Identification and characterization of a ribose transport system in Leishmania donovani promastigotes

A transport system for ribose in Leishmania donovani promastigotes was identified and characterized by measuring the uptake of radioisotope-labeled ribose. The pentoses arabinose, 2-deoxyribose and xylose inhibited ribose uptake, whereas hexoses (glucose, alpha-methylglucoside, thioglucose, galactose, lactose, maltose, mannose), adenosine, and proline did not inhibit uptake, indicating that the transporter exhibited substrate specificity. Intracellular ribose exchanged with 2-deoxyribose. Uptake of ribose showed saturation kinetics with an apparent Km = 2 mM and Vmax = 11 nmol (mg protein)-1 min-1. Both N-ethylmaleimide and p-hydroxymercuribenzoate inhibited ribose uptake which was prevented by dithiothreitol. The uncoupling agents 2,4-dinitrophenol and carbonylcyanide p-(trifluoromethoxy)phenylhydrazone and a variety of inhibitors of energy-driven transport had no significant effect on ribose uptake. Following transport, the intracellular ribose pool contained two-thirds of the sugar in the phosphorylated form and one-third in the neutral form. These cumulative results indicate that a specific carrier mediates ribose uptake via a facilitated diffusion system in L. donovani promastigotes.