Fig. 4. Singlet oxygen generates ceramide in sphingomyelin-containing liposomes. (A) HPTLC of NDPO₂-induced ceramide formation in liposomes. Liposomes were treated with 0 (lanes 1 and 2), 5 (lanes 3 and 4) and 10 mM NDPO₂ (lanes 5 and 6), and chloroform/methanol lipid extracts prepared immediately thereafter and analysed. Iodine vapour was used to stain ceramides. (B) Analysis of NDPO₂-induced ceramide formation in liposomes by HPLC/mass spectroscopy. Liposomes were treated for 30 min with increasing doses (0–10 mM) of NDPO₂ and lipid extracts prepared immediately thereafter. (C) Analysis of ceramide in long-term cultured human keratinocytes by HPLC/mass spectroscopy. Lipid extracts were prepared 30 min after stimulation of cells with 30 J/cm² UVA or 1 mM NDPO₂. C24, white bars; C18, grey bars; C16, black bars.