The Gas That Regulates Pregnancy: A Landmark Map of Hydrogen Sulfide Research Reveals a Half-Empty Picture of the Placenta

Hydrogen sulfide is best known as the gas that gives rotten eggs their notorious smell, yet inside the human body it is one of the most important signalling molecules ever discovered. Nowhere is its influence more consequential than in the placenta, the transient organ that sustains every growing fetus. A new scoping review published in Reproductive Sciences has now mapped, for the first time, exactly where the scientific literature on placental hydrogen sulfide stands, and the results reveal a field that is both promising and strikingly lopsided. Led by Anord Nkuba and Bryony A. McNeill of Deakin University in Australia, the review systematically charted 64 primary studies spanning human placental tissue, trophoblast cell lines and animal pregnancy models, following the rigorous Joanna Briggs Institute methodology and the PRISMA-ScR reporting standard, with a protocol registered on the Open Science Framework.

The biological stakes are considerable. Hydrogen sulfide is produced endogenously by three principal enzymes: cystathionine beta-synthase, known as CBS, cystathionine gamma-lyase, known as CSE or CTH, and 3-mercaptopyruvate sulfurtransferase, known as 3-MST or MPST. But production is only half of the story. Sulfide is continuously cleared by a mitochondrial oxidation pathway that begins with sulfide:quinone oxidoreductase, or SQOR, and continues through persulfide dioxygenase, ETHE1, and thiosulfate sulfurtransferase, TST, with sulfite oxidase, SUOX, finishing the job by oxidising sulfite to sulfate. Net hydrogen sulfide bioavailability therefore reflects a dynamic balance between synthesis and clearance. Assessing only one side of this equation, the authors argue, provides an incomplete and potentially misleading picture of what is actually happening in placental tissue.

That incomplete picture is precisely what the review uncovered. Of the 64 studies included, CBS was investigated in 34 and CSE/CTH in 29, mostly at the level of protein and mRNA expression, while 3-MST/MPST appeared in only seven studies. Most remarkably, not a single included study measured SQOR, ETHE1 or TST in placental tissue at either the mRNA or protein level. SUOX was reported in just two studies, both at the mRNA level. This means that the entire clearance arm of placental sulfide metabolism, the machinery that determines how much sulfide actually accumulates inside cells, remains essentially unexamined. Because reduced expression of a biosynthetic enzyme does not necessarily mean reduced sulfide signalling, conclusions drawn from synthesis alone may underestimate, overestimate or misattribute changes in hydrogen sulfide bioavailability in complicated pregnancies.

The review also exposed a heavy concentration of research interest in a single pregnancy complication. Pre-eclampsia dominated the literature, accounting for 27 of the 78 non-mutually exclusive condition assignments, compared with 12 studies of normal pregnancy and only seven on fetal growth restriction. Gestational diabetes and recurrent pregnancy loss were each represented by a single study, and preterm birth by just three. This clustering makes it difficult to know whether findings reported in pre-eclampsia apply to other placental disorders, and it leaves large areas of reproductive medicine, including premature rupture of membranes and other understudied conditions, almost entirely unmapped with respect to hydrogen sulfide biology.

Measurement itself emerged as a major methodological problem. Only 23 of the 64 studies reported sulfide concentrations, relative assay signals or hydrogen sulfide production, and they used six different analytical approaches. Methylene blue assays accounted for 18 of the 28 identified assay applications, with the remainder using lead acetate detection, fluorescent probes, sulfide-sensitive electrodes, RP-HPLC with monobromobimane derivatisation, or gas chromatography-mass spectrometry. These methods differ fundamentally in what they detect. Acidification-based methylene blue methods can release sulfide from acid-labile sulfur pools, so the resulting measurement may include sulfide that was never present as free hydrogen sulfide. Fluorescent probes generally provide relative signals rather than absolute concentrations, while substrate-supplemented incubations measure generating capacity rather than steady-state sulfide availability. Units, calibration standards and sample-processing procedures varied across studies, making direct quantitative comparison between them effectively impossible.

