| Abstract: | Na(+) was required for the aerobic growth of Salmonella typhimurium on citrate, but not on l-malate, glucose, or glycerol. The maximal growth rate and the maximal total growth occurred with 6 to 7 mm Na(+). Na(+) could not be replaced by K(+), NH(4) (+), Li(+), Rb(+), or Cs(+). Sonically treated extracts of citrate-grown cells contained the enzymes of the citrate fermentation pathway (citritase and oxalacetate decarboxylase) and all of the enzymes of the citric acid cycle. Thus, two separate routes of citrate catabolism appeared to be operational in the cells. Two discrete oxalacetate (OAA) decarboxylases were also demonstrated. One was of the "classic" type, being activated by Mn(++) and inhibited by ethylenediaminetetracetate (EDTA). It was present in the cell sap. The second decarboxylase closely resembled the Na(+)-activated OAA decarboxylase of citrate-grown Aerobacter aerogenes, whose growth also requires, or is increased, by Na(+). This decarboxylase was EDTA-insensitive, specifically activated by Na(+) and inhibited by avidin, and it had a high affinity for OAA. It was induced by growth on citrate, but not l-malate or glycerol. It is suggested that the Na(+) requirement for growth reflects the need to activate this OAA decarboxylase as a component of the citrate fermentation pathway and that citrate catabolism via the citric acid cycle, which should be independent of Na(+), is somehow dependent upon the activity of the Na(+)-activated enzyme. |
