Distribution of lipid and protein in human semen fractions

Human semen is formed by the secretions of different glands. We fractionated semen by centrifugation and obtained four main fractions: (a) spermatozoa, (b) material precipitating at 10 000× g, (c) prostasomes (precipitate at 105 000× g), and (d) a soluble fraction. When required, fractions were purified further. We find that most semen protein (about 85%) is in the soluble fraction, 7% in spermatozoa and the remainder is scattered in the other fractions. We compared the electrophoretic pattern of soluble protein with the protein of prostasomes and found marked differences. On the other hand, prostasomes, that comprises only about 3% of total semen protein, contain about 45% of cholesterol and almost 15% of lipid phosphorus with a cholesterol to phospholipid molar ratio greater than 2. On the contrary, phospholipid is largely bound to the fraction containing spermatozoa (about 46% of total lipid phosphorus). This fraction is poor in cholesterol and has a cholesterol to phospholipid molar ratio of about 0.2. The distribution of lipid phosphorus among lipid classes shows some similarity in the soluble fraction and in prostasomes; in both fractions, sphingomyelin is the most abundant phospholipid (about 50%). On the other hand, phosphatidylcholine is the main phospholipid in spermatozoa-enriched fractions (about 35% of total lipid phosphorus). We conclude that the various fractions of seminal plasma obtained by centrifugation differ markedly from each other as to lipid and protein content.