Carbon-13 and phosphorus-31 NMR study of gluconeogenesis: utilization of carbon-13-labeled substrates by perfused liver from streptozotocin-diabetic and untreated rats

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Abstract
The metabolism of 13C-labeled substrates were followed by 13C and 31P NMR in perfused liver from the streptozotocin-treated rat model of insulin-dependent diabetes. Comparison was made with perfused liver from untreated littermates, fasted either 24 or 12 h. The major routes of pyruvate metabolism were followed by a 13C NMR approach that provided for the determination of the metabolic fate of several substances simultaneously. The rate of gluconeogenesis was 2-4-fold greater and .beta.-hydroxybutyrate production was 50% greater in liver from the chronically diabetic rats as compared with the control groups. Large differences in the distribution of 13C-label in hepatic alanine were measured between diabetic and control groups. The biosyntheses of 13C-labeled glutathione and N-carbamoylaspartate were monitored in time-resolved 13C NMR spectra of perfused liver. Assignments for the resonances of glutathione and N-carbamoylaspartate were made with the aid of 13C NMR studies of perchloric acid extracts of the freeze-clamped livers. 13C NMR spectroscopy of the perfusates provided a convenient, rapid assay of the rate of oxidation of [2-13C]ethanol, the hepatic output of [2-13C]acetaldehyde, and the accumulation of [2-13C]acetate in the perfusate. By 31P NMR spectroscopy, carbamoyl phosphate was measured in all diabetic livers and an unusual P,P''-diesterified pyrophosphate was observed in one-fourth of the diabetic livers examined. Neither of these phoshorylated metabolites was detected in control liver. Both 13C and 31P NMR were useful in defining changes in hepatic metabolism in experimental diabetes.