Approach to the quantitation of alkylated amino acids in haemoglobin by gas chromatography mass spectrometry

As a result of the increasing concern over the environmental exposure of humans to carcinogenic alkylating agents, assay methods are being developed in order to quantitate the extent of in vivo alkylating reactions. Such methods will be of value also in connection with the use of alkylating agents as anti-cancer drugs. The reactions carried out by these compounds include the alkylation of cysteine in proteins, and a procedure has now been developed for the estimation of S-methylcysteine in haemoglobin following exposure of rats to the carcinogen methyl methanesulphonate. Haemoglobin is particularly appropriate for the analysis of alkylating agent reactions as this protein is readily available from blood and has a long in vivo half-life. The analytical method involved isolation of globin from blood, hydrolysis in 6 M hydrochloric acid, partial purification of S-methylcysteine by chromatography on Dowex 50 ion exchange resin, and subsequent analysis by gas chromatography mass spectrometry. The amino acid mixtures were derivatized by n-butyl ester formation and N-heptafluorobutyrylation, and were chromatographed on a capillary column coated with OV-101 or Chirasil-Val using a solid injection system. For the mass spectrometric determination [2H3]-S-methylcysteine was employed as an internal standard. Monitoring of the [MH]+ ion of derivatized S-methylcysteine in chemical ionization (isobutane) allowed the detection of 10 pg of the compound with a signal-to-noise ratio of 20:1.