The consequences of this heterogeneity are visible in the conflicting results the review catalogued. Some studies reported lower placental hydrogen sulfide production in pre-eclampsia, and reduced placental sulfide was also reported after gestational cigarette-smoke exposure and maternal high-fat diet. Yet one study reported approximately two-fold higher placental hydrogen sulfide content in early-onset pre-eclampsia than in preterm controls, and another found no difference in maternal plasma sulfide between normal pregnancy and early- or late-onset pre-eclampsia using GC-MS. Differences in sample compartment, assay method, sulfide pool and reporting units prevented any pooling of these results, leaving the field without a consensus on whether sulfide is genuinely depleted in the diseased placenta.

On the therapeutic side, the review charted 85 experimental intervention applications across 38 studies, and the overall pattern was encouraging. Fast-releasing sulfide donors such as sodium hydrosulfide accounted for 26 applications, with slow-releasing donors, a mitochondria-targeted donor, organosulfur compounds, substrate supplementation, enzyme inhibition and genetic approaches making up the rest. Donor-based studies were predominantly associated with improved placental and trophoblast outcomes, including better angiogenic balance, enhanced trophoblast migration and invasion, improved mitochondrial function and reduced inflammation and cell death, while inhibition or genetic suppression of hydrogen sulfide-producing enzymes generally worsened outcomes. Notably, no clinical study has yet tested a hydrogen sulfide-modulating intervention directly in human pregnancy, so all of this evidence remains preclinical.

Yet the authors caution against reading donor studies as proof of endogenous deficiency. A sulfide measurement was reported in only 23 of the 85 intervention applications, and six of those involved maternal plasma alone. In many studies the intervention was intended to raise hydrogen sulfide availability, but the resulting change in sulfide within the relevant placental compartment was never experimentally confirmed, so interpretation rested on the expected action of the donor rather than a measured exposure. Complicating matters further, effects may occur at multiple biological compartments: in one cited example, sodium hydrosulfide showed direct cardioprotective effects in isolated chicken embryo hearts, independent of any maternal or placental influence. Without concurrent sulfide measurements, the magnitude of biological responses cannot be reliably linked to the sulfide actually delivered.

The review also highlighted a striking temporal blind spot. Human placental studies were heavily weighted toward late gestation, relying on tissue collected at delivery or near-term explants, while first-trimester evidence was limited to a small number of studies. During the first trimester the placenta develops in a relatively low-oxygen, histiotrophic environment before the maternal arterial circulation is fully established, a fundamentally different setting from the mature organ examined after birth. Findings from late-gestation tissue therefore cannot establish when hydrogen sulfide pathway alterations arose, or whether they preceded the development of placental disease, a distinction that matters enormously for identifying causes rather than consequences of pathology.

The authors conclude that placental hydrogen sulfide research has concentrated on biosynthetic enzyme expression and pharmacological modulation while leaving sulfide clearance, early gestation and standardised measurement as major gaps. Future work, they argue, should assess enzyme expression, enzyme activity, hydrogen sulfide production and oxidation capacity within the same preparation, state precisely which sulfide pool is being measured, and extend investigation beyond pre-eclampsia into gestational diabetes, recurrent pregnancy loss and preterm birth. Only then can the field resolve whether altered signalling in complicated pregnancies reflects reduced synthesis, impaired clearance, or a shift between compartments, and only then can the long-hypothesised promise of hydrogen sulfide-based therapies for placental disease be properly evaluated.

Subject of Research: Hydrogen sulfide biosynthesis and oxidation pathways in placental health and pregnancy disease

Article Title: Mapping Hydrogen Sulfide Biosynthesis and Oxidation Pathways in Placental Health and Disease: a Scoping Review

Article References: Mapping Hydrogen Sulfide Biosynthesis and Oxidation Pathways in Placental Health and Disease: a Scoping Review. (n.d.). https://doi.org/10.1007/s43032-026-02236-y

Image Credits: AI Generated

DOI: 10.1007/s43032-026-02236-y

Keywords: hydrogen sulfide, placenta, pre-eclampsia, trophoblast, CBS, CSE, 3-MST, sulfide oxidation, SQOR, fetal growth restriction, scoping review, sulfide measurement