Analysis of 6-hydroxy-2-aminocaproic acid (HACA) as a specific marker of protein oxidation: the use of N(O,S)-ethoxycarbonyl trifluoroethyl ester derivatives and gas chromatography/mass spectrometry.

An alteration of low density lipoprotein (LDL) apolipoprotein (apo) B-100 structure by direct oxidative modification is an important mechanism involved in atherogenesis. There is difficulty in quantifying this type of modification because a lack of specific assays. The use of N(O,S)-ethoxycarbonyl trifluoroethyl amino acid esters for a rapid and sensitive determination of 6-hydroxy-2-aminocaproic acid (HACA), a highly specific marker of metal catalyzed protein oxidation, by using standard gas chromatography/electron impact mass spectrometry, is discussed. The derivatives are formed by the unlabored reaction of amino acids with ethylchloroformate plus trifluoroethanol plus pyridine. Femtomole levels of HACA can be reproducible measured in different LDL preparations subjected to oxidative damage in the presence of iron or copper. HACA determination compares well with the measurement of carbonyl groups that are generally accepted as a nonspecific index of protein oxidation. Thus, the method could prove to be a sensitive assay for studying specific apoB-100 modification.