Gangliosides in membranes from Torpedo electric organ

The electric organ membrane has been the subject of many studies, due principally to its rich content of nicotinic acetylcholine receptor (AChR). Knowing its lipid composition is clearly important. Although its major membrane lipids have been characterized, its ganglioside composition has not been as well-described. In this study, gangliosides were characterized in membranes prepared from two species of electric organ, Torpedo californica and T. nobiliana. The ganglioside content of total electric organ membranes and AChR-enriched membranes was similar in both species, accounting for from 0.9 to 1.5% of membrane lipid by weight. However, the AChR-enriched membranes contained significantly less ganglioside relative to AChR than did the total membrane preparations. Five major gangliosides were purified from T. californica and identified as II3NeuNAc-GgOse3 (GM2); II3(NeuNAc)2-GgOse3 (GD2), IV3NeuNAc, II3NeuNAc-GgOse4 (GD1a), IV3NeuNAc, II3(NeuNAc)2-GgOse4 (GT1b), and IV3(NeuNAc)2,II3(NeuNAc)2-GgOse4 (GQ1b). Together these five gangliosides accounted for over 90% of the total ganglioside present in the two membrane preparations from both species. The most abundant ganglioside by far was GM2, which accounted for about one-half of the ganglioside content, followed by GD2. Determination of the N-fatty acid composition was performed on gangliosides purified from T. nobiliana. The lower-order gangliosides, GM2, GD2, and GD1a, contained substantial amounts of very long chain fatty acids (> 20 carbons), including alpha-hydroxynervonic acid (15-21% of total). In contrast, unsubstituted, 14-18 carbon chains accounted for about 90% of the fatty acids on the two higher-order gangliosides, GT1b and GQ1b.