ABSTRACT
The glycine cleavage system (GCS), the major pathway of glycine catabolism in liver, is found only in the mitochondria matrix and is regulated by the oxidized nicotinamide adenine dinucleotide (NAD(+) )/reduced nicotinamide adenine dinucleotide (NADH) ratio. In conjunction with serine hydroxymethyltransferase, glycine forms the 1 and 2 positions of serine, while the 3 position is formed exclusively by GCS. Therefore, we sought to exploit this pathway to show that quantitative measurements of serine isotopomers in liver can be used to monitor the NAD(+) /NADH ratio using (13) C NMR spectroscopy. Rat hepatocytes were treated with modulators of GCS activity followed by addition of 2-(13) C-glycine, and the changes in the proportions of newly synthesized serine isotopomers were compared to controls. Cysteamine, a competitive inhibitor of GCS, prevented formation of mitochondrial 3-(13) C-serine and 2,3-(13) C-serine isotopomers while reducing 2-(13) C-serine by 55%, demonstrating that ca. 20% of glycine-derived serine is produced in the cytosol. Glucagon, which activates GCS activity, and the mitochondrial uncoupler carbonyl cyanide-3-chlorophenylhydrazone both increased serine isotopomers, whereas rotenone, an inhibitor of complex I, had the opposite effect. These results demonstrate that (13) C magnetic resonance spectroscopy monitoring of the formation of serine isotopomers in isolated rat hepatocytes given 2-(13) C-glycine reflects the changes of mitochondrial redox status.