Transport and deamination of amino acids by a gram-positive, monensin-sensitive ruminal bacterium

Abstract

Strain F, a recently isolated ruminal bacterium, grew rapidly with glutamate of glutamine as an energy source in the presence but not the absence of Na. Monensin, a Na+/H+ antiporter, completely inhibited bacterial growth and significantly reduced ammonia production (85%), but 3,3',4',5-tetrachlorosalicylanide (a protonophore) and valinomycin had little effect on growth or ammonia production. Dicyclohexylcarbodiimide, a H+-ATPase, inhibitor had no effect. The kinetics of glutamate and glutamine transport were biphasic, showing unusually high rates at high substrate concentrations. On the basis of low substrate concentrations (<100 μM), the K(m) values for glutamate and glutamine were 4 and 11 μM, respectively. Strain F had separate carriers for glutamate and glutamine which could be driven by a chemical gradient of Na. An artificial ΔΨ was unable to drive transport even when Na was present. The glutamate carrier had a single binding site for Na with a K(m) of 21 mM; the glutamine carrier appeared to have more than one binding site, and the K(m) was 2.8 mM. Neither carrier could use Li instead of Na. Histidine and serine were also rapidly transported by Na-dependent systems, but serine alone did not allow growth even when Na was present. Because exponentially growing cells at pH 6.9 had little ΔΨ (-3 mV) and a slightly reversed ZΔpH (+17 mV), it appeared that the membrane bioenergetics of strain F were solely dependent on Na circulation.