Thermal behavior of sphingomyelins with very long chain polyunsatturated fatty acids (VLCPUFA)

Abstract

Rat germ cells and spermatozoa contain species of SM and Cer with 28:4n-6 and 30:5n-6, followed by 32:5n-6, as main nonhydroxy (n) and 2-hydroxy (2-OH) VLCPUFA. The aim of this study is to gain some insight into the properties of these unique sphingolipids in membranes. Total SM from rat testis was partially separated by TLC into two main fractions: SM1, containing n-VLCPUFA, and SM2, containing similar proportions of 16:0 and 18:0 as well as 2- OH VLCPUFA. Each was then separated into molecular species by HPLC. Liposomes formed with these fractions or species were prepared and the generalized polarization of the fluorescent probe Laurdan as a function of temperature was followed. The effects of adding various % of these SM upon the transition temperature (Tt) of dipalmitoyl and dimiristoyl phosphatidylcholine (DPPC, DMPC) were also determined. The Tt of SM1 was significantly lower than that of SM2. In turn, the Tt of SM2 was lower than that of 16:0-SM, 18:0-SM, or a 50:50 mixture of 16:0-SM/18:0-SM, indicating that the presence of SM with 2-OH VLCPUFA in SM2 accounts for this decrease. The Tt of DPPC and DMPC decreased and increased with SM2, respectively, and decreased when SM1 or species with n-VLCPUFA were added. The potential increase in Tt that could have resulted from their unusually long carbon chains is thus overriden by the effect the double bonds have in decreasing it.