Sodium-dependent transport of branched-chain amino acids by a monensin-sensitive ruminal peptostreptococcus

Abstract

A recently isolated ruminal peptostreptococcus which produced large amounts of branched-chain volatile fatty acids grew rapidly with leucine as an energy source in the presence but not the absence of Na. Leucine transport could be driven by an artifical membrane potential (Δψ) only when Na was available, and a chemical gradient of Na+ (ΔuNa+) also drove uptake. Because Na+ was taken up with leucine and a ZΔpH could not serve as a driving force (with or without Na), it appeared that leucine was transported in symport with Na+. The leucine carrier could use Li as well as Na and had a single binding site for Na+. The K(m) for Na was 5.2 mM, and the K(m) and V(max) for leucine were 77 μM and 328 nmol/mg of protein per min, respectively. Since valine and isoleucine competitively inhibited (K(i)s of 90 and 49 μM, respectively) leucine transport, it appeared that the peptostreptococcus used a common carrier for branched-chain amino acids. Valine or isoleucine was taken up rapidly, but little ammonia was produced if they were provided individually. The lack of ammonia could be explained by an accumulation of reducing equivalents. The ionophore, monensin, inhibited growth, but leucine was taken up and deaminated at a slow rate. Monensin caused a loss of K, an increase in Na, a slight increase in Δψ, and a decrease in intracellular pH. The inhibition of growth was consistent with a large decrease in ATP. The capacity of the ruminal peptostreptococcus to transport and deaminate branched-chain amino acids and its sensitivity to monensin clarifies the observation that monensin decreases branched-chain volatile fatty acid production in vivo.