Unique features of pancreatic beta‐cell mitochondrial NADH metabolism

In association with the glucokinase, the role of mitochondrial metabolism is widely recognized in glucose sensing. It is thought that the low affinity glucokinase, which is insensitive to feedback inhibition by ATP, increases the flux through glycolysis and enhances pyruvate production. Pyruvate is suggested to be metabolized by TCA cycle to generates electron donors (NADH & FADH2 for respiratory Complexes I & II respectively) for ATP production, and provide additional signals for sustained release of insulin. Although imaging based studies have suggested that pyruvate metabolism within mitochondria may not be efficient in generating NADH, its implication on mitochondrial function has not been explored in details. Using an in situ respirometry assay with permeabilized INS1E and dispersed islet cells, we have explored the ability of β‐cells to generate NADH on different substrates. Our data show that β‐cell mitochondria are unable to display a detectable level of respiration on NADH‐generating substrates, while the mitochondria from lung fibroblasts, astrocytes and neurons display robust respirations under the same experimental conditions. Further, the respiration rates on succinate and α‐glycerophosphate, the substrates for Complexes II & III respectively, were very robust. These data clearly suggest a unique regulation of NADH metabolism within β‐cell mitochondria, which could be due to limited NADH production and/or consumption by enzymes other than Complex I. NADH levels are lower compared to that found in fibroblast and astrocyte mitochondria. We propose that β‐cells regulate the NADH output per glucose within mitochondria by negatively regulating key steps to favor the reliance on redox shuttles, and help export of malate and citrate to cytosol for tight coupling of glucose metabolism with insulin secretion.