H2O2 is a common substrate for the peroxidation reaction of peroxiredoxins and also a well-known cause for their hyperoxidation.22 In our study, serotonin caused excessive production of H2O2 in PAH-hPASMCs, as well as decreased activity of H2O2 reducing enzymes, catalase, and glutathione, in PAH-hPASMCs versus control hPASMCs. Here, CAT is linked to pulmonary arterial hypertension.