Natural auxins 



This inference tends to be supported by the studies with ionizing radiation. 

 The enzymatic conversion of indolepyruvate to lAA is not appreciably 

 affected by ionizing radiation. It will be recalled from Figures 2, 6, and 7 that 

 low X-ray dosages will result in the inhibition of both native auxin formation 

 and the formation of lAA from tryptophan. Conversely, the enzymatic 

 transformation of indolepyruvate to lAA is not significantly affected by 

 irradiation of mung bean plants with more than 10 kr X-ray doses. Thus, if 

 we accept the view that indoleacetaldehyde is the immediate precursor of 

 lAA in tryptophan conversion, a view supported by the radiation studies, the 

 lack of inhibition of IPyA conversion to lAA by radiation makes it difficult 

 to accept the function of IPyA in the sequence. 



The conversion of IPyA to lAA without apparent formation of free 

 aldehyde in most plant materials or preparations raises the interesting 

 question of how the keto-acid is oxidized. By-pass of the aldehyde suggests 

 the ad hoc explanation of 'simultaneous oxidative decarboxylation'. A 

 mechanism whereby this may be accomplished was suggested by Krebs ( 1 936) 

 from his work on amino-acid oxidases. For example, kidney preparations 

 will catalyse in vitro the oxidation of a-amino acids to yield the corresponding 

 keto-acid and hydrogen peroxide. In the presence of catalase or a coupled 

 peroxidative action to reduce the peroxide, the keto-acid accumulates. In 

 the absence of a mechanism for peroxide utilization, the keto-acid undergoes 

 further non-enzymatic oxidative decarboxylation to yield the next lower 

 saturated acid. In terms of tryptophan conversion to lAA, this sequence can 

 be represented as follows: 



R_C— C— C00H + 02^^--R— G— C— COOH+HoOo .... (1) 



NH2 NH 



R— C— C— COOH + HgO-^^^^R— C— C— COOH + NH3 .... (2 



NH O 



) 



R— C— C— COOH+H2O2 -R— C— COOH + CO,+HoO .... (3) 



I II I 



O 



While it appears likely that reactions (2) and (3) may occur in the plant, 

 evidence so far is against the occurrence of reaction (1). Plant amino-acid 

 oxidases active on tryptophan are notable by their absence, or, if indicated, 

 are of so low activity as to be of questionable significance. Of course, the low 

 concentration of lAA in tissues and the low yields of lAA obtained from 

 tryptophan might be considered evidence for the function of an enzyme in low 

 concentration. At the moment, however, I wovild tend to discount the 

 idea of an active tryptophan oxidase participating in IPyA synthesis. 



If IPyA does function in lAA formation from tryptophan, a transa.mma.s,e 

 active on tryptophan might be considered as a mechanism whereby the 

 keto-acid is formed. This seems plausible, since IPyA will replace tryptophan 



72 



