Prostasomes are membranous vesicles (150-200 nm diameter) present in human semen. They are secreted by the prostate and contain large amounts of cholesterol, sphingomyelin and Ca2+. In addition, some of their proteins are enzymes. Prostasomes enhance the motility of ejaculated spermatozoa and are involved in a number of additional biological functions. The possibility that they may fuse to sperm has never been proved. In this work, we studied the fusion of sperm to prostasomes by using various methods (relief of octadecyl Rhodamine B fluorescence self-quenching, fluorescence microscopy and flow cytometry) and we found that it occurs at acidic pH (4-5), but not at pH 7.5 pH-dependent fusion relies on the integrity of one or more proteins and is different from the Ca2+-stimulated fusion between rat liver liposomes and spermatozoa that does not require any protein and occurs at neutral pH. We think that the H+-dependent fusion of prostasomes to sperm may have physiological importance by modifying the lipid and protein pattern of sperm membranes.

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Abstract— The calcium‐dependent incorporation of l‐[3‐3H]serine and [1,2‐14C]ethanol‐amine into the phospholipid of isolated subcellular fractions from chick brain was studied and the properties of incorporation were examined. The microsomal fraction was found to possess the highest rate of incorporation and was able to convert under optimal conditions about 120 nmol of labelled serine and 220 nmol of ethanolamine/g of fresh brain microsomes/h. The requirement for Ca2+ ion appeared to be absolute. Mg2+ ion caused a gradual reduction in the existing enzymic activity, only when pre‐incubated with microsomes and labelled bases before adding Ca2+ ion. The incorporation of serine and ethanolamine was actively inhibited by Hg2+, Co2+, Cu2+ and Mn2+ ions, and was abolished by ethylenediamine tetra‐acetate treatment. Ethanolamine, but not choline, inositol or carnitine, competitively inhibited serine incorporation, while d‐serine had slight effect. Conversely, l‐serine inhibited competitively the incorporation of ethanolamine. The greater part of the incorporated serine (85 per cent) was localized in microsomal phosphatidylserine, while a small percentage was found in phosphatidylethanolamine. Similarly, 90 per cent of the incorporated ethanolamine was confined to phosphatidylethanolamine and a small percentage was found in the plasmalogen derivative. The mechanism of serine and ethanolamine incorporation was investigated. The results are compared with those published for similar mammalian and non‐mammalian systems.

Human saliva contains nitrate that is converted into nitrite by the activity of facultative, anaerobic bacteria of the oral cavity. Nitrite can be reduced to NO in the acidic gastric milieu; some NO may also form in the mouth at acidic pH values. In this paper, we show that bacteria (S. salivarius, S. mitis and S. bovis) isolated from saliva, may contribute to NO production in human saliva. NO formation by bacteria occurs at neutral pH values and may contribute to the antibacterial activity of saliva.