title: Ornithine transport and exchange in Streptococcus lactis.
abstract: Resting cells of Streptococcus lactis 133 appeared to accumulate [14C]ornithine to a high concentration in the absence of an exogenous energy source. However, analysis of intracellular amino acid pool constituents and results of transport experiments revealed that the accumulation of ornithine represented a homoexchange between extracellular [14C]ornithine and unlabeled ornithine in the cell. The energy-independent exchange of ornithine was not inhibited by proton-conducting uncouplers or by metabolic inhibitors. Intracellular [14C]ornithine was retained by resting cells after suspension in a buffered medium. However, addition of unlabeled ornithine to the suspension elicited rapid exit of labeled amino acid. The initial rate of exit of [14C]ornithine was dependent on the concentration of unlabeled ornithine in the medium, but this accelerative exchange diffusion process caused no net loss of amino acid. By contrast, the presence of a fermentable energy source caused a rapid expulsion of and net decrease in the concentration of intracellular ornithine. Kinetic analyses of amino acid transport demonstrated competitive inhibition between lysine and ornithine, and data obtained by two-dimensional thin-layer chromatography established the heteroexchange of these basic amino acids. The effects of amino acids and of ornithine analogs on both entry and exit of [14C]ornithine have been examined. The data suggest that a common carrier mediates the entry and exchange of lysine, arginine, and ornithine in cells of S. lactis.
mesh_terms: D000596:Amino Acids / Q000378:metabolism; D001692:Biological Transport; D002855:Chromatography, Thin Layer; D002956:Citrulline / Q000378:metabolism; D007700:Kinetics; D013294:Lactococcus lactis / Q000254:growth & development / Q000378:metabolism*; D008239:Lysine / Q000378:metabolism; D009952:Ornithine / Q000378:metabolism*
chemical_list: D000596:Amino Acids; D002956:Citrulline; D009952:Ornithine; D008239:Lysine