Human seminal plasma displays significant phospholipid transfer activity due to the presence of active phospholipid transfer protein

The lipid composition of germ cell membranes is considerably modified during spermatogenesis, sperm maturation and capacitation. Some of these modifications are caused by exchanges between soluble lipid donors or acceptors and cell membranes. The aim of this study was to assess whether significant lipid transfers between lipoprotein structures are detectable in human seminal plasma. Phospholipid and cholesteryl ester (CE) transfer activities were measured by specific fluorescence and isotopic assays. Seminal plasma samples did not display significant CE transfer. Substantial levels of phospholipid transfer activity were detected in all samples studied, levels were similar to25% of the phospholipid transfer activity measured in human blood plasma. Concordantly, CE transfer protein was not detected in seminal plasma, while the presence of the phospholipid transfer protein (PLTP) was confirmed by Western blot analysis. Enzyme-linked immunosorbent assay indicated that seminal PLTP concentrations represented 25% of the concentration measured in blood plasma. Blockade of phosphatidylcholine and phosphatidyl-ethanolamine transfer by a 60 min, 56degreesC heating step or with anti-PLTP antibody revealed that PLTP accounts for almost 80% of the phospholipid transfer activity present in seminal plasma. As shown by gel-permeation chromatography and Western blot analysis, seminal PLTP activity was partially associated with prostasomes. Significantly higher PLTP activity levels were measured in seminal plasma samples with low seminal vesicle secretions. The latter observation may reflect the sustained secretion of active PLTP that is diluted in a variable volume of PLTP-free seminal vesicle secretion. In conclusion, human seminal plasma displays significant phospholipid transfer activity due to the presence of active PLTP.