Histidine is a source of the antioxidant, α-ketoglutarate, in Pseudomonas fluorescens challenged by oxidative stress

The role of alpha-ketoglutarate (KG) in the detoxification of reactive oxygen species (ROS) has only recently begun to be appreciated. This ketoacid neutralizes ROS in an NADPH-independent manner with the concomitant formation of succinate and CO2. To further probe this intriguing attribute of KG in living systems, we have evaluated the significance of histidine metabolism in the model organism, Pseudomonas fluorescens, challenged by hydrogen peroxide (H2O2). Here, we show that this amino acid does contribute to KG homeostasis and appears to be earmarked for the production of KG during oxidative stress. Both the NAD- and the NADP-dependent glutamate dehydrogenases were upregulated in the stressed cells despite the sharp decline in the activities of numerous enzymes mediating the tricarboxylic acid cycle and oxidative phosphorylation. Enzymes such as isocitrate dehydrogenase-NAD dependent, succinate dehydrogenase, alpha-ketoglutarate dehydrogenase, Complex I, and Complex IV were severely affected in the P. fluorescens grown in the presence of H2O2. Studies with fluorocitrate, a potent inhibitor of citrate metabolism, clearly revealed that histidine was preferentially utilized in the production of KG in the H2O2-challenged cells. Regulation experiments also helped confirm that the metabolic reprogramming, resulting in the enhanced production of KG was induced by H2O2 stress. These data further establish the pivotal role that KG plays in antioxidative defense.