Transport systems for acyclic polyols and monosaccharides in Torulopis candida

Anomeric specificity of the monosaccharide carrier in yeasts and yeast-like organisms

The anomeric specificity of monosaccharide uptake was investigated in 42 species of yeasts and related mycelium-forming fungi. Differences in the uptake of anomers were determined by the following methods. (1) Shift of anomeric equilibrium in the outer medium caused by preferential uptake of one of the anomeric forms was monitored polarimetrically as induced mutarotation. (2) The uptake of 14C-D-glucose by cells was examined after addition of freshly prepared solutions of alpha- or beta-D-glucose. Most of the organisms examined display the Saccharomyces-type preference for the alpha-anomers of glucose and xylose which is caused by the higher affinity of the monosaccharide carrier for the alpha-pyranose configuration. The following genera show this type of preference (the number of species is given in parenthesis): Saccharomyces (5), Schizosaccharomyces (1), Endomycopsis (2), Eremascus (1), Endomyces (1), Pichia (1), Hansenula (1), Debaryomyces (2), Lipomyces (1), Willia (1), Nematospora (1), Kluyveromyces (2), Candida (5), Torulopsis (5), Cryptococcus (1). No anomeric specificity was shown by the following genera: Nadsonia (1), Dipodascus (2), Rhodotorula (5), Sporobolomyces (2), Bullera (1), Rhodosporidium (1). A parallel investigation of the concentration dependence of glucose uptake indicates that most yeasts possess a constitutive monosaccharide carrier characterized by the following features: a high maximum rate of uptake, a relatively low affinity, and preference for alpha-anomers. Besides this carrier the majority of these microorganisms possess a glucose-transporting carrier with a higher affinity and a lower capacity.

Transport of ribitol and D-glucose in the yeast Candida guillermondii

The uptakes of the linear polyol ribitol and of D-glucose by Candida guillermondii were found to be carrier-mediated and to require metabolic energy. In glucose-grown cells ribitol possibly enters by simple diffusion but after an induction period a specific transport system is synthesized, inhibitable by higher concentrations of arabinitols, xylitol, mannitol and sorbitol. Actidione blocks the synthesis of the inducible ribitol transport system. Two systems of different affinity for substrate were found to operate in the uptake of both glucose and of ribitol. Counter-transport experiments with ribitol, D-glucose and 3-O-methyl-D-glucose support the carrier nature of the uptake system.

Monosaccharide transport systems in the yeast Rhodotorula glutinis

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 (Km's and Ki'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 "Km" 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.

A note on the kinetics of uptake of D-glucose by the food yeast, Candida utilis

Unlike other yeasts so far investigated, the D-glucose carrier of Candida utilis (strain NCYC 737) appears to change affinity for D-glucose according to its exogenous concentration. When the concentration of D-glucose was less than 0.4 mM, the apparent Km approximately 0.2 mM; at greater than 0.4 mM, the Km approximately 10 mM.

[Transport and utilization of alditols in the yeast Rhodotorula gracilis glutinis (author's transl)]

The obligate aerobic yeast Rhodoturula gracilis was found to take up the alditols D-glucitol, D-mannitol, ribitol, xylitol, D-arabinitol, L-arabinitol and erythritol by means of a constitutive mobile membrane carrier. This uptake involved active transport, that is, it was dependent on the supply of metabolic energy, leading to the accumulation of alditols inside the cells. The accumulation ratio (intracellular concentration to extracellular concentration, Si/SO) was much lower for alditols than for monosaccharides. As for monosaccharides, this ratio decreased with increasing extracellular concentration, even to values below 1. The kinetic data showed that the carrier system for alditols was identical to that for monosaccharides, though it had a much lower affinity and maximum velocity for alditols. Hence the uptake of alditols was blocked in the presence of monosaccharides. Only ribitol and L-arabinitol were catabolized following enzyme induction. The other alditols were not broken down.