# Untargeted Urinary Volatilomics Reveals Hexadecanal as a Potential Biomarker for Preeclampsia
## 3 Discussion  
The identification of hexadecanal as a statistically significant compound associated with PE underscores its potential as a biomarker for this complex pregnancy disorder [19; 20]. This finding is particularly important as the role of oxidative stress and lipid peroxidation in the pathophysiology of PE is well established. Hexadecanal, a product of lipid peroxidation, may serve as a key indicator of oxidation-induced placental dysfunction, which is a hallmark of PE [11]. Previous studies have emphasized the importance of lipid-derived aldehydes, such as 4-hydroxy-2-nonenal (HNE) and malondialdehydedehyde (MDA), in the context of oxidative  
Figure 3: Metabolomic set enrichment analysis identifying key pathways and (**a**) cellular locations in preeclampsia (**b**). stress and carbonyl stress, both of which are closely associated with placental aging and damage in PE. While hexadecanal itself has been less extensively studied compared to these aldehydes, its formation during sphingolipid metabolism makes it a promising candidate for further investigation. The role of hexadecanal in oxidative damage is supported by its ability to form protein adducts and alter membrane stability, similar to other reactive aldehydes [12; 21; 22]. Hexadecanal is formed as a metabolic by-product of the degradation of sphingosine-1-phosphate (S1P) by S1P lyase, a critical enzyme in sphingolipid metabolism [23]. Sphingolipids are important bioactive molecules that regulate various cellular processes, including apoptosis, proliferation and inflammatory processes, which are of great importance for the development of PE [24; 25]. The disruption of sphingolipid metabolism in PE, particularly through the formation of reactive aldehydes such as hexadecanal, may contribute to endothelial dysfunction and inflammatory responses, both of which are key features of the disease [19]. In addition, oxidative stress induced by poor placentation and repeated ischemia-reperfusion injury is a known mechanism for the progression of PE [5; 20; 26]. The increased production of reactive oxygen species (ROS) leads to the peroxidation of polyunsaturated fatty acids, which in turn produces a number of reactive aldehydes, including hexadecanal. As [PERSON] et al. have shown, oxidative stress plays a central role in placental dysfunction, particularly through the induction of apoptosis and inflammation [4]. The involvement of hexadecanal in lipid peroxidation places it at the interface of these pathological processes, suggesting that it may be an important biomarker for the detection of early signs of oxidative damage in the placenta. The two-stage model of PE, as outlined by [PERSON] et al., underpins the relevance of hexadecanal as a potential biomarker [1]. Stage 1 is characterized by placental dysfunction due to defective trophoblast invasion and oxidative stress, while stage 2 is characterized by maternal symptoms such as hypertension and proteinuria. Hexadecanal may play a key role in stage 1 by reflecting early oxidative damage and lipid peroxidation in the placenta, which could make it a valuable biomarker for early diagnosis or risk stratification in pregnant women. In addition, the enrichment analysis revealed that sulfate/sulfite metabolism plays an important role in PE. The cellular mechanisms responsible for maintaining redox homeostasis and antioxidant defense systems are highly dependent on the regulated reactivity of sulfur atoms, especially those derived from the amino acids cysteine and methionine. Under oxidative stress, cells strive to generate reducing agents to counteract the damaging effects of ROS. Sulfur-containing molecules play a crucial role in this process as they are involved in antioxidant metabolic pathways. As [PERSON] et al. pointed out, the reactivity of sulfur in these molecules is essential for neutralizing oxidative damage [27]. The fact that oxidative stress has been identified in several studies as the main cause of PE reinforces the need for biomarkers that reflect this pathological process. [27; 28].