Leishmania tropica: Uptake of methionine by promastigotes

Abstract

Promastigotes of Leishmania tropica transport methionine against a concentration gradient through a saturable system. This concentrative uptake requires metabolic energy and is sensitive to temperature and sulfhydryl reagents such as N-ethyl maleimide. Intracellular methionine exchanges with external methionine. Preformed methionine pools are lost at low temperatures and in the presence of energy uncouplers such as 2,4-dinitrophenol. Glucose stimulates uptake rates presumably by providing additional energy. In short term uptake experiments lasting 10 min, the incorporation of methionine into cellular material is negligible. Thus almost all methionine taken up goes into the amino acid pools where a small fraction (less than 20%) is metabolized, chiefly into l-methionine sulfoxide and cystathionine. In 10-min experiments, over 80% of the total intracellular label constitutes unaltered methionine. At steady state, unaltered methionine in the intracellular pool is at a concentration about 12–17 times that found in the external medium.