Quantitative ¹³C Traces of Glucose Fate in Hepatitis B Virus-Infected Hepatocytes

Quantitative characterization of ¹³C-labeled metabolites is an important part of the stable isotope tracing method widely used in metabolic flux analysis. Given the long relaxation time and low sensitivity of ¹³C nuclei, direct measurement of ¹³C-labeled metabolites using one-dimensional ¹³C NMR often fails to meet the demand of metabolomics studies, especially with large numbers of samples and metabolites having low abundance. Although HSQC-based 2D NMR methods have improved sensitivity with inversion detection, they are time-consuming and thus unsuitable for high-throughput absolute quantification of ¹³C-labeled metabolites. In this study, we developed a method for absolute quantification of ¹³C-labeled metabolites using naturally abundant TSP as a reference with the first increment of the HMQC pulse sequence, taking polarization transfer efficiencies into consideration. We validated this method using a mixture of ¹³C-labeled alanine, methionine, glucose, and formic acid together with a mixture of alanine, lactate, glycine, uridine, cytosine, and hypoxanthine, which have natural ¹³C abundance with known concentrations. We subsequently applied this method to analyze the flux of glucose in HepG2 cells infected with hepatitis B virus (HBV). The results showed that HBV infection increased the cellular uptake of glucose, stimulated glycolysis, and enhanced the pentose phosphate and hexosamine pathways for biosynthesis of RNA and DNA and nucleotide sugars to facilitate HBV replication. This method saves experimental time and provides a possibility for absolute quantitative tracking of the ¹³C-labeled metabolites for high-throughput studies.