Measuring enzyme activities under standardized in vivo-like conditions for systems biology.
Realistic quantitative models require data from many laboratories. Therefore, standardization of experimental systems and assay conditions is crucial. Moreover, standards should be representative of the in vivo conditions. However, most often, enzyme-kinetic parameters are measured under assay conditions that yield the maximum activity of each enzyme. In practice, this means that the kinetic parameters of different enzymes are measured in different buffers, at different pH values, with different ionic strengths, etc. In a joint effort of the Dutch Vertical Genomics Consortium, the European Yeast Systems Biology Network and the Standards for Reporting Enzymology Data Commission, we have developed a single assay medium for determining enzyme-kinetic parameters in yeast. The medium is as close as possible to the in vivo situation for the yeast Saccharomyces cerevisiae, and at the same time is experimentally feasible. The in vivo conditions were estimated for S. cerevisiae strain CEN.PK113-7D grown in aerobic glucose-limited chemostat cultures at an extracellular pH of 5.0 and a specific growth rate of 0.1 h(-1). The cytosolic pH and concentrations of calcium, sodium, potassium, phosphorus, sulfur and magnesium were determined. On the basis of these data and literature data, we propose a defined in vivo-like medium containing 300 mM potassium, 50 mM phosphate, 245 mM glutamate, 20 mM sodium, 2 mM free magnesium and 0.5 mM calcium, at a pH of 6.8. The V(max) values of the glycolytic and fermentative enzymes of S. cerevisiae were measured in the new medium. For some enzymes, the results deviated conspicuously from those of assays done under enzyme-specific, optimal conditions.
van Eunen
Karen
K
Department of Molecular Cell Physiology, Vrije Universiteit Amsterdam, The Netherlands.
Bouwman
Jildau
J
Daran-Lapujade
Pascale
P
Postmus
Jarne
J
Canelas
André B
AB
Mensonides
Femke I C
FI
Orij
Rick
R
Tuzun
Isil
I
van den Brink
Joost
J
Smits
Gertien J
GJ
van Gulik
Walter M
WM
Brul
Stanley
S
Heijnen
Joseph J
JJ
de Winde
Johannes H
JH
de Mattos
M Joost Teixeira
MJ
Kettner
Carsten
C
Nielsen
Jens
J
Westerhoff
Hans V
HV
Bakker
Barbara M
BM
D003470:Culture Media / Q000592:standards*; D003600:Cytosol / Q000201:enzymology; D005285:Fermentation / Q000235:genetics; D006019:Glycolysis / Q000235:genetics; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D012441:Saccharomyces cerevisiae / Q000201:enzymology*; D049490:Systems Biology / Q000592:standards*
D003470:Culture Media