Strategy of oxidative metabolism in bull spermatozoa

Manipulation of metabolism to improve liquid preservation of mammalian spermatozoa

Reproductive success in mammals hinges on the ability of sperm to generate sufficient energy through cellular metabolism to perform the energy-intensive processes required for fertilisation, including motility, maturation, and oocyte interactions. It is now widely accepted that sperm exhibit metabolic flexibility, utilising a combination of glycolysis and oxidative phosphorylation (supported by the Krebs cycle and other complementary pathways) to meet their energy demands. However, the preferred pathway for energy production varies significantly among species, making it challenging to map species-specific metabolic strategies, particularly in species with high metabolic flexibility, like the ram. Additionally, differences in methodologies used to measure metabolism have led to biased interpretations of species' metabolic strategies, complicating the development of liquid storage methods aimed at preserving spermatozoa by manipulating energy generation based on species-specific requirements. This review examines sperm energy requirements, current methods for assessing metabolic capacity, and the current research on species-specific metabolism. Future research should focus on establishing a standardised approach for determining metabolic preferences to accurately map species-specific strategies, a critical step before developing effective liquid preservation methods. By identifying species-specific regulatory points, strategies can be designed to temporarily inhibit metabolic pathways, conserving resources and reducing the accumulation of metabolic by-products. Alternatively, supplementation with depleted metabolites can be guided by understanding areas of excessive consumption during prolonged metabolism. Applying this knowledge to develop tailored preservation techniques will help minimise sperm damage and improve survival during in vitro processing and liquid storage, ultimately enhancing the success of artificial breeding programs.

Mitochondria, spermatogenesis, and male infertility - An update

The incorporation of mitochondria in the eukaryotic cell is one of the most enigmatic events in the course of evolution. This important organelle was thought to be only the powerhouse of the cell, but was later learnt to perform many other indispensable functions in the cell. Two major contributions of mitochondria in spermatogenesis concern energy production and apoptosis. Apart from this, mitochondria also participate in a number of other processes affecting spermatogenesis and fertility. Mitochondria in sperm are arranged in the periphery of the tail microtubules to serve to energy demand for motility. Apart from this, the role of mitochondria in germ cell proliferation, mitotic regulation, and the elimination of germ cells by apoptosis are now well recognized. Eventually, mutations in the mitochondrial genome have been reported in male infertility, particularly in sluggish sperm (asthenozoospermia); however, heteroplasmy in the mtDNA and a complex interplay between the nucleus and mitochondria affect their penetrance. In this article, we have provided an update on the role of mitochondria in various events of spermatogenesis and male fertility and on the correlation of mitochondrial DNA mutations with male infertility.

Analysis of metabolic flux in felid spermatozoa using metabolomics and 13C-based fluxomics†

Spermatozoa from three feline species-the domestic cat (Felis catus), the cheetah (Acinonyx jubatus), and the clouded leopard (Neofelis nebulosa)-were analyzed using metabolomic profiling and 13C-based fluxomics to address questions raised regarding their energy metabolism. Metabolic profiles and utilization of 13C-labeled energy substrates were detected and quantified using gas chromatography-mass spectrometry (GC-MS). Spermatozoa were collected by electroejaculation and incubated in media supplemented with 1.0 mM [U13C]-glucose, [U13C]-fructose, or [U13C]-pyruvate. Evaluation of intracellular metabolites following GC-MS analysis revealed the uptake and utilization of labeled glucose and fructose in sperm, as indicated by the presence of heavy ions in glycolytic products lactate and pyruvate. Despite evidence of substrate utilization, neither glucose nor fructose had an effect on the sperm motility index of ejaculated spermatozoa from any of the three felid species, and limited entry of pyruvate derived from these hexose substrates into mitochondria and the tricarboxylic acid cycle was detected. However, pathway utilization was species-specific for the limited number of individuals (four to seven males per species) assessed in these studies. An inhibitor of fatty acid beta-oxidation (FAO), etomoxir, altered metabolic profiles of all three felid species but decreased motility only in the cheetah. While fluxomic analysis provided direct evidence that glucose and fructose undergo catabolic metabolism, other endogenous substrates such as endogenous lipids may provide energy to fuel motility.

