Selective labelling of apolipoproteins A–I and C–I at methionine residues by (3H) methyl exchange

Abstract
Apolipoproteins C–I and A–I were radioactively labelled with tritium by (3H)-methyl exchange. The methionine residues were first methylated with (3H)-methyl iodide at pH4 and the reaction products were purified by gel filtration and cation exchange chromatography. The products were then demethylated with 2-mercaptoethanol (6M) at pH 8.6 to regenerate the apolipoproteins in an unmodified but tritiated form. The specific radioactivity for apolipoprotein C–I and A–I was 3.5 times 106 and 1.5 times 107 dpm/pmol respectively. The properties of (3H)-apolipoprotein C–I were examined by reversed phase HPLC and by incorporation into very low density lipoproteins (VLDL).