Hexose‐specificity of hexokinase and ADP‐dependence of pyruvate kinase play important roles in the control of monosaccharide utilization in freshly diluted boar spermatozoa

Abstract

Incubation of boar sperm from fresh ejaculates in a minimal medium with 10 mM glucose induced a fast and intense activation of glycolysis, as indicated by the observed increases in the intracellular levels of glucose 6-phosphate (G 6-P) and ATP and the rate of formation of extracellular L-lactate. The effect of glucose was much more intense than that induced by fructose, sorbitol, and mannose. The greater utilization of glucose was related to a much greater sensitivity to hexokinase when compared with the other monosaccharides. Thus, the presence of 0.5 mM glucose induced total hexokinase activity in supernatants from sperm extracts of 1.7 +/- 0.1 mIU/mg protein, while the same concentration of both fructose, mannose, and sorbitol induced total hexokinase activity from 0.3 +/- 0.1 mIU/mg protein to 0.60 +/- 1 mIU/mg protein. Kinetic analysis of the total pyruvate kinase activity indicated that this activity was greatly dependent on the presence of ADP and also showed a great affinity for PEP, with an estimated Km in supernatants of 0.15-0.20 mM. Immunological location of proteins closely related to glycolysis, like GLUT-3 hexose transporter and hexokinase-I, indicated that these proteins showed the trend to be distributed around or in the cellular membranes of both head and midpiece in a grouped manner. We conclude that glycolysis is regulated by both the specific availability of a concrete sugar and the internal equilibrium between ATP and ADP levels. Furthermore, localization of proteins involved in the control of monosaccharide uptake and phosphorylation suggests that glycolysis starts at concrete points in the boar-sperm surface.

Energy Management of Mature Mammalian Spermatozoa

“…The first point is the fact that the optimal utilization of sugars by sperm is reached to determined concentrations of this sugars. For instance, the optimal utilization rate of glucose by sperm from dog and boar is reached to very low concentrations of the sugar, at about 0.1 mM [16,36,37,45]. This indicates that the addition of low concentrations of sugar to the extenders would be enough to maintain sperm energy levels.…”

“…In fact, both fructose and sorbitol are typical vegetal sugars, without any significant presence in animals, excepting in mammalian seminal plasma. It is noteworthy that the utilization of sugars like fructose, sorbitol and mannose by sperm of species like boar and dog of is in fact less effective in order to obtain energy than that of glucose [36,45]. The greater effectiveness of glucose is mainly linked to a greater sensitivity to the hexokinase system, which phosphorylates sugars as a first step in their metabolization pathway [36,45].…”

“…It is noteworthy that the utilization of sugars like fructose, sorbitol and mannose by sperm of species like boar and dog of is in fact less effective in order to obtain energy than that of glucose [36,45]. The greater effectiveness of glucose is mainly linked to a greater sensitivity to the hexokinase system, which phosphorylates sugars as a first step in their metabolization pathway [36,45]. Taking into account this lower efficiency, it is difficult to understand the biological logics to utilize sugars like fructose or sorbitol as mere energy sources for sperm.…”

“…In this way, it is important to remind that this sensitivity changes upon a sugar-and species-specific basis. Thus, as described above and taking porcine sperm as a basis, it is important to remind that the velocity by which glucose is phosphorylated and then incorporated to the glycolytic flux is greater than that observed by other sugars like fructose, sorbitol and mannose [36]. The ability of each species to utilize each separate sugar will be then different, depending on the specific machinery that sperm have in order to uptake and further phosphorylate monosaccharides.…”

“…However, it is difficult to understand why seminal plasma in all species does not contain glucose as their main energy source, since glucose is the most important energy-producing monosaccharide for all of mammalian tissues. Instead of this, seminal plasma contains other sugars, specially fructose, but also sorbitol and other [6,28,32,33,36,42,45], which are not as efficient as glucose as primary energy sources (see as an example in boar sperm [36]. Again, investigators can only speculate on this point.…”