Composition and metabolism of carbohydrates and lipids in Sparus aurata semen and its relation to viability expressed as sperm motility when activated

The present study investigated aspects of lipid and carbohydrate metabolism in Sparus aurata semen and tested the effect of lipids, carbohydrates and related metabolites on sperm viability using in vitro incubation experiments. Sparus aurata semen contained enzyme systems to metabolize sugars and lipids. Also key enzymes of the tricarboxylic acid cycle and enzymes involved in ATP metabolism were detected. When spermatozoa were incubated in sperm motility inhibiting saline solution for 48 h phospholipid levels decreased constantly and triglycerides levels during the first 24 h of incubation indicating that spermatozoa utilize lipids as energy resources. After 24 h triglycerides levels started to re-increase indicating a change in sperm metabolism, in particular the onset of triglycerides synthesis by the fatty acid synthase complex. In the incubation period from 0 to 24 h glucose levels were constant, and decreased thereafter. Glycogen levels did not change at all. Semen contained also considerable amounts of sialic acid, glucuronic acid and hexosamines, components of mucopolysaccharides. To find out whether lipids, carbohydrates, and related metabolites had a positive effect on sperm functionality semen was incubated together with the described compounds in sperm motility inhibiting saline solution and motility when activated was determined. In the control 37.2 ± 10.1% of the spermatozoa were locally motile and 38.3 ± 13.3% motile after 24 h, 36.4 ± 5.2% were locally motile and 9.6 ± 4.5% were motile after 48 h. The swimming velocity was 89.0 ± 13.1 µm/s after 24 h and 61.3 ± 12.6% after 48 h. Different types of lipids (arachidic acid, linoleic acid, and glycerol trimyristate) and metabolites acting as fuel for the tricarboxylic acid cycle (hydroxybutyrate, ketoglutarate, and pyruvate) had a positive effect on the sperm viability. Tested carbohydrates (fucose, galactose, glucosamine, glucose, glucoheptose, glycogen, and sialic acid) had no effect. Also lactate and fructose-6-phosphate had no effect on sperm viability while glucose-6-phosphate, oxalacetate, and phosphoglycerate had negative effects.