The uptake and incorporation of leucine and cystine by the mycelial and yeastlike phases of Blastomyces dermatitidis

Abstract

Uptake and incorporation of L-leucine-C¹⁴ and L-cystine-S³⁵ was studied in the mycelial [MP] and yeastlike [YP] phases of the dimorphic fungal pathogen,Blastomyces dermatitidis. Both amino acids entered the cells of the two morphological forms ofB. dermatitidis by a permease-like system at low external concentrations of substrate. At high substrate levels, the amino acids entered the cells by a simple diffusion-like process in addition to the permease-like system. Michaelis-Menten constants [K_m] for L-leucine was found to be 1.1×10⁻⁵ M and 4.4×10⁻⁵ M for the MP and YP phases, respectively. The K_m for L-cystine was found to be 1.0×10⁻⁵ M for the MP and 0.5×10⁻⁵ M for the YP. A requirement for energy supplied by metabolic activity was demonstrated by the inhibition of uptake and incorporation of the amino acids by cells incubated with either 2,4-dinitrophenol or sodium azide. Amino acid uptake was broadly tolerant of hydrogen ion concentration, but definite optima were demonstrated at pH 7.0 to 7.5.