Abstract
Very long-chain (C24 to C34) polyunsaturated fatty acids (VLCPUFA) are important constituents of sphingomyelin (SM) and ceramide (Cer) in testicular germ cells. In the present paper we focused on the SM and Cer and their fatty acids in spermatozoa and their main regions, heads and tails. In bull and ram spermatozoa, SM was the third most abundant phospholipid and VLCPUFA were the major acyl groups ( approximately 70%) of SM and Cer. In rat epididymal spermatozoa the SM/Cer ratio was low in the absence of and could be maintained high in the presence of the cation chelator EDTA, added to the medium used for sperm isolation. This fact points to the occurrence of an active divalent cation-dependent sphingomyelinase. Bull and rat sperm had an uneven head-tail distribution of phospholipid, with virtually all the VLCPUFA-rich SM located at the head, the lower SM content in the rat being determined by the lower sperm head/tail size ratio. Most of the SM from bull sperm heads was readily solubilized with 1% Triton X-100 at 4 degrees C. The detergent-soluble SM fraction was richer in VLCPUFA than the nonsoluble fraction and richer in saturated fatty acids. Cer was produced at the expense of SM, thus decreasing severalfold the SM/Cer ratio in rat spermatozoa incubated for 2 h in presence of the sperm-capacitating agents, calcium, bicarbonate, and albumin. The generation of Cer from SM in the sperm head surface may be an early step among the biochemical and biophysical changes known to take place in the spermatozoon in the physiological events preceding fertilization.
Highlights
We unexpectedly found that VLCPUFA were major fatty acid components of both sphingolipids in fresh spermatozoa from adult, fertile bulls (Ն70%) (Table 1)
There was a net predominance of n-3 hexaenoic VLCPUFA, the three major fatty acids of SM and Cer being 32:6n-3, 30:6n-3, and 34:6n-3 in agreement with the GC-MS tracings shown in Ref. 1
Because Cer had a high percentage of VLCPUFA, and the SM/Cer ratio was in the order of 2.5 for the spermatozoa whose fatty acids are shown in Table 1, the sum of SM ϩ Cer contributed a total of ϳ12–15% of VLCPUFA to the total fatty acid weight of whole bull and ram spermatozoa, a far from negligible amount
Summary
In the testis of various mammals, we focused on the fatty acids of the ceramide (Cer), a lipid molecule with which SM bears a close precursor-product relationship, showing that SM and Cer species containing VLCPUFA are a specific feature of cells of the spermatogenic lineage [3] Because these testicular cells are predecessors of spermatozoa, the question arose as to the quantitative importance of these molecules in spermatozoa, where they could play a role in sperm functions related to fertilization. As a prerequisite to fertilization, mammalian spermatozoa must undergo the time-dependent priming process known as capacitation, as a result of which sperm change their motility patterns and migrate to the site of fertilization, in the process becoming competent to undergo the acrosomal reaction This reaction entails a specific fusion event in the spermatozoal head whereby the plasma membrane of the apical region and the outer membrane of the underlying acrosome fuse at many phospholipid; EGP, ethanolamineglycerophospholipids; DPG, diphosphatidylglycerol; ros, rod outer segment(s); PC, phosphatidylcholine. The molecular mechanisms underlying these processes have been studied in great molecular detail over the years (see Refs. 5– 8 for reviews)
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