Simultaneous assay of isotopic enrichment and concentration of guanidinoacetate and creatine by gas chromatography-mass spectrometry

A gas chromatography-mass spectrometry (GC-MS) method for the simultaneous measurement of isotopic enrichment and concentration of guanidinoacetate (GAA) and creatine in plasma sample for kinetic studies is reported. The method, based on preparation of the bis(trifluoromethyl)pyrimidine methyl ester derivatives of GAA and creatine, is robust and sensitive. The lowest measurable m1 and m3 enrichment for GAA and creatine, respectively, was 0.3%. The calibration curves for measurements of concentration were linear over ranges of 0.5 to 250 μM GAA and 2 to 500 μM for creatine. The method was reliable for inter- and intraassay precision, accuracy, and linearity. The technique was applied in a healthy adult to determine the in vivo fractional synthesis rate of creatine using primed-constant rate infusion of [1-13C]glycine. It was found that isotopic enrichment of GAA reached a plateau by 30 min of infusion of [1-13C]glycine, indicating either a small pool size or a rapid turnover rate (or both) of GAA. In contrast, the tracer appearance in creatine was slow (slope = 0.00097), suggesting a large pool size and a slow rate of synthesis of creatine. This method can be used to estimate the rate of synthesis of creatine in vivo in human and animal studies.