Lipid biology of plasmalogen-derived halolipids: Signature molecules of myeloperoxidase and eosinophil peroxidase activity

Myeloperoxidase and eosinophil peroxidase exert their antimicrobial functions through the oxidative actions of their hypohalous acid products. Plasmalogen phospholipids are particularly susceptible to oxidation of their vinyl ether functional group by hypohalous acids. This produces a family of halogenated lipid products with pro-inflammatory roles and potential biomarker utility. The initial product of plasmalogen oxidation by HOCl is 2-chlorofatty aldehyde, which has been shown to play a key role at the blood-endothelium interface. In vitro and in vivo studies indicate increased endothelial barrier permeability, neutrophil chemotaxis, neutrophil and platelet adherence to endothelium, and promotion of erythrocyte lysis as some of its effects. These effects may be due to protein modification by 2-chlorofatty aldehyde. 2-Chlorofatty aldehyde is metabolized by host dehydrogenases to 2-chlorofatty acid. While it is less chemically reactive, 2-chlorofatty acid has partial overlap of pro-inflammatory effects with 2-chlorofatty aldehyde and unique actions such as induction of neutrophil extracellular trap formation. The stability of 2-chlorofatty acid in plasma also makes it well-suited as a biomarker of HOCl generation, and its plasma levels may be predictive of disease outcomes. 2-Bromofatty aldehydes and acids are produced analogously from HOBr reaction with plasmalogens. Their functions have yet to be well-elucidated, though similarities with chlorolipids have been observed, and increased reactivity with proteins is expected through enhanced electrophilicity of the alpha carbon. Altogether, these halogenated lipids represent underexplored mediators of diseases involving excess hypohalous acid production.