title: Comparative physiologial studies of the yeast and mycelial forms of Histoplasma capsulaum: uptake and incorporation of L-leucine.
abstract: l-Leucine entered the cells of both morphological forms of Histoplasma capsulatum by a permease-like system at low external concentrations of substrate. However, at levels greater than 5 x 10(-5)m l-leucine, the amino acid entered the cells both through a simple diffusion-like process and the permease-like system. The rate of the amino acid diffusion into yeast and mycelial forms appeared to be the same, whereas the initial rate of accumulation through the permease-like system was 5 to 10 times faster in the mycelial phase than it was in the yeast phase. The Michaelis constants were 2.2 x 10(-5)m in yeast phase and 2 x 10(-5)m in mycelial phase cells. Transport of l-leucine at an external concentration of 10(-5)m showed all of the characteristics of a system of active transport, which was dependent on temperature and pH. Displacement or removal of the alpha-amino group, or modification of the alpha-carboxyl group abolished amino acid uptake. The process was competitively inhibited by neutral aliphatic side-chain amino acids (inhibition constants ranged from 1.5 x 10(-5) to 6.2 x 10(-5)m). Neutral aromatic side-chain amino acids and the d-isomers of leucine and valine did not inhibit l-leucine uptake. These data were interpreted to mean that the l-leucine transport system is stereospecific and is highly specific for neutral aliphatic side-chain amino acids. Incorporation of l-leucine into macromolecules occurred at almost the same rate in both morphological forms of the fungus. The mycelial phase but not the yeast phase showed a slight initial lag in incorporation. In both morphological forms the intracellular pool of l-leucine was of limited capacity, and the total uptake of the amino acid was a function of intracellular pool size. The initial rate of l-leucine uptake was independent of the level of intracellular pool. Both morphological forms deaminated and degraded only a minor fraction of the accumulated leucine.
mesh_terms: D000596:Amino Acids / Q000494:pharmacology; D001151:Arsenic / Q000494:pharmacology; D001345:Autoradiography; D001386:Azides / Q000494:pharmacology; D001692:Biological Transport; D002247:Carbon Isotopes; D002855:Chromatography, Thin Layer; D003470:Culture Media; D004058:Diffusion; D004140:Dinitrophenols / Q000494:pharmacology; D005374:Filtration; D005782:Gels; D006658:Histoplasma / Q000201:enzymology / Q000254:growth & development / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007930:Leucine / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000378:metabolism; D012822:Silicon Dioxide; D012964:Sodium; D013237:Stereoisomerism; D013696:Temperature; D013997:Time Factors; D014238:Trichloroacetic Acid
chemical_list: D000596:Amino Acids; D001386:Azides; D002247:Carbon Isotopes; D003470:Culture Media; D004140:Dinitrophenols; D005782:Gels; D026901:Membrane Transport Proteins; D014238:Trichloroacetic Acid; D012822:Silicon Dioxide; D012964:Sodium; D007930:Leucine; D001151:Arsenic