In activated macrophages, an unknown dehdyrogenase abstracts hydrogen from 13-hydroxy-docosahexaenoic acid to form the electrophilic oxo-derivative (EFOX) 13-oxo-DHA (Groeger et al. 2010). Potential candidates are cellular dehydrogenases such as 3α-hydroxysteroid dehydrogenases (3α-HSDs), which can convert 13- and 17-HDHA into their corresponding oxo-derivatives in the presence of NAD(P)+ in vitro (supplementary data, Groeger et al. 2010) or 5- and 15-hydroxyeicosanoid dehydrogenases (5- and 15-HEDH), which convert LOX products to 5-and 15-oxoETE (Erlemann et al. 2007). EFOXs can act as peroxisome proliferator-activated receptor-γ (PPARγ) agonists and inhibit pro-inflammatory cytokine and nitric oxide production, confirming their anti-inflammatory actions (Groeger et al. 2010).