Ophthalmate detection in human plasma with LC-MS-MS.

Based on animal experimentations, ophthalmate (OPH) has recently been suggested as a potential plasma biomarker to probe hepatic GSH homeostasis. Up until now, the inability to accurately determine OPH concentrations in human plasma prohibited further studies of OPH metabolism in humans. This study therefore aimed to study the influence of delayed sample preparation on OPH concentrations using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Venous plasma samples from 5 healthy human volunteers were incubated for varying times (5, 30, 60 and 120 min) at temperatures of 4 °C and 37 °C to investigate potential enzymatic degradation. At 37 °C, the decrease in OPH reached significance after 120 min (74.6% (range: 56.2-100.0%; p<0.0001)). At 4 °C, the same trend was observed but did not reach significance. These findings indicate ongoing enzymatic activity, stressing the need for immediate sample deproteinization to obtain reliable plasma concentrations. To investigate the feasibility of the here developed method, baseline arterial plasma values of 21 patients scheduled for partial liver resection were determined to be 0.06±0.03 μmol/l (mean±s.d.). In addition, in pooled samples from 3 patients, an OPH calibration curve was spiked to arterial plasma, arterial whole blood and liver biopsy material, resulting in a linear calibration curve in all cases. Individual measurements of baseline samples revealed that both arterial whole blood and liver biopsy material contained significant levels of endogenous OPH, namely 16.1 (11.8-16.4) μmol/l and 80.0 (191.8-349.2) μmol/kg, respectively. In conclusion, the present LC-MS/MS assay enables the accurate measurement of OPH in human plasma, whole blood and liver biopsies. Freshly prepared samples and immediate deproteinization are mandatory to block enzymatic degradation.