Enzymatic Conversion of Indole Acetaldehyde and Naphthalene Acetaldehyde to Auxins.

A neutral substance, convertible to an acid auxin, is present in various plant extracts (5, 9) and chemical preparations (9). As indicated by a number of biological tests, the neutral substance is in all likelihood 3-indole acetaldehyde and will for convenience be designated by this name in the present paper. In a previous paper (11) it was shown that 3-indole acetaldehyde is rapidly converted to indoleacetic acid in excised Avena coleoptiles and in fresh, but not in boiled, coleoptile juice. It was concluded that the conversion is caused by an enzyme system present in the coleoptiles. The indole acetaldehyde used in those experiments was prepared from tryptophan and isatin using a method described previously (9, p. 85). GORDON and NIEVA (5, 6), using indole acetaldehyde extracted from pineapple leaves and purified via the bisulphite addition-product, demonstrated the presence of an enzyme system, capable of converting indole acetaldehyde to indoleacetic acid, in breis and protein preparations obtained from pineapple leaf bases. WILDMAN et al. (17) failed to obtain any conversion of indole acetaldehyde to indoleacetic acid by enzyme preparations from spinach leaves, altlhough such preparations produced indoleacetic acid readily from tryptophan. As already pointed out by GORDON and NIEVA (6), however, the synthetic preparation of indole acetaldehyde used by WILDMAN et al. (17) probably contained very little, if any, of the desired aldehyde. Since indole acetaldehyde seems to be the immediate precursor of indoleacetic acid in the biosynthesis of this auxin, a method for the determination of indole acetaldehyde in plant extracts would be highly desirable. This aldehyde, however, lhas not yet been prepared in the chemically pure state; so we are unable to construct a standard activity curve, relating biological effect, such as curvature in the Avena test, to actual concentration of indole acetaldehyde. Amounts of indole acetaldehyde have been expressed in terms of amounts of indoleacetic acid obtained after treatment of the preparation or extract with soil (5, 6, 9, 10, 15). Owing to losses occurring during the treatment this procedure is unsatisfactory. Since indole acetaldehyde does produce curvatures in the Avena test these curvatures can be compared with curvatures produced by pure indoleacetic acid. As an arbitrary, biological unit of indole acetaldehyde was proposed the y-"equiva-