D-Glucose Generation from [2-13C]Pyruvate in Rat Hepatocytes: Implications in Terms of Enzyme-to-Enzyme Channelling

In rat hepatocytes exposed to [2-13C]pyruvate, newly formed glucose was more efficiently labeled in the carbon C5than C2, as well as in the carbon C6than C1, suggesting enzyme-to-enzyme channeling ofD-glycer- aldehyde 3-phosphate between glyceraldehyde-3-phos- phate dehydrogenase and phosphofructoaldolase. Likewise the C1/C2and C6/C5ratios for13C abundance in newly formed glucose, which largely exceeded the C3/C2ratio of lactate or alanine and could reflect reversibility in the fumarase reaction, were compatible with the enzyme-to-enzyme tunneling of symmetrical Krebs cycle intermediates in the sequence of reactions catalyzed by succinyl-CoA synthetase, succinate dehydrogenase, and fumarase. This study further indicates that the major fraction of pyruvate is metabolized via pyruvate carboxylase rather than pyruvate dehydrogenase.