## 4 Discussion
No method, however, was able to capture all lipids perfectly. The method chosen in each acquisition mode was selected because it was best able to separate and analyze fatty acid-like molecules, which are a priority for further investigation in psoriatic disease. The initial conditions for solvent B were then optimized and achieved the narrowest peaks at smaller initial percentages. A concave downward gradient was selected because it reduced retention times while preserving peak shape. After developing and optimizing the final method in positive and negative modes, the methods were applied to serum samples collected from a cohort of patients with PsC, PsA, and healthy controls. The analytical run was evaluated using instrumental quality control standards, deuterated internal standards, and pooled quality control samples. A low variation in the RSD of the peak and retention time of the standards indicated a stable run. We examined significant metabolite differences between PsC and PsA patients to help understand how PsA patients may be able to be better detected among patients with psoriatic disease. Tentatively annotated features classified as fatty acids, carboxylic acids, glycerophospholipids, steroids, and sphingolipids were statistically significant between the PsC and PsA groups using both multivariate and univariate approaches. The short-chain acylcarnitine, valerylcarnitine, was the only compound that had its identity confirmed using MS/MS spectra (See Figures S1-S3). Another acylcarnitine, acetyl-D-carnitine, was found to differentiate PsA and PsC patients; however, this tentative feature was not able to be confirmed using MS/MS. Acylcarnitines are involved in transporting fatty acids into the mitochondria to be broken down to produce energy via beta-oxidation. Previous studies have found changes in the levels of acylcarnitines in patients with diabetes [44; 45], obesity [45], and heart disease [46], which are all comorbidities of psoriatic disease. Interestingly, a recent study by [PERSON] revealed that PsC patients with insulin resistance had a distinct acylcarnitine profile from patients without insulin resistance, reflecting impaired beta-oxidation [47]. It appears acylcarnitines may be a possible connection between dysregulated fatty acid metabolism seen in previous metabolomic studies and the high prevalence of metabolic abnormalities in patients with psoriatic disease. Other tentatively identified features included tryptophly-histidine, isoleucyl-isoleucine, phenylalaninely-tryptophan, and arginyl-histidine. These four compounds are dipeptides, formed as a result of an incomplete breakdown in protein catabolism. No previous studies have been published regarding these compounds. Additionally, we found that levels of both lysophosphatidic acid (18:2(9Z,12Z)/0:0) and sphingomyelin (d18:1/24:0) distinguished PsA and PsC patients. Lysophosphatidic acid (LPA) is an important intercellular lipid mediator controlling wound healing and the inflammatory cascade [48]. A previous study revealed LPA levels at significantly higher levels in psoriatic patients' serum and suggested LPA may mediate the pathogenesis of psoriasis by activating keratinocytes [49]. The role of LPA in PsA has not yet been reported in the literature; however, our study points to a possible implication in the disease. Sphingomyelin (SM) is a sphingolipid and a component of animal cell membranes [50]. A previous metabolomic study detected an increase in serum concentrations of sphingolipids in PsC patients with severe disease activity [51]. It is important to recognize, however, that this study and untargeted metabolomic studies in general are not without their limitations. Identification of features remains a roadblock in untargeted metabolomics. In this study, we employed several databases for feature identification; however, we were only able to tentatively identify approximately 20% of significant features. Out of the 18 tentatively identified features, we were able to confirm the identity of one compound using MS/MS spectra.