Cell Autonomous and Nonautonomous Function of CUL4B in Mouse Spermatogenesis

CUL4B ubiquitin ligase belongs to the cullin-RING ubiquitin ligase family. Although sharing many sequence and structural similarities, CUL4B plays distinct roles in spermatogenesis from its homologous protein CUL4A. We previously reported that genetic ablation ofCul4ain mice led to male infertility because of aberrant meiotic progression. In the present study, we generated Cul4bgerm cell-specific conditional knock-out (Cul4b(Vasa)),as well asCul4bglobal knock-out (Cul4b(Sox2)) mouse, to investigate its roles in spermatogenesis. Germ cell-specific deletion of Cul4bled to male infertility, despite normal testicular morphology and comparable numbers of spermatozoa. Notably, significantly impaired sperm mobility caused by reduced mitochondrial activity and glycolysis level were observed in the majority of the mutant spermatozoa, manifested by low, if any, sperm ATP production. Furthermore,Cul4b(Vasa)spermatozoa exhibited defective arrangement of axonemal microtubules and flagella outer dense fibers. Our mass spectrometry analysis identified INSL6 as a novel CUL4B substrate in male germ cells, evidenced by its direct polyubiquination and degradation by CUL4B E3 ligase. Nevertheless,Cul4bglobal knock-out males lost their germ cells in an age-dependent manner, implying failure of maintaining the spermatogonial stem cell niche in somatic cells. Taken together, our results show that CUL4B is indispensable to spermatogenesis, and it functions cell autonomously in male germ cells to ensure spermatozoa motility, whereas it functions non-cell-autonomously in somatic cells to maintain spermatogonial stemness. Thus, CUL4B links two distinct spermatogenetic processes to a single E3 ligase, highlighting the significance of ubiquitin modification during spermatogenesis.

What sperm can teach us about energy production

Mammalian sperm have evolved under strict selection pressures that have resulted in a highly polarized and efficient design. A critical component of that design is the compartmentalization of specific metabolic pathways to specific regions of the cell. Although the restricted localization of mitochondria to the midpiece is the best known example of this design, the organization of the enzymes of glycolysis along the fibrous sheath is the primary focus of this review. Evolution of variants of these metabolic enzymes has allowed them to function when tethered, enabling localized energy production that is essential for sperm motility. We close by exploring how this design might be mimicked to provide an energy-producing platform technology for applications in nanobiotechnology.

Characterization of the proteomes associating with three distinct membrane raft sub-types in murine sperm

Mammalian sperm are transcriptionally and translationally inactive. To meet changing needs in the epididymis and female tract, they rely heavily on post-translational modifications and protein acquisition/degradation. Membrane rafts are sterol and sphingolipid-enriched micro-domains that organize and regulate various pathways. Rafts have significance in sperm by transducing the stimulus of sterol efflux into changes in intracellular signaling that confer fertilization competence. We recently characterized three biochemically distinct sub-types of sperm rafts, and now present profiles for proteins targeting to and associating with these sub-types, along with a fraction largely comprised of "non-raft" domains. Proteomics analysis using a gel-based LC-MS/MS approach identified 190 strictly validated proteins in the raft sub-types. Interestingly, many of these are known to be expressed in the epididymis, where sperm membrane composition matures. To investigate potential roles for rafts in epididymal protein acquisition, we compared the expression and localization of two different sterol-interacting proteins, apolipoprotein-A1 (apoA1) and prominin-1 (prom1) in sperm from different zones. We found that apoA1 was gradually added to the plasma membrane overlying the acrosome, whereas prom1 was not, suggesting different mechanisms for raft protein acquisition. Our results define raft-associating proteins, demonstrate functional similarities and differences among raft sub-types, and provide insights into raft-mediated epididymal protein acquisition.

