The Mechanism of Aconitase Action EVIDENCE FOR AK EKZYR’IE ISOMERIZATIOK BY STUDIES OF IKHIBITIOK BY TRICARBOXYLIC ACIDS *

Tricarboxylic acids were shown to be both competitive and noncompetitive inhibitors of aconitase with the inhibition pattern dependent upon which substrate was used. Tricarballyate and fluorocitrate were linear competitive inhibitors when citrate or isocitrate was the substrate but noncompetitive inhibitors when cis-aconitate was the substrate. transAconitate was a linear competitive inhibitor with cis-aconitate as substrate and noncompetitive when citrate or isocitrate was substrate. These data support a kinetic scheme which involves an enzyme isomerization between two forms, one of which preferentially interacts with cis-aconitate and the other with citrate and isocitrate.