Supplementary key words fatty aldehydes • plasmalogens • myeloperoxidase • reactive chlorinating species • hypochlorous acid • infl ammationNeutrophils are the most abundant of the innate immune system's cellular arsenal that combats invading pathogens ( 1-3 ). Upon activation, neutrophils undergo a respiratory burst leading to the production and release of hydrogen peroxide (H 2 O 2 ) and concomitant release of myeloperoxidase (MPO) from the azurophilic granules. MPO amplifi es the oxidant response of neutrophils by converting H 2 O 2 to hypochlorous acid ( 4, 5 ). Hypochlorous acid is in equilibrium with chlorine gas ( 4 ). Collectively, hypochlorous acid, its conjugate anion, hypochlorite, and chlorine gas comprise the reactive chlorinating species (RCS) produced by activated neutrophils. In addition to targeting invading organisms, these RCS also attack host macromolecules, including proteins, nucleic acids, and lipids ( 1, 6-13 ). RCS attack of the vinyl ether bond of plasmalogens results in the release of ␣ -chlorofatty aldehyde and unsaturated lysophosphatidylcholine production ( 14 ). One of the ␣ -chlorofatty aldehyde molecular species, 2-chlorohexadecanal (2-ClHDA), is produced by activated neutrophils and monocytes and has been shown to accumulate in infarcted myocardium ( 8 )

Although acute myocardial infarction (MI) is consistently among the top causes of death in the United States, the spatial distribution of lipids and metabolites following MI remains to be elucidated. This work presents the investigation of an in vivo rat model of MI using mass spectrometric imaging (MSI) and multivariate data analysis. MSI was conducted on cardiac tissue following a 24-hour left anterior descending coronary artery ligation in order to analyze multiple compound classes. First, the spatial distribution of a small metabolite, creatine, was used to identify areas of infarcted myocardium. Second, multivariate data analysis and tandem mass spectrometry were used to identify phospholipid (PL) markers of MI. A number of lysophospholipids demonstrated increased ion signal in areas of infarction. In contrast, select intact PLs demonstrated decreased ion signal in the area of infarction. The complementary nature of these two lipid classes suggest increased activity of phospholipase A2, an enzyme that has been implicated in coronary heart disease and inflammation.

Numerous studies have suggested relationships between myeloperoxidase (MPO), inflammation, and atherosclerosis. MPO-derived reactive chlorinating species attack membrane plasmalogens releasing ␣-chloro fatty aldehydes including 2-chlorohexadecanal (2-ClHDA), which have been found to accumulate in activated neutrophils, activated monocytes, infarcted myocardium and human atheromas. The present study employed synthetically prepared 2-Cl-

There is an urgent need to identify which COVID-19 patients will develop life-threatening illness so that medical resources can be optimally allocated and rapid treatment can be administered early in the disease course, when clinical management is most effective. To aid in the prognostic classification of disease severity, we perform untargeted metabolomics on plasma from 339 patients, with samples collected at six longitudinal time points. Using the temporal metabolic profiles and machine learning, we build a predictive model of disease severity. We discover that a panel of metabolites measured at the time of study entry successfully determine disease severity. Through analysis of longitudinal samples, we confirm that the majority of these markers are directly related to disease progression and that their levels return to baseline upon disease recovery. Finally, we validate that these metabolites are also altered in a hamster model of COVID-19.

Myeloperoxidase-derived HOCl targets tissue-and lipoprotein-associated plasmalogens to generate ␣-chlorinated fatty aldehydes, including 2-chlorohexadecanal. Under physiological conditions, 2-chlorohexadecanal is oxidized to 2-chlorohexadecanoic acid (2-ClHA). This study demonstrates the catabolism of 2-ClHA by -oxidation and subsequent ␤-oxidation from the -end. Mass spectrometric analyses revealed that 2-ClHA is -oxidized in the presence of liver microsomes with initial -hydroxylation of 2-ClHA. Subsequent oxidation steps were examined in a human hepatocellular cell line (HepG2). Three different ␣-chlorinated dicarboxylic acids, 2-chlorohexadecane-(1,16)-dioic acid, 2-chlorotetradecane-(1,14)-dioic acid, and 2-chloroadipic acid (2-ClAdA), were identified. Levels of 2-chlorohexadecane-(1,16)-dioic acid, 2-chlorotetradecane-(1,14)-dioic acid, and 2-ClAdA produced by HepG2 cells were dependent on the concentration of 2-ClHA and the incubation time. Synthetic stable isotope-labeled 2-ClHA was used to demonstrate a precursorproduct relationship between 2-ClHA and the ␣-chlorinated dicarboxylic acids. We also report the identification of endogenous 2-ClAdA in human and rat urine and elevations in stable isotopelabeled urinary 2-ClAdA in rats subjected to intraperitoneal administration of stable isotope-labeled 2-ClHA. Furthermore, urinary 2-ClAdA and plasma 2-ClHA levels are increased in LPStreated rats. Taken together, these data show that 2-ClHA is -oxidized to generate ␣-chlorinated dicarboxylic acids, which include ␣-chloroadipic acid that is excreted in the urine.