208 R. L. Hinman and P. Frost 



of a crude peroxidase from horseradish root, both at pH 3.7. The 

 principal differences are at 254 m^, where Ray observed only a shoul- 

 der, and at 285 m^, where a new peak appeared in our experiments 

 after 3 days. Since Ray's experiments were for periods under 2 hrs., 

 it cannot be determined whether the maximum at 285 m^u, would have 

 appeared. These minor differences may be due in part to differences 

 in pH (at pH 7.5, Figure 1, the spectrum of the product(s) of lAA and 

 peroxidase is quite different from that at pH 5), or to the presence 

 of hydrogen peroxide in Ray's experiments. We found that the pres- 

 ence of hydrogen peroxide in the mixture resulted in a general blur- 

 ring of the spectrum, particularly in the 250 m/i, region, where the 

 peak at 254 m^^ became a shoulder. 



Despite these minor differences the otherwise striking resemblance 

 of the spectra from the peroxidase reaction and those from the lAA 

 oxidase reaction confirm again the close relationship between the 

 two, and the relevancy of using peroxidase as a model enzymatic sys- 

 tem for studies of lAA oxidase. 



In addition to lAA, a number of related indoles were examined 

 as possible substrates for peroxidase in the absence of added hydrogen 

 peroxide. Under the conditions described for lAA in the experimental 

 section, the spectra of (3-(indole-3-)propionic acid (IPA), ■y-(indole-3-)- 

 n-butyric acid (IBA), a,a-dimethylindole-3-acetic acid, tryptophan, 

 tryptamine, and skatole were unchanged during periods extending up 

 to 90 hrs. 



THE MODEL CHEMICAL SYSTEM 



From other studies of the oxidation of indoles we have found that 

 the rate of oxidation of some indoles by oxygen or hydrogen per- 

 oxide is greatly accelerated by the presence of acid. The action of 

 oxygen on lAA in aqueous solution at pH 1 produced red pigments, 

 but there was little change in the ultraviolet portion of the spectrum. 

 When lAA and hydrogen peroxide were mixed at pH 1, however, the 

 spectrum of lAA underwent immediate changes, as shown in Figme 

 2. The peaks at 245 to 248 and 254 m/x are remarkably like those ob- 

 served in the experiments with peroxidase. At pH 1 maximum ab- 

 sorbancy was attained by the two peaks in the 250 m/x region in 

 about 24 hrs., after which a slow decrease occurred. The indole peak 

 at 279 m/x decreased very slowly while a new maximum appeared at 

 297 m^, complete in 72 to 90 hrs. In short, rapid changes took place 

 at first in the 250 m^a region while the characteristic indole spectrum 

 (278 to 288 m/x) underwent little change. Then, much more slowly, 

 the indole portion of the spectrum ciianged completely, \\ith little 

 disturbance of the peaks which had previously appeared in tlie 250 

 m/x region. 



