A radioisotopic-exchange method for quantitation of short-chain (acid-soluble) acylcarnitines

A reliable and sensitive method for the quantitation of picomole amounts of acetylcarnitine, propionylcarnitine, aliphatic 4-carbon, and 5-carbon acylcarnitines has been developed. The procedure requires the measurement of the amounts of carnitine and acid-soluble carnitine and then the enzymatic exchange of 3H- or 14C-labeled L-carnitine into the acylcarnitine pool using commercial carnitine acetyltransferase, essentially free of acyl-CoA hydrolase activity. After isotopic equilibrium is obtained, the radioactive acylcarnitines are separated using either HPLC or thin-layer chromatography. Procedures for both are described. After separation, the amounts of radioactivity in the acylcarnitines are determined and the amount of individual acylcarnitines can be calculated from the specific activity of the initial total carnitine pool or from the ratio of dpm in the acylcarnitine fraction/dpm in free carnitine X (nanomoles L-carnitine) in the sample. The method has several advantages over current procedures, including rapidity, use of small sample sizes, simplicity, and reliability.