Background: Nicotinamide adenine dinucleotide (NAD⁺), its precursors, and its derivatives (collectively NADome) play a crucial role in cellular processes and maintain redox homeostasis. Understanding the dynamics of these metabolic pools and redox reactions can provide valuable insights into metabolic functions, especially cellular regulation and stress response mechanisms. The accurate quantification of these metabolites is challenging due to the interconversion between the redox forms. Methods: Our laboratory previously developed a zwitterionic hydrophilic interaction liquid chromatography (zic-HILIC)–tandem mass spectrometry method for the quantification of five essential pyridine nucleotides, including NAD⁺ derivatives and it’s reduced forms, with ¹³C isotope dilution and matrix-matched calibration. In this study, we have improved the performance of the chromatographic method and expanded its scope to twelve analytes for a comprehensive view of NAD⁺ biosynthesis and utilization. The analytical method was validated and applied to investigate Escherichia coli BL21 under varying oxygen supplies including aerobic, microaerobic, and anaerobic conditions. Conclusions: The intracellular absolute metabolite concentrations ranged over four orders of magnitude with NAD⁺ as the highest abundant, while its precursors were much less abundant. The composition of the NADome at oxygen-limited conditions aligned more with that in the anaerobic conditions rather than in the aerobic phase. Overall, the NADome was quite homeostatic and E. coli rapidly, but in a minor way, adapted the metabolic activity to the challenging shift in the growth conditions and achieved redox balance. Our findings demonstrate that the zic-HILIC-MS/MS method is sensitive, accurate, robust, and high-throughput, providing valuable insights into NAD⁺ metabolism and the potential significance of these metabolites in various biological contexts.

Background: High-yielding recombinant protein expression systems often face challenges due to the metabolic burden caused by the competition for cellular resources, resulting in reduced growth and, hence, limiting their industrial applicability. Furthermore, industrial recombinant protein production is also affected by the occurrence of oxygen gradients, which is a prevalent issue in large-scale bioreactors. These gradients create a heterogeneous environment in the bioreactor, which affects cell growth and metabolism, having severe consequences on the process performance. Both these factors alter cellular physiology and metabolism, thereby affecting recombinant protein yields. Understanding metabolic adaptations to these stress conditions is crucial for uncovering the underlying cellular mechanisms, which can direct further optimization of the recombinant strains. In this study, we aimed to explore the combined response of the central metabolism of Escherichia coli to metabolic burden and microaerobic conditions. Two recombinant protein-producing E. coli BL21 strains carrying XylS/ Pm vectors with low (A2-mCh) and medium plasmid copy numbers (A3-mCh), and producing mCherry protein, were studied by introducing oxygen limitation. Central metabolite pools were analyzed by three targeted LC–MS/MS methods, using the isotope dilution strategy for absolute quantification. Results: Both recombinant strains exhibited different levels of metabolic burden, with the strain possessing a higher plasmid copy number showing more pronounced growth retardation and a stronger impact on metabolite pools. Both strains, however, showed a similar response to oxygen limitation, with significant adaptations in the central metabolite pools. The low plasmid copy number strain showed an increase in the concentration of lower glycolytic and tricarboxylic acid cycle metabolites, while the pools of upper glycolytic and pentose phosphate pathways and nucleoside phosphates were mostly unaffected. However, a more extreme response was seen in A3-mCh, where the majority of the metabolite pools were increased. Oxygen limitation caused lower metabolic activity, but the energy charge and redox balance were maintained, and no negative effect was observed on mCherry production rates. Conclusion: This study provides insights into metabolic adaptations in E. coli BL21 recombinant strains, having quite robust mechanisms to maintain intracellular metabolic homeostasis during both internal and external perturbations.