Carrier-mediated transport of D-ribose by Rhodotorula glutinis

The kinetics of D-ribose transport by Rhodotorula glutinis were investigated over a 1 000-fold range of sugar concentrations. Analysis of the saturation isotherm revealed the presence of two carrier systems for D-ribose in the Rhodotorula plasma membrane. These two carriers exhibited Km values of 1.3 and 30 mM. At saturating concentrations of D-ribose, the low Km carrier contributes less than 10% to the total rate of transport. Although D-ribose is metabolized rapidly by Rhodotorula, intracellular free sugar concentrations exceed those in the medium. In addition, the transport of this pentose is inhibited by the proton-conductors, 2,4-dinitrophenol and carbonylcyanide m-chlorophenylhydrazone. These data suggest that Rhodotorula cells are capable of an energy-dependent, concentrative transport of D-ribose.