Surface behaviour of sphingomyelines with very long chains (C28-C32) PUFAs and their interaction with premixed or enzymatically generated ceramides

Abstract

Molecular species of sphingomyelin (SM) with nonhydroxy (n) and 2-hydroxy (h) very long-chain polyunsaturated fatty acids (n- and h- 28:4, 30:5 and 32:5) abound in rat spermatogenic cells and spermatozoa. These SMs are exclusively located on the sperm head, where they are converted into the corresponding Cer by sphingomyelinase after completion of the acrosomal reaction. The aim of this study was to gain some insight into the surface properties of this unique type of sphingolipids and how such properties change by the SM  Cer conversion. SM and Cer species were isolated by HPLC [1] and organized in Langmuir films, alone and in SM/Cer mixtures. Compression isotherms for all six SMs under study were compatible with a liquid-expanded state and showed large mean molecular areas. Only the longest SMs (n- and h-32:5 SM) underwent phase transition upon cooling. h-28:4 SM show typical general properties whereas h-28:4 Cer exhibited an easily compressible liquid condensed phase [2] which may results from its higher conformational freedom in such phase. In premixed and enzymatically generated h-28:4 SM / h-28:4 Cer films, Cer-rich domains with a high incorporation of SM were formed. In conclusion, while the SMs from sperm behave in a regular way, the corresponding Cers show atypical properties that may be relevant for the structural rearrangement occurring in the acrosome-reacted sperm membrane.