Requirement for Sodium in the Anaerobic Growth of Aerobacter aerogenes on Citrate

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Anaerobic growth of Aerobacter aerogenes on citrate as a carbon source required the presence of Na+. The growth rate increased with increasing Na+ concentration and was optimal at 0.10 m Na+. The requirement was specific for Na+, which could not be replaced by K+, NH4+, Li+, Rb+, or Cs+. K+ was required for growth in the presence of Na+, the optimal K+ concentration being 0.15 mm. Enzyme profiles were determined on cells grown in three different media: (i) intermediate Na+, high K+ concentration, (ii) high Na+, high K+ concentration, and (c) high Na+, low K+ concentration. All cells contained the enzymes of the citrate fermentation pathway, namely, citritase and the Na+-requiring oxalacetate (OAA) decarboxylase. All of the enzymes of the citric acid cycle were present, except α-ketoglutarate dehydrogenase which could not be detected. The incomplete citric acid cycle was, in effect, converted into two biosynthetic pathways leading to glutamate and succinate, respectively. The specific activities of citritase and OAA decarboxylase were lowest in medium (i), and under these conditions the activity of OAA decarboxylase appeared to be limited in vivo by the availability of Na+. Failure of A. aerogenes to grow anaerobically on citrate in the absence of Na+ can be explained at the enzymatic level by the Na+ requirement of the OAA decarboxylase step of the citrate fermentation pathway and by the absence of an alternate pathway of citrate catabolism.

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