Uptake of Cystine by the Yeast Phase of Histoplasma capsulatum.

This report deals with factors affecting the uptake of cystine by the yeast phase of Histoplasma capsulatum. The kinetics of uptake showed a saturation at 70 muM and an average K(m) value of 3 x 10(-5)m. The optimal pH and temperature for transport of cystine were 6.5 and 37 C, respectively. The energy of activation was 14.1 kcal/mole, and the temperature coefficient value was 2.1. A requirement for energy supplied by metabolic activity was demonstrated by the inhibition of incorporation of the amino acid by cells preincubated with either 2,4-dinitrophenol or sodium azide. Although uptake was not inhibited by any single amino acid, a combination of amino acids did cause a decrease in uptake. Thus, the data show that the uptake of cystine by yeast cells of H. capsulatum has the characteristics of a system of transport that requires the expenditure of energy by the cells.