title: Kinetic analysis of hexose uptake in Saccharomyces cerevisiae cultivated in continuous culture.
abstract: As the expression of the hexose transporters in Saccharomyces cerevisiae is regulated by the external fructose concentration, in vivo fructose transport was investigated at a number of different but constant fructose concentrations in a continuous culture. The in vitro uptake rate and kinetic parameters were studied by uptake of 14C-labelled fructose. From Eadie-Hofstee plots, based on the results of the in vitro uptake experiment, K(m) and Vmax for the different steady-state conditions were determined. These results demonstrate that as fructose concentrations increase, fructose consumption rates and K(m) increase and Vmax slightly decreases. However, remarkably a large discrepancy was observed between in vivo determined consumption rates in the continuous culture and the in vitro uptake rates from the 14C-labelled fructose uptake experiment on cells from an identical steady-state condition. In every condition the in vivo consumption rate was approx. 2-3 times higher than the in vitro uptake rate. A comparable discrepancy was observed for glucose uptake rates; however, in vivo and in vitro galactose uptake rates were identical. These observations imply that kinetic analyses of sugar transport mechanisms can only be performed correctly when accompanied by a comparison with in vivo determined consumption rates.
authors.affiliation: Department of Molecular Cell Biology, Utrecht University, The Netherlands. michelle@emsaserv.biol.ruu.nl
authors.forename: M M
authors.initials: MM
authors.lastname: Meijer
authors.forename: J
authors.initials: J
authors.lastname: Boonstra
authors.forename: A J
authors.initials: AJ
authors.lastname: Verkleij
authors.forename: C T
authors.initials: CT
authors.lastname: Verrips
mesh_terms: D002250:Carbon Radioisotopes; D005632:Fructose / Q000378:metabolism / Q000494:pharmacology; D005690:Galactose / Q000378:metabolism; D005947:Glucose / Q000378:metabolism; D006601:Hexoses / Q000378:metabolism*; D007700:Kinetics; D009004:Monosaccharide Transport Proteins / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000254:growth & development / Q000378:metabolism*
chemical_list: D002250:Carbon Radioisotopes; D006601:Hexoses; D009004:Monosaccharide Transport Proteins; D005632:Fructose; D005947:Glucose; D005690:Galactose