Rapid determination of lysine in biological samples by isocratic liquid chromatography

A simple, rapid, and sensitive method was developed for detection and quantitation of lysine (Lys) in various biological samples by isocratic liquid chromatography (LC). Samples containing Lys and other amino acids were derivatized with 9-fluorenylmethyl chloroformate (FMOC-CI). The mobile phase used for isocratic elution was 50 mmol/L sodium acetate buffer (pH 4.20)-acetonitrile (43 + 57, v/v). Lys was detected with a UV detector at 265 nm. The derivatized Lys eluted from a LiChrospher 100 RP-18 (150 x 4.0 mm id) column at a retention time of 5.6 min. The limit of detection was 0.73 mumol/L (signal-to-noise [S/N] ratio, 3:1), and the limit of quantitation was 2.37 mumol/L (S/N ratio, 10:1). Lys recoveries from fortified biological samples were > 97.5%. Average Lys contents found in rumen fluid samples collected before the morning feeding and at 2.0, 4.0, and 6.0 h after feeding were 4.26, 3.34, 3.58, and 3.82 mumol/L, respectively. The hydrolysate of a sample of mixed rumen microorganisms collected before the morning feeding was determined to contain 1.372 mumol/mg microbial nitrogen in the form of Lys. The Lys concentrations of human plasma, goat plasma, human urine, and goat urine were 140.0, 102.0, 58.0, and 32.0 mumol/L, respectively.