Abstract
In rat germ cells and spermatozoa, sphingomyelin (SM) contains molecular species with nonhydroxy (n) and 2-hydroxy (h) very-long-chain polyunsaturated fatty acids (V), the most abundant being SMs with (n- and h-) 28:4n-6, 30:5n-6, and 32:5n-6 as acyl chains. The aim of this study was to gain information about their thermotropic behavior and interactions with other lipids. After isolation from rat testis, multilamellar and giant unilamellar vesicles from these SMs were examined using fluorescent probes. Only n-32:5 SM and h-32:5 SM displayed a gel-liquid transition temperature (Tt ∼ 21-22°C), the rest remaining in the liquid state in the 5°C-45°C range. The degree of order was larger in bilayers of any of the h-V SMs than in those of their chain-matched n-V SMs. Both, but n-V SM relatively more than h-V SM, decreased the Tt of dimyristoylphosphatidylcholine as their proportion increased in binary phosphatidylcholine:SM liposomes. In contrast to the established ability of 16:0 SM to form lateral cholesterol/SM-rich ordered domains in ternary dioleoylphosphatidylcholine:cholesterol:SM bilayers, neither n-V SM nor h-V SM showed a tendency to do so. Thus, these SMs are in the fluid state and are not involved in this type of domains in spermatozoa at physiological temperatures. However, this state could be altered at the very low temperatures at which these gametes are usually preserved.
Highlights
In rat germ cells and spermatozoa, sphingomyelin (SM) contains molecular species with nonhydroxy (n) and 2-hydroxy (h) very-long-chain polyunsaturated fatty acids (V), the most abundant being SMs with (n- and h-) 28:4n-6, 30:5n-6, and 32:5n-6 as acyl chains
Two major fatty acids were representative of each fraction: n-28:4 SM followed by n-30:5 SM in the SM species with nonhydroxy very-long-chain PUFA (VLCPUFA) (n-V) SM group, h-30:5 SM followed by h-28:4 SM in the h-V SM group, and 16:0 SM followed by 18:0 SM in the S+M SM group
In order to obtain information about acyl chain order in the bilayer and Tt values for the SMs in these subfractions, the thermal behavior of liposomes prepared with them was assessed by determining the Generalized polarization (GP) of Laurdan and the anisotropy of DPH as a function of temperature (Fig. 1)
Summary
In rat germ cells and spermatozoa, sphingomyelin (SM) contains molecular species with nonhydroxy (n) and 2-hydroxy (h) very-long-chain polyunsaturated fatty acids (V), the most abundant being SMs with (n- and h-) 28:4n-6, 30:5n-6, and 32:5n-6 as acyl chains. In contrast to the established ability of 16:0 SM to form lateral cholesterol/SM-rich ordered domains in ternary dioleoylphosphatidylcholine: cholesterol:SM bilayers, neither n-V SM nor h-V SM showed a tendency to do so These SMs are in the fluid state and are not involved in this type of domains in spermatozoa at physiological temperatures. The testis and spermatozoa of various mammals including man contain SM species with an infrequent series of very-long-chain polyunsaturated fatty acids (VLCPUFAs) [2]. Whereas SMs from bovine and ovine spermatozoa are exceedingly rich in n-3 VLCPUFA with up to 34 carbon atoms, those from rodent germ and sperm cells mostly contain tetraenoic and pentaenoic fatty acids of the n-6 series with 28 to 32 carbon atoms, representative components being 28:4n-6, 30:5n-6, and 32:5n-6 [2, 3]
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