Saccharomyces cerevisiae was grown on different concentrations of sucrose and glucose mixtures after adapting on sucrose. The yeast cells were found to have two different mechanisms by which sucrose was utilised: hydrolysis outside the cell membrane and direct transport into the cells. The mechanism by which sucrose was utilised depended on the initial concentration of glucose in the mixture and the adaptation state of the cells. In both cases, glucose was utilised first and invertase secretion was repressed when the glucose concentration was higher than 2 g l-'. The major important finding in our results is that, for fully sucrose-adapted cells, even in the presence of a repressive glucose concentration, the yeast cells were able to utilise sucrose, and most importantly, without first being hydrolysed to its constituent monosaccharides. Previously, it has been thought that direct transport of sucrose contributes only a small part of the process of utilisation of sucrose, but as our results indicate it can be the major mechanism of sucrose utilisation in S. cerevisiae if the cells are adapted on sucrose for long periods, glucose concentrations in the medium are high and/or invertase activities are limiting.The initial steps in the utilisation of sugars by yeast is either the passage of intact sugar molecules across the cell membrane or the hydrolysis outside the cell followed by entry of all or some of the hydrolytic products into the cell. Sugar transport into microbial cells across the plasmalemma can take place by active transport, facilitated diffusion and group translocation (19).Sucrose utilisation involves its hydrolysis outside the yeast cell by extracellular enzyme invertase ($-D-fructosidase) into glucose and fructose which are then transported into the cells and metabolised. Most Saccharomyces cerevisiae strains have two forms of invertase: an external large invertase, which is a glycoprotein, and an internal small invertase (9,15). Some strains of S. cerevisiae have been reported to transport sucrose directly into the cell (14,17,20).
A sucrose-adapted inoculum of Saccharomyces cerevisiae growing in a batch fermenter in a sucrose medium was dosed with a high concentration of the enzyme invertase ($-D-fructosidase) during the mid-exponential phase of growth. The culture then underwent a short lag of about 20 min before dramatically decreasing its growth rate. The sugar analysis profiles showed that, during the lag phase, all the sucrose was hydrolyzed to glucose and fructose. The specific growth rate decreased from 0.50 h ' when growing on sucrose (before addition of invertase) to 0.44 h ' on the resulting mixture of glucose and fructose. These results help in explaining the mechanism of utilization of sucrose by actively growing yeast cells in a batch culture and suggest a direct uptake of sucrose molecules into yeast cells.
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