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l -Leucine entered the cells of both morphological forms ofHistoplasma capsulatum by a permease-like system at low external concentrations of substrate. However, at levels greater than 5 × 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 × 10−5 m in yeast phase and 2 × 10−5 m in mycelial phase cells. Transport ofl -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 andp H. Displacement or removal of the α-amino group, or modification of the α-carboxyl group abolished amino acid uptake. The process was competitively inhibited by neutral aliphatic side-chain amino acids (inhibition constants ranged from 1.5 × 10−5 to 6.2 × 10−5 m ). Neutral aromatic side-chain amino acids and thed -isomers of leucine and valine did not inhibitl -leucine uptake. These data were interpreted to mean that thel -leucine transport system is stereospecific and is highly specific for neutral aliphatic side-chain amino acids. Incorporation ofl -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 ofl -leucine was of limited capacity, and the total uptake of the amino acid was a function of intracellular pool size. The initial rate ofl -leucine uptake was independent of the level of intracellular pool. Both morphological forms deaminated and degraded only a minor fraction of the accumulated leucine.

Comparative Physiological Studies of the Yeast and Mycelial Forms of Histoplasma capsulatum : Uptake and Incorporation of l -Leucine