Addition of exogenous γ-glutamyl hydrolase eliminates the need for lengthy incubation of whole blood lysate for quantitation of folate vitamers by high performance liquid chromatography-tandem mass spectrometry

Abstract

BackgroundMeasurement of folate monoglutamates by HPLC-MS/MS in whole blood lysate (WBL) requires lengthy incubation prior to analysis, risking degradation of labile folate vitamers. ObjectiveWe explored whether addition of a commercially available recombinant exogenous γ-glutamyl hydrolase (exoGGH) enzyme reduced the required incubation time of WBL for measurement of folate as monoglutamates. MethodsFor conventional deglutamylation of polyglutamates, WBL was incubated 4 h at 37°C. Alternatively, we added exoGGH to WBL at varying concentrations (1–10 µg/mL) and incubation times (0–90 min). We also investigated modifications to the sample diluent (pH, ascorbic acid vs. sodium ascorbate, and ascorbate concentration). Finally, we tested the effect of the enzyme in different sample types: WBL from frozen whole blood vs. frozen WBL or vs. frozen washed red blood cells (RBC). Samples (n ≤ 15 per experiment) were analyzed by HPLC-MS/MS for 6 folate monoglutamates and 5-methyltetrahydrofolate diglutamate. ResultsOptimal deconjugation of folate polyglutamates was achieved using 1% ascorbic acid and 5 µg enzyme/mL WBL, requiring ≤ 30 min incubation time to achieve complete folate recovery as monoglutamates. This treatment resulted in similar folate concentrations as conventional deglutamylation (4 h at 37°C). The exoGGH enzyme was effective in samples stored frozen as whole blood and as WBL. However, extended thaw time of whole blood resulted in 5-methyltetrahydrofolate loss and unacceptable changes to non-methyl folate concentration. Total folate (with exoGGH) measured in washed RBC was ∼15% lower than RBC folate calculated from WBL concentrations (conventional deglutamylation). ConclusionsUse of exoGGH minimized incubation time and thus may avoid degradative losses of labile folate forms during sample preparation. The lower folate results in washed RBC may be due to inadequate packing of RBC among other unidentified factors. A larger study is required to confirm lack of differences in folate concentrations determined with and without the use of exoGGH.