Presence of ceramidase activity in electronegative LDL

Electronegative LDL (LDL(-)) is a minor modified fraction of human plasma LDL with several atherogenic properties. Among them, LDL(-) has increased content of bioactive lipid mediators, such as lysophosphatidylcholine (LPC), non-esterified fatty acids (NEFA), ceramide (Cer), and sphingosine (Sph), which are related to the presence in LDL(-) of some phospholipolytic activities, including platelet-activating factor acetylhydrolase (PAF-AH), phospholipase C (PLC) and sphingomyelinase (SMase). However, the activity of these enzymes does not explain the increased content of Sph, which should derive from Cer degradation. In the present study we analyzed the putative presence of a ceramidase (CDase) activity that could explain the increased content of Sph. Thin layer chromatography (TLC) and lipidomic analysis showed that Cer, Sph and NEFA spon-taneously increased in LDL(-) incubated alone at 37ºC, in contrast with native LDL(+). An inhibitor of neutral CDase prevented the formation of Sph and, in contrast, increased the content of Cer in LDL(-). In addition, a fluorescently-labelled Cer (NBD-Cer) was efficiently degraded by LDL(-) to form Sph and NEFA. These observations point to the existence of a CDase activity associated to LDL(-). However, neither proteomic analysis nor western blot detected the presence of a known CDase enzyme. Further studies are warranted to define the origin of the CDase activity detected in LDL(-).