Monosaccharide transport systems in the yeast Rhodotorula glutinis

Abstract

By using d-glucose, d-xylose, d-galactose and d-fructose in the strictly aerobic yeast Rhodotorula glutinis and by comparing the half-saturation constants with inhibition constants the yeast was shown to possess a single common system for d-xylose and d-galactose (K_m's and K_i's all between 0.5 and 1.1 mM) but another distinct transport system for d-fructose. The transport of d-glucose has a special position in that glucose blocks apparently allotopically all the other systems observed although it uses at least one of them for its own transport. The different character of d-glucose uptake is underlined by its relative independence of pH (its “K_m” is completely pH-insensitive) in contrast with all other sugars. At low concentrations, all sugars show mutual positive cooperativity in uptake, suggesting at least two transport sites plus possibly a modifier site on the carrier.