Identification of biochemical pathways for the metabolism of oxidized low-density lipoprotein derived aldehyde-4-hydroxy trans-2-nonenal in vascular smooth muscle cells

Oxidation of low-density lipoproteins (LDL) generates high concentrations o f unsaturated aldehydes, such as 4-hydroxy trans-2-nonenal (HNE). These ald ehydes are mitogenic to vascular smooth muscle cells and sustain a vascular inflammation. Nevertheless, the processes that mediate and regulate the va scular metabolism of these aldehydes have not been examined. In this commun ication, we report the identification of the major metabolic pathways and p roducts of [H-3]-HNE in rat aortic smooth muscle cells in culture. High-per formance liquid chromatography separation of the radioactivity recovered fr om these cells revealed that a, large (60-65%) proportion of the metabolism was linked to glutathione (GSH). Electrospray mass spectrometry showed tha t glutathionyl-1,4 dihydroxynonene (GS-DHN) was the major metabolite of HNE in these cells. The formation of GS-DHN appears to be due aldose reductase (AR)-catalyzed reduction of glutathionyl 4-hydroxynonanal (GS-HNE), since inhibitors of AR (tolrestat or sorbinil) prevented GS-DHN formation, and in creased the fraction of the glutathione conjugate remaining as GS-HNE. Gas chromatography-chemical ionization mass spectroscopy of the metabolites ide ntified a subsidiary route of HNE metabolism leading to the formation of 4- hydroxynonanoic acid (HNA). Oxidation to HNA accounted for 25-30% of HNE me tabolism. The formation of HNA was inhibited by cyanamide, indicating that the acid is derived from an aldehyde dehydrogenase (ALDH)-catalyzed pathway . The overall rate of HNE metabolism was insensitive to inhibition of AR or ALDH, although inhibition of HNA formation by cyanamide led to a correspon ding increase in the fraction of HNE metabolized by the GSH-linked pathway, indicating that ALDH-catalyzed oxidation competes with glutathione conjuga tion. These metabolic pathways may be the key regulators of the vascular ef fects of HNE and oxidized LDL. (C) 2001 Elsevier Science Ireland Ltd. All r ights reserved.