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We previously reported that lysine inhibits in vivo homocitrate synthesis in the lysine bradytroph,Penicillium chrysogenum L1 , and that such feedback inhibition could explain the known lysine inhibition of penicillin formation. In the present study, it was found that dialyzed cell-free extracts of mutant L1 converted [1-14 C]acetate to homocitrate. This homocitrate synthase activity was extremely labile but could be stabilized by high salt concentrations. The pH optimum of the reaction was 6.9, and theKm was 5.5 mM with respect to α-ketoglutarate. The reaction was also dependent upon the presence of Mg2+ , adenosine 5′-triphosphate, and coenzyme A. Surprisingly, the activity in these crude extracts was not inhibited by lysine. Benzylpenicillin at a high concentration (20 mM) partially inhibited the enzyme, an effect that was enhanced by lysine. Casein hydrolysate also partially inhibited the enzyme.

Insensitivity of Homocitrate Synthase in Extracts of Penicillium chyrosogenum to Feedback Inhibition by Lysine