The energy metabolism of a cell type should be related to its function. This relation has been termed as the metabolic strategy of the cell. In the case of sperm, the question that may be asked is: is sperm's metabolic strategy geared to the series of reactions related to the process of fertilization including motility, or does the maintenance of motility dominate the metabolic strategy of the spermatozoon? These questions can be clarified by a close look at the kinetic properties of pyruvate kinase in sperm, as compared with that of liver and muscle, as well as by looking at the metabolic coupling of pyruvate kinase with flagellar ATPase. Pyruvate kinase has access to its substrates in permeabilized epididymal sperm while remaining bound to the sperm cytoskeleton and suffering minimal perturbation from the pemeabilization procedure , Epididymal sperm are usually immotile but motility is restored by suspending sperm in KCl medium containing Mg2+ upon addition of 3 mM ATP, according to the method of Gibbons and Gibbons , and adding 50 mM phospho-enolpyruvate to maintain ATP levels . Under these conditions, phosphoenolpyru-vate is converted to pyruvate. This conversion is readily measured, so that the activity of flagellar ATPase in permeabilized epididymal sperm can be quantified and putative regulators assessed.
Flagellar ATPase in mammalian spermatozoa has an activity comparable to that of somatic cell pyruvate kinase. Sperm flagellar ATPase requires Mg2+ for activity . However, if Mg2+ is omitted from the medium, then sperm remain immotile even in the presence of 3 mM ATP and 50 mM phosphoenolpyruvate. Motility is restored following addition of 3 mM Mg2+. The calculated Km values for ATP and Mg2+ at 37°C were 0.22 and 0.25 mM, respectively, thus suggesting that the substrate for flagellar ATPase is MgATP, as previously shown by Tibbs for sperm tails from perch [ 42] and by Gibbons and Gibbons [ 40] and Hayasi for sea urchin axonemes .
Pyruvate kinase in mammalian spermatozoa is kinetically very similar to that of muscle and it is affected by the same inhibitors and activators as in the case of the muscle enzyme. The question of whether or not this similarity extends to the protein structure of the enzyme remains to be elucidated. The difficulty of extracting and purifying this and other enzymes from sperm without damaging their structure [44, 45] makes its characterization difficult using immunological methodology. Immunological cross-reactions can occur among some of the pyruvate kinase isozymes [46 ] and, therefore, protein characterization is rendered a near impossible task. For the purpose of metabolic analysis, however, the similarity of the kinetic properties and control characteristics of the sperm and muscle pyruvate kinases implies that control of the glycolytic pathway in sperm operates in a similar fashion to that of skeletal muscle. Both cells require ATP for mechanical work by contractile proteins. Both cells have access to glucose reserves: the muscle cell through glycogenolysis and the sperm cell through an inexhaustible supply of glucose in the oviduct . In mature spermatozoa, the energy-producing and energy-utilizing pathways seem fairly well balanced. The activities of pyruvate kinase and flagellar ATPase and lactate dehydrogenase are all of the same order of magnitude and higher than the activity of mitochondrial lactate or pyruvate oxidation system . The metabolic strategy of mature sperm depends on a high rate of glycolysis to provide ATP for motility rather than maximal efficiency through the Krebs cycle, as in the case of skeletal muscle for contractibility [8, 17].
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