Transport of -Lactate, -Lactate, and Glycolate by the LldP and GlcA Membrane Carriers of Escherichia coli

To examine the substrate specificity of the membrane transport carriers LldP (l-lactate permease) and GlcA (glycolate permease) of Escherichia coli, a mutant strain lacking their structural genes and blocked in the metabolism of the tested substrates was constructed and transformed with a plasmid bearing either the lldP or the glcA gene. Each transformant acquired the ability to accumulate l-lactate, d-lactate, and glycolate against a high concentration gradient. Substrate accumulation was inhibited by carbonyl cyanide m-chlorophenylhydrazone, a hydrophobic proton conductor that dissipates proton motive force. Competition of 14C-l-lactate transport by nonradioactive l-lactate, d-lactate, and glycolate in LldP synthesizing cells and competition of 14C-glycolate transport by the same three substrates in GlcA synthesizing cells showed that both carriers effectively transported all three substrates with a Ki value ranging from 10 to 20 μM. d-Lactate does not appear to have a permease of its own. Utilization of the compound depends mainly on LldP.