Liquid chromatographic assessment of total and protein-bound homocysteine in human plasma

Homocysteine present in human blood plasma is derivatized with thiol selective ultraviolet labelling reagent, 2-chloro-1-methylpyridinium iodide, and separated from other plasma thiol derivatives by high-performance liquid chromatography (HPLC) with detection at 312 nm. The separation is carried out isocratically on LiChrospher RP-18 column using mobile phase consisting of pH 2.5 0.04 M trichloroacetic acid buffer and methanol in the ratio 9:1 (v/v) pumped at 0.5 ml min−1 at 40°C. The homocysteine S-pyridinium derivative elutes at 6.5 min. To determine total and protein-bound homocysteine it is necessary to cleave disulphide bounds by the use of tri-n-butylphosphine in order to form free sulfhydryl group. The method provides quantitative information on total and protein-bound homocysteine based on assays with derivatization after reduction of whole plasma, and derivatization after reduction of acid precipitated proteins. The calibration graph is linear over the concentration range covering most experimental and clinical cases. The assay has a low pmol sensitivity and is reproducible; intra- and inter-day, relative standard deviation range from 1.79 to 5.09% and from 2.80 to 5.60%, respectively. The method is applied to the determination of total and protein-bound homocysteine in the plasma of healthy individuals.