BACKGROUND: Hydrogen sulfide (H2S) is established as the third gaseous signaling molecule and is known to be overproduced in down syndrome (DS) due to the extra copy of the CBS gene on chromosome 21, which has been suggested to contribute to the clinical manifestation of this condition. We recently discovered trimethylsulfonium (TMS) in human urine and highlighted its potential as a selective methylation metabolite of endogenously produced H2S, but the clinical utility of this novel metabolite has not been previously investigated. We hypothesize that the elevation of H2S production in DS would be reflected by an elevation in the methylation product TMS.
METHODS: To test this hypothesis, a case-control study was performed and the urinary levels of TMS were found to be higher in the DS group (geo. mean 4.5 nM, 95 % CI 2.4-3.9) than in the control (N) group (3.1 nM, 3.5-6.0), p-value 0.01, whereas the commonly used biomarker of hydrogen sulfide, thiosulfate, failed to reflect this alteration in H2S production (15 µM (N) vs. 13 µM (DS), p-value 0.24.
RESULTS: The observed association is in line with the proposed hypothesis and provides first clinical evidence of the utility of TMS as a novel and more sensitive biomarker for the endogenous production of the third gaseous signaling molecule than the conventionally used biomarker thiosulfate, which is heavily dependent on bacterial hydrogen sulfide production.
CONCLUSIONS: This work shows that TMS must be explored in clinical conditions where altered metabolism of hydrogen sulfide is implicated.

Hydrogen sulfide is a toxic gas but also recognized as an endogenously produced metabolite in humans playing key roles. We previously identified trimethylsulfonium, which can be a methylation product of hydrogen sulfide but the stability in the production of trimethylsulfonium has not been investigated. In the present work, the intra- and inter-individual variability in the excretion of trimethylsulfonium over 2 months in a group of healthy volunteers was investigated. Urinary levels of trimethylsulfonium (mean: 56 nM, 95% CI: 48-68 nM) were > 100-fold lower than the conventional hydrogen sulfide biomarker thiosulfate (13 µM, 12-15 µM) and the precursor for endogenous hydrogen sulfide production cystine (47 µM, 44-50 µM). There was no correlation between urinary trimethylsulfonium and thiosulfate. Higher intra-individual variability in the excretion of trimethylsulfonium (generally 2-8 fold) than that for cystine (generally 2-3 fold) was found. Trimethylsulfonium displayed significant inter-individual variability with two concentration clusters at 117 nM (97-141) and 27 nM (22-34). In conclusion, the observed inter- and intra-individual variability must be considered when using urinary trimethylsulfonium as a biomarker.

BACKGROUND: Hydrogen sulfide (H2S), an endogenous gas, can also be generated from organics putrefaction. It is difficult for suspected cases of H2S poisoning to determine whether H2S in specimens is ingested by antemortem poisoning or generated from organics putrefaction. The aim of this study was to find the biomarkers of acute H2S poisoning via comparing the concentrations of H2S and its metabolites over time in specimens.
METHODS: The H2S-spiked blood and blank blood group were established. The decomposition kinetics and the postmortem production of H2S were studied due to organics putrefaction. The specimens were placed under 4 conditions of 37, 20, 4 and - 20 ℃. The content of H2S in specimens was quantified by gas chromatography-mass spectrometry, and the contents of its metabolites (thiosulfate and trimethylsulfonium) were measured by liquid chromatography-mass spectrometry, and the variation of its concentration was evaluated.
RESULTS: In H2S-spiked blood, H2S decreased sharply in the initial stage at 37, 20 and 4 °C, and increased first and then decreased later; but it was relatively stable at - 20 °C. In spiked blood, thiosulfate was 9-fold higher than endogenous concentrations, which increased at first and then decreased during storage. Except for thiosulfate at 37 °C, H2S and thiosulfate in blank blood both increased at first and then decreased in storage; but trimethylsulfonium (TMS) gradually decreased over time in both groups.
CONCLUSIONS: Thiosulfate is a reliable biomarker of acute H2S poisoning at - 20℃ within 7 days. But H2S, because of instability and volatility, is not an ideal poisoning marker. TMS is not an appropriate biomarker due to extremely low concentration in blood.