Acetyl-L-carnitine ameliorates mitochondrial dysfunction following contusion spinal cord injury

In the present study, we evaluated the therapeutic efficacy of acetyl-l-carnitine (ALC) administration on mitochondrial dysfunction following tenth thoracic level contusion spinal cord injury (SCI) in rats. Initial results from experiments in vitro with naïve mitochondria showed that, in the absence of pyruvate, ALC can be used as an alternative substrate for mitochondrial respiration. Additionally, when added in vitro to mitochondria isolated from 24 h injured cords, ALC restored respiration rates to normal levels. For administration studies in vivo, injured rats were given i.p. injections of saline (vehicle) or ALC (300 mg/kg) at 15, 30 or 60 min post-injury, followed by one booster after 6 h. Mitochondria were isolated 24 h post-injury and assessed for respiration rates, activities of NADH dehydrogenase, cytochrome c oxidase and pyruvate dehydrogenase. SCI significantly (p < 0.05) decreased respiration rates and activities of all enzyme complexes, but ALC treatment significantly (p < 0.05) maintained mitochondrial respiration and enzyme activities compared with vehicle treatment. Critically, ALC administration in vivo at 15 min and 6 h post-injury versus vehicle, followed once daily for 7 days, significantly (p < 0.05) spared gray matter. In summary, ALC treatment maintains mitochondrial bioenergetics following contusion SCI and, thus, holds great potential as a neuroprotective therapy for acute SCI.

Heparin and quercitin generate differential metabolic pathways that involve aminotransferases and LDH-X dehydrogenase in cryopreserved bovine spermatozoa

Heparin and quercetin induce capacitation in spermatozoa through membrane receptor binding and inhibition of Ca-ATPase of the plasma membrane, respectively. Although capacitation is energy intensive, ammonia from amino acid metabolism can inhibit respiration and Krebs cycle activity. The objective was to determine activities of key enzymes in bull spermatozoa that contribute to the redox state and supply energy for capacitation. Malate dehydrogenase (MDH-NAD(+)), alanine and aspartate aminotransferases (ALT, AST), and lactate dehydrogenase-X (LDH-X) were measured spectrophotometrically (340 nm); mean (+/-S.D.) activities in control spermatozoa were 7.65+/-1.67, 0.45+/-0.05 and 0.74+/-0.14x10(-2)U/10(8) spermatozoa for MDH-NAD(+), ALT and AST, respectively, and were 2.83+/-0.66U/10(8) spermatozoa for LDH-X. Heparin decreased (P<0.05) activities of MDH-NAD(+), ALT, AST and LDH-X (78, 53, 66 and 66% of control levels, respectively); we inferred that amino acid catabolism was decreased. Quercetin decreased (P<0.05) activities of MDH-NAD(+) and ALT (60 and 49% of control levels), but activities of AST and LDH-X were not significantly different from controls; apparently maintenance of LDH-X activity supplied pyruvate for cellular metabolism. The proportion of capacitated spermatozoa in controls (8.5+/-1.73%) was substantially increased (P<0.05) by treatment with either heparin (36.2+/-4.5%) or quercetin (32.8+/-4.7%), there was no significant difference among groups for acrosomal integrity and sperm viability. In conclusion, heparin- or quercetin-induced capacitation affected different metabolic pathways that modulated the redox state and oxidative metabolism in cryopreserved bovine spermatozoa.

Glycolysis and sperm motility: does a spoonful of sugar help the flagellum go round?

It is doubtful that diffusion can deliver sufficient ATP from the mitochondria to sustain activity at the distal end of the sperm flagellum. Glycolytic enzymes bound to the fibrous sheath could provide energy along the flagellum at the point it is required. An obligatory role for glycolysis is supported by the lack of progressive motility in sperm from mice where the gene for sperm-specific glyceraldehