Bioavailable Sulforaphane Quantitation in Plasma by LC-MS/MS Is Enhanced by Blocking Thiols

Quantifying sulforaphane (SFN) and its thiol metabolites in biological samples using liquid chromatography-tandem mass spectrometry is complicated by SFN's electrophilic nature and the facile dissociation of SFN-thiol conjugates. SFN can be lost during sample preparation due to conjugation with protein thiols, which are precipitated and discarded. We observe that only 32 ± 3% of SFN is recovered 2 h after spiking into fetal bovine serum. The SFN-glutathione conjugate prepared at 10 mM in 0.1% formic acid in water (pH 3) dissociated by approximately 95% to free SFN, highlighting the difficulty in preparing thiol metabolite standards. We used the alkylating agent iodoacetamide (IAA) to both release SFN from protein thiols and force the dissociation of SFN metabolites. This thiol-blocking method increased SFN percent recovery from serum from 32 to 94 ± 5%, with a 4.7 nM method limit of quantitation. Applying the method to clinical samples, SFN concentrations were on average 6 times greater than when IAA was omitted. The IAA thiol-blocking method streamlines the analysis of bioavailable SFN in plasma samples.