First, we report the application of stable isotope dilution theory in metabolome characterization of aerobic glucose limited chemostat culture of S. cerevisiae CEN.PK 113-7D using liquid chromatography - electrospray ionization MS/MS (LC-ESI-MS/MS). A glucose-limited chemostat culture of S. cerevisiae was grown to steady state at a specific growth rate (μ) = 0.05 h^-1 in a medium containing only naturally labeled (99% U-¹²C, 1% U- ¹³C) carbon source. Upon reaching steady state, defined as 5 volume changes, the culture medium was switched to chemically identical medium except that the carbon source was replaced with 100% uniformly (U) ¹³C labeled stable carbon isotope, fed for 4 h, with sampling every hour. We observed that within a period of 1 h ∼80% of the measured glycolytic metabolites were U-¹³C-labeled. Surprisingly, during the next 3 h no significant increase of the U-¹³C-labeled metabolites occurred. Second, we demonstrate for the first time the LC-ESI-MS/MS-based quantification of intracellular metabolite concentrations using U-¹³C-labeled metabolite extracts from chemostat cultivated S. cerevisiae cells, harvested after 4 h of feeding with 100% U-¹³C-labeled medium, as internal standard. This method is hereby termed "Mass Isotopomer Ratio Analysis of U-¹³C Labeled Extracts" (MIRACLE). With this method each metabolite concentration is quantified relative to the concentration of its U-¹³C-labeled equivalent, thereby eliminating drawbacks of LC-ESI-MS/MS analysis such as nonlinear response and matrix effects and thus leads to a significant reduction of experimental error and work load (i.e., no spiking and standard additions). By coextracting a known amount of U- ¹³C labeled cells with the unlabeled samples, metabolite losses occurring during the sample extraction procedure are corrected for. ... Read more

In vivo kinetics of Saccharomyces cerevisiae are studied, in a time window of 150 s, by analyzing the response of O2 and CO2 in the fermentor off-gas after perturbation of chemostat cultures by metabolite pulses. Here, a new mathematical method is presented for the estimation of the in vivo oxygen uptake rate (OUR) and carbon dioxide evolution rate (CER) directly from the off-gas data in such perturbation experiments. The mathematical construction allows effective elimination of delay and distortion in the off-gas measurement signal under highly dynamic conditions. A black box model for the fermentor off-gas system is first obtained by system identification, followed by the construction of an optimal linear filter, based on the identified off-gas model. The method is applied to glucose and ethanol pulses performed on chemostat cultures of S. cerevisiae. The estimated OUR is shown to be consistent with the independent dissolved oxygen measurement. The estimated in vivo OUR and CER provide valuable insights into the complex dynamic behavior of yeast and are essential for the establishment and validation of in vivo kinetic models of primary metabolism. ... Read more