Abstract
The study of triglyceride (TG) metabolism using stable isotope tracers would be facilitated by being able to detect low 13C enrichment. To meet this goal, we developed a gas chromatography/isotope ratio-mass spectrometry technique to measure the enrichment of palmitate in nonesterified fatty acids (NEFA) and TG as its methyl derivative. This method allows accurate and reproducible measurements of enrichment as low as 0.009 mole percent excess (MPE), in a range between 0-0.65 MPE. The usefulness of this method is shown by two studies of lipid metabolism in human beings. First, we studied the metabolic fate of an oral TG load labeled with [1,1,1-13C3]tripalmitin. Labeled palmitate appeared concurrently in plasma NEFA and TG, and four hours after the load, the labeling was higher in NEFA than in TG (MPE NEFA: 1.53 +/- 0.31 vs. MPE TG: 0.78 +/- 0.06, P < 0.05). In a second study, the hepatic reesterification of NEFA was estimated by measuring the appearance of infused [1-13C]palmitate in circulating TG. The estimated contribution of plasma NEFA to circulating TG increased to a maximum of 22%. Thus, gas chromatography/isotope ratio-mass spectrometry appears to be a useful tool for future studies of lipid metabolism in humans.
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