Oxidative Decarboxylation of Indole-3-acetic Acid by Mangani-Versene and by Wheat Leaf Enzyme

Abstract

THERE is little agreement concerning either the pathway or the final products of the enzymic oxidation of heteroauxin¹. The stoichiometry of the oxidative decarboxylation seems to be reasonably well established¹, one mole of oxygen being taken up and one of carbon dioxide evolved for every mole of indole-3-acetic acid (IAA) reacting. Galston² reported that no indole-3-aldehyde was formed when IAA was destroyed by a pea brei (see, however, Racusen³); two principal products were formed with R_F very close to that of o-aminoacetophenone (I) and o-formamidoacetophenone (II) and giving similar colour reactions. On the basis of a ferric chloride test, Galston concluded that these compounds were hydroxylated derivatives of (I) and (II). Indole-3-aldehyde has an R_F intermediate between those of (I) and (II) and does give a brownish-pink colour with ferric chloride; the colour reported by Galston is undoubtedly due to the spot for the indolealdehyde overlapping on to the other two spots. It also seems most unlikely that the R_F of phenolic derivatives of both (I) and (II) should be the same as those of (I) and (II). Finally, should such phenols be formed, the uptake of oxygen should be at least 1.5 times that actually reported. Using an oxidizing enzyme from Omphalia flavida, Stowe et al.⁴ rejected the possibility that (I), (II) or indolealdehyde was formed in any amount and suggested that the product (not isolated) might be 3-methyldioxindole. From their chemical evidence it seems rather unlikely that their product was 3-methyldioxindole. Most of their observations can be explained on the basis of our present results. With Lupinus seedlings Stutz⁵ obtained a number of products (one of them being indolealdehyde) chromatographically similar to those obtained by Galston². Using substituted 3-indoleacetic acids Stutz concluded that the ring system (probably C₂) and not the side chain was the primary site of dehydrogenation.