Simultaneous determination of plasma levels of alpha-ketoisocaproic acid and leucine and evaluation of alpha-[1-13C]ketoisocaproic acid and [1-13C]leucine enrichment by gas chromatography-mass spectrometry.

A sensitive method for the simultaneous analysis of plasma alpha-ketoisocaproic acid (KIC) and leucine using GC-MS is described. Plasma was deproteinized after addition of both alpha-ketovaleric acid and norleucine as internal standards. Derivatization of the compounds involved protection of the keto groups with methoxyamine and tert-butyldimethylsilylation under optimized conditions. Mass spectral analyses were carried out in electron impact ionization mode and the gas chromatographic conditions selected prevented interference by other keto acids and amino acids normally present in plasma or by the biological matrix. Determination of both plasma levels and isotopic enrichment of KIC and leucine required a single injection and was easily accomplished in 7-9 min. The good reproducibility of the method, in addition to rapidity and simplicity, permits several samples to be accurately analyzed daily. Therefore, the method is particularly useful for studies on the metabolism of amino acids.