Hyperpolarized δ-[1-¹³ C]gluconolactone as a probe of the pentose phosphate pathway

The pentose phosphate pathway (PPP) is thought to be upregulated in trauma (to produce excess NADPH) and in cancer (to provide ribose for nucleotide biosynthesis), but simple methods for detecting changes in flux through this pathway are not available. MRI of hyperpolarized ¹³ C-enriched metabolites offers considerable potential as a rapid, non-invasive tool for detecting changes in metabolic fluxes. In this study, hyperpolarized δ-[1-¹³ C]gluconolactone was used as a probe to detect flux through the oxidative portion of the pentose phosphate pathway (PPP_ox ) in isolated perfused mouse livers. The appearance of hyperpolarized (HP) H¹³ CO₃^- within seconds after exposure of livers to HP-δ-[1-¹³ C]gluconolactone demonstrates that this probe rapidly enters hepatocytes, becomes phosphorylated, and enters the PPP_ox pathway to produce HP-H¹³ CO₃^- after three enzyme catalyzed steps (6P-gluconolactonase, 6-phosphogluconate dehydrogenase, and carbonic anhydrase). Livers perfused with octanoate as their sole energy source show no change in production of H¹³ CO₃^- after exposure to low levels of H₂ O₂ , while livers perfused with glucose and insulin showed a twofold increase in H¹³ CO₃^- after exposure to peroxide. This indicates that flux through the PPP_ox is stimulated by H₂ O₂ in glucose perfused livers but not in livers perfused with octanoate alone. Subsequent perfusion of livers with non-polarized [1,2-¹³ C]glucose followed by ¹ H NMR analysis of lactate in the perfusate verified that flux through the PPP_ox is indeed low in healthy livers and modestly higher in peroxide damaged livers. We conclude that hyperpolarized δ-[1-¹³ C]gluconolactone has the potential to serve as a metabolic imaging probe of this important biological pathway.