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J Bacteriol. 1971 March; 105(3): 690-700
Copyright © 1971 American Society for Microbiology. All Rights Reserved.

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

^a Department of Medical Microbiology and Immunology, School of Medicine, University of California, Los Angeles, California 90024

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^–5M 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^–5M in yeast phase and 2 x 10^–5M in mycelial phase cells. Transport of L-leucine at an external concentration of 10^–5M showed all of the characteristics of a system of active transport, which was dependent on temperature and pH. Displacement or removal of the -amino group, or modification of the -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^–5M). 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.

¹ Present address: Department of Microbiology, School of Medicine, Yale University, New Haven, Conn. 06510.