Transport of sugars and amino acids in bacteria. XIII. Mechanism of selective inhibition of the active transport reactions for proline, leucine, and succinate by zinc ions.

A mutant (S-1) of Escherichia coli which lacks succinate dehydrogenase [EC 1.3.99.1] but has normally succinate transport system, and various oxidase activities other than succinate, was isolated from the strain U24. Using these strains studies were made on the effects of zinc ions on the active transport reactions for succinate proline, and leucine. Zinc ions inhibited succinate transport reaction and succinate oxidation by intact cells. The active transport reaction for proline, which was supported by endogenous energy source in intact cells, was slightly inhibited by zinc ions. This inhibition was not reversed by large excess of magnesium ions. The initial rates of active uptakes of proline were increased by exogenous energy sources such as succinate and glycerol and zinc ions greatly inhibited these. However, D-lactate dependent proline uptake was enhanced slightly by the presence of zinc ions. It was found that zinc ions at a high concentration enhanced the steady level of proline accumulation in cells. This mechanism was studied in detail and we concluded that zinc ions inhibited completely the exchange-exit reaction for proline. Effects of zinc ions on the active transport reaction for leucine were rather simple when compared with these for proline. Zinc ions inhibited strongly the initial rates of leucine uptakes which were driven by endogenous and exogenous energy sources. The ions also inhibited the exchange-exit reaction.