Effects of Asparagusate and Lipoate on Enzymes of the Tricarboxylic Acid Cycle and Related Metabolic Pathways

1. The effects of lipoate and asparagusate on animal and plant enzymes of the TCA cycle and related metabolic pathways were studied. 2. Lipoate inhibited bovine liver glutamate dehydrogenase [EC 1.4.1. 3]. The inhibition may play a role in metabolic regulation. 3. Asparagusate inhibited lipoyl dehydrogenase [EC 1.6.4.3] from asparagus and lettuce competitively with respect to lipoate. Asparagusate had practically no effects on other asparagus enzymes. 4. Asparagusate strongly inhibited lipoyl dehydrogenase, glutamate dehydrogenase, and isocitrate dehydrogenase [EC 1. 1. 1. 42] from animal sources, in competion with the corresponding substrate. 5. Asparagusate and lipoate also inhibited yeast glutamate dehydrogenase. 6. Based upon kinetic studies, the mode of these inhibitions is discussed.