144 P. L. Goldacre 



such as catalase (4, 6), certain peroxidase substrates (6, 13), or manga- 

 nese (4) in the presence of the peroxidase of the preparation. The no- 

 tion that peroxidase is obligatorily implicated in the process is sup- 

 ported by the total inhibitions produced by cyanide, azide, and other 

 agents which bind heavy metals (17, 19). 



Thus, we may picture that lAA is subject to two successive oxida- 

 tive steps, one mediated directly or indirectly by oxygen, the other by 

 hydrogen peroxide: 



lAA -\- HviOo > Pi (peroxidase) 



V 



Pi + O2 > Po + H2O2 (oxidase) 



The peroxide must be produced in one of the steps in the oxida- 

 tion of lAA itself, for if the oxidation of an exogenous metabolite 

 were involved obligatorily, two and not one mole of oxygen per mole 

 of lAA would be consumed. Since catalase can inhibit oxygen up- 

 take completely, we may infer that the peroxidase moiety deals -with 

 lAA itself and the oxidase with the reaction product. Otherwise we 

 would expect a maximum inhibition of 50 per cent oxygen uptake 

 while the oxidase step continued independently. 



Dialysis (8) or ultrafiltration (13) of enzyme preparations reveals 

 that activity is enhanced by the presence of a diffusible cofactor. 

 However, exhaustive dialysis may not reduce the activity to zero, and 

 for a particular tissue, there remains a consistent residue of activity 

 which is not cofactor-dependent (8). This residue may amount to 

 about 40 per cent in macerates from the epicotyls of peas grown in 

 weak red light, 10 per cent for those grown in darkness, and zero for 

 pea roots. The pineapple enzyme appears to have no cofactor require- 

 ment (9). This suggests that there are at least two systems for lAA 

 destruction, acting in parallel. However, as oxidation of lAA is 

 totally inhibitablc by catalase, cyanide, or guaiacol, the total activity 

 depends on peroxidation. Activity lost by dialysis can be re]:)laced, or 

 even increased above the original level, by adding a monohydric 

 phenol, e.g., 2,4-dichlorophenol (DCP), optimum concentration 3 X 

 10-5 M. As DCP can overcome the inhibition by catalase (6, 7), it is 

 concluded that it stimulates production of peroxide or promotes its 

 more effective utilization. 



A property of great interest in the pea epicotyl preparation is 

 its activation by light. Though there is considerable activity in dark- 

 ness, up to fourfold promotion may be obtained by 250 foot candles 

 of \\)n(c i;<rhi ^1) ^']•^^, ;,riion sj)cctrum for the increment of activity 



