1. Intermediary metabolites, arising from the glyoxylate by-pass, were tested for their effect on the activity of isocitrate lyase, the first enzyme of the by-pass. 2. Oxaloacetate and pyruvate inhibited the enzyme when present at concentrations similar to that of the substrate. 3. Inhibition was competitive and did not depend on a non-optimum pH. The affinities determined at the pH optimum were: Ki (oxaloacetate), 3·7×10−5m; Ki (pyruvate), 6·6×10−5m. 4. The significance of the inhibitions is discussed with particular reference to the known inhibition by phosphoenolpyruvate.
