The biogenesis of natural auxins 



corresponding aldehyde have been shown to occur in plants and animals^ 

 certain plants cannot utilize tryptamine in the formation of auxin or lAA. 

 No auxin is formed from tryptamine by the spinach leaf. Neither mung bean 

 seedlings nor cultures of U. zea will produce lAA from tryptamine. Perhaps 

 more conclusive evidence for the non-participation of the amine is available 

 from recent experiments with amine-oxidase inhibitors by Dr. R. Moss in our 

 laboratory. In Table 1 are shown the effects of both enzyme inhibitors and 



Table 1 

 Amine-oxidase inhibitors and auxin formation 





X 



In all experiments, tryptamine and tryptophan 2-5 x- 10 M. 



competitive substrates on tryptamine oxidation and auxin formation. 

 It may be seen that the first two compounds, marsalid and dipyridyl, 

 inhibited almost completely the oxidation of tryptamine and the conversion 

 of tryptamine to lAA. However, these compounds had no observable effect 

 on the enzymatic conversion of tryptophan to lAA. Similarly, the mono- 

 amines and the diamine inhibited both tryptamine oxidation and conversion, 

 but had no effect on the tryptophan reaction. If one may extrapolate to other 

 plant materials from these experiments, it appears highly unlikely that trypta- 

 mine functions as an intermediate in the formation of lAA from tryptophan. 

 On the basis of comparative biochemistry, one might anticipate that 

 indolepyruvate would be the more probable intermediate. Here again a 

 scepticism is warranted on the basis of the experimental evidence available. 

 Experiments with the keto-acid are complicated by its high degree of spon- 

 taneous breakdown to both lAA and the aldehyde as well as to other indole 

 compounds. If corrections are made for spontaneous breakdown, or if 

 enzymatic examination is facilitated by removal of the unreacted keto-acid 

 as the phenylhydrazone after incubation, certain generalizations may be 

 made. Several plant species can accelerate enzymatically the conversion of 

 indolepyruvate to lAA. In this conversion, it has been shown with a number 

 of plant tissues or plant preparations that no significant rise occurs in the level 

 of the neutral component possessing the characteristics of the aldehyde. 

 On the other hand, a similar treatment of tryptophan results in quite 

 significant rises in level of the neutral component considered to be indole- 

 acetaldehyde. At first glance this would suggest that the keto-acid is not an 

 intermediate in the conversion of tryptophan to lAA. 



71 



