and 

 above) it inhibits growth both of coleoptiles and of pea stems. To the 

 plant tissue poisoned with iodoacetate, then, malonic acid acts like an 

 auxin, promoting growth at low concentrations and inhibiting it in 

 high. 



4) The 4-carbon acid enzymes, including succinic dehydrogenase, are 

 of critical importance to the hfe of cells. To protect them against 

 poisoning is essential not only for growth but for hfe. But the action of 

 a protecting substance implies that it is adsorbed on the enzyme more 

 readily than the inhibitor against which it protects. Hence when the 

 concentration of a protecting substance becomes high its molecules begin 

 to be adsorbed on the enzyme to a degree comparable with that of the 

 substrate; in other words the protecting substance now begins to inhibit. 

 As the concentration increases at first only growth is inhibited, but 

 when inhibition becomes complete the cells are killed. In this way the 

 fact that at high concentrations the auxins become toxic and herbicidal 

 would be explained. 



This hypothesis has the advantage of bringing together two hitherto 

 unrelated aspects of the auxin studies, namely the role of auxm in 

 metabolism and the chemistry of the active substances. Perhaps also 

 it may be suggestive to workers in other fields of biology which involve 

 the fascinating relation between structure and activity. 



REFERENCES 



1. Bonner, D. M., Botan. Gaz., 100:200-14 (1938)- 



2. Havinga, H. and Nivard, R. J. P., Rec. trav. chim. Pays-Bas, 67:846-54 



3. KoE?FLi,* J. B., Thimann, K. v., and Went, F. W., /. Biol. Chem., 



122:763-80 (1938). 



4. KoGL, F., Ber. deut. Chem. Ges., 68:16-28 (1935). 



5. , and Kostermans, D. G. F. R., Zeii. Physiol. Chem., 235:201-16 



(1935)- 



