. The chemistry and mode of action of plant growth substances; proceedings of a symposium held at Wye College, University of London, July 1955. Plant regulators; Auxin; Growth (Plants). 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 bi
. The chemistry and mode of action of plant growth substances; proceedings of a symposium held at Wye College, University of London, July 1955. Plant regulators; Auxin; Growth (Plants). 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-
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