Determination of leucine flux in vivo by gas chromatography—mass spectrometry utilizing stable isotopes for trace and internal standard

A simple and reliable method is described for the determination of leucine flux in vivo using two stable isotopes of leucine and gas chromatography—mass spectrometry (GCMS). [6,6,6-²H₃]Leucine is administered as a primed-dose constant infusion in vivo and dl-[²H₇]leucine is added to plasma as an internal standard. Plasma leucine concentration and moles per cent enrichment of [²H₃]leucine can be determined simultaneously by GCMS and selected ion monitoring. Leucine flux calculated from the [6,6,6-²H₃]leucine data was nearly identical to that obtained with l-[U-¹⁴C]leucine in dogs. This method is readily applicable to the study of leucine metabolism in humans of all ages and laboratory animals.