154 



E. R. Waygood and G. A. Maclachlan 



40r-- 



Concn. Retarder, M. 



Fig. 4. Relative rales of iiidole-3-acetic acid oxidation in the presence of various 

 concentrations of retarders. Systems standard containing: A, hydroqninonc; B, 

 ^-quinone; C, riboflavinphosphate; D, scopoletin. 



tration of lAA, since the further addition of lAA at equilibrium 

 caused a resumption of oxygen uptake. Both hydroquinone and its 

 oxidation product p-benzoquinone retarded the oxidation to the 

 same extent. The coimiarin derivative scopoletin, already shown to 

 be a competitive inhibitor of lAA oxidation (1), had effects re- 

 sembling riboflavinphosphate. 



The maximum initial rates of lAA oxidation in the presence of 

 each of these retarders are compared as a function of retarder con- 

 centration in Figure 4. Unlike the inhibition induced by catechol, 

 etc., inhibition caused by retarders did not appear to depend on 

 the age of reagents, enzyme, etc., but only on the concentration. 

 Irrespective of the absolute rate of the unretarded control, close to 

 50 per cent retardation occurred at the following concentrations: 

 hydroquinone 3.0 X ^0'^ ^t'> p-quinone 3.5 X 10'^ ^^^; riboflavin- 

 l)hosphatc 7.5X10"^"' ^^J', scopoletin 1.25x10^ ^^^- As indicated 



