Gluconeogenesis in Saccharomyces cerevisiae: determination of fructose-1,6-bisphosphatase activity in cells grown in the presence of glycolytic carbon sources

Abstract

The activity of fructose-1,6-bisphosphatase (FBP), a gluconeogenic enzyme, was determined in wild-type Saccharomyces cerevisiae X2180 grown in the presence of the glycolytic carbon sources, glucose, fructose, and galactose. The activities of phosphofructokinase (PFK), a glycolytic enzyme, and phosphoglucose isomerase (PGI), an enzyme functioning both in glycolysis and gluconeogenesis, were determined for purposes of comparison. A measurable amount of FBP activity was present in 20-h-old cells grown with moderate shaking in 1% glucose-nutrient or minimal medium. This activity increased significantly in 40 and 60-h-old cells. Similar levels of FBP activity were also present in 20-, 40-, and 60-h-old cells grown in 1% fructose-nutrient medium. A higher level of FBP activity was present in 20-h-old cells grown in 1% galactose-nutrient medium than in 20-h-old cells grown in 1% glucose- or fructose-nutrient medium. The FBP activity in glucose- or fructose-grown cells was higher than the corresponding activity in cells grown under similar conditions for 40 and 60 h in the presence of ethanol, a gluconeogenic carbon source. The PFK activity was significantly less in galactose- and ethanol-grown cells. The PGI activity was relatively constant in 20-, 40-, and 60-h-old cells grown in the presence of glucose, fructose, and galactose, but this activity was reduced approximately 50% in ethanol-grown cells. It is concluded from these results that, depending upon the concentration of carbon source and the time of incubation, FBP, a strictly gloconeogenic enzyme, is synthesized by S. cerevisiae grown in the presence of glycolytic carbon sources.