Transport of some mono- and di-saccharides into yeast cells

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Abstract
The permeability of the cell membranes of some yeast cells for various mono- and di-saccharides has been studied. The influx of D-galactose and D-arabinose into the cells of Saccharomyces cerevisae followed an exponential curve; an apparent equilibrium concentration of these sugars in the cells was reached within 60 minutes. Free glucose could be demonstrated intracellularly when the metabolism of this sugar was inhibited, e.g. by m[image]-iodoacetate. The maximum rate of penetration of D-galactose intoj S. cerevisiae cells was 2-06 [plus or minus] 0-2 [mu]g/ mg dry weight/minute. This rate was sufficient to account for the metabolism of galactose in this yeast, partly adapted to the utilization of galactose by aerobic incubation with this sugar. The apparent galactose or arabinose space in S. cerevisiae was found to be 56-4 and 59-1% of the cell volume. In the calculation of the apparent sugar space no account was taken of the considerable changes of the intracellular water volume on addition of sugar. The Q10 values for both influx and efflux of D-galactose in Saccharomyces cerevisiae cells was of the same order, i.e. 2-9 and 2-4 respectively; at a temperature of 1[degree] practically no fluxes of sugar were observed. The influx of D-galactose or D-arabinose into S. cerevisiae was inhibited by the presence of glucose and glucosamine. The efflux of D-galactose from S. cerevisiae cells was not inhibited by glucose unless glucose could be simultaneously demonstrated intracellularly in the presence of 1-5 m[image]-iodoacetate. Preliminary aeration of anaerobically grown S. cerevisiae cells decreased the influx of D-galactose. The permeability of the cell membrane of S. fragilis for [alpha]-D-methylglucoside, but not for maltose, and of S. cerevisiae for trehalose, was demonstrated. The results obtained are considered from the point of view of a carrier-linked transport of D-galactose and D-arabinose across the cell membrane of S. cerevisiae cells, with glucose acting as a competitive inhibitor of considerably higher affinity for the carrier than galactose. The possible significance of the results for the phenomenon of diauxie is discussed.