The biogenesis of natural auxins 



as an essential amino acid in animals. It is of interest in this connection that 

 no enhancement whatsoever of tryptophan conversion to lAA is obtained 

 when a-keto-glutarate is added to suitable enzyme preparations, even with 

 the amino acid in relatively high concentration. Hence one would also tend 

 to discount a transaminase reaction on tryptophan as a primary reaction 

 in the biogenesis of auxin. Verification that IPyA occurs as such in plants is 

 needed. If we accept the chromatographic evidence that it does occur, there 

 appears to be no experimental basis for assuming that it is formed from 

 tryptophan. 



I would like to consider at this point a mechanism of pyruvate dehydro- 

 genase action that may tie together a number of observations. Let us assume 



h" 



COz + 



Aldehyde 

 DPT 



Acetaldehyde 



Acetaldehj'de 



Pyruvate 



— ►Acetom 

 f-COj 

 Acetolactate 



Diacetyl 



" Acetj'l acetoin 



2Fe(CN)e 

 2:e-Cl2-<;)indo<(i 

 Oj* particles 



Lipoic 

 acid 



-►Acetate 



OX 



DPNH + H^ 







DPN* 



-* FP — *HA 



Lipoic 

 acid red 



Acetate 



Pyruvate 



Lactate 



Acyl PO^ 

 Figure 9. Pyruvate dehydrogenase mechanism (adapted from Gunsalus, 1954; 1955). 



that IPyA occurs in plants and that it is derived from tryptophan. It may be 

 suggested that the subsequent steps of auxin formation proceed by a sequence 

 analogous to the dehydrogenase mechanism proposed for pyruvic acid 

 oxidation {Figure 9). The 'core' of these reactions is the formation of an 

 intermediate 'aldehyde-diphosphothiamine' or 'aldehyde-DPT-enzyme' 

 compound, followed by donor reactions for the aldehyde. Oxidation to the 

 acyl-CoA would occur via reduction of lipoic acid without the liberation of 

 free aldehyde or acetate. Hydrolysis of the CoA ester would then yield the 

 free acid. Alternatively, the free acid may arise directly from the aldehyde- 

 DPT either enzymatically or non-enzymatically in the presence of suitable 

 electron acceptors. 



The above sequence would account for the conversion of IPyA to lAA 

 by some tissues and tissue preparations either with or without the concomitant 

 formation of free aldehyde. It is pertinent that free aldehyde also may be 

 liberated from the DPT-complex in the presence of proton donors; possibly 



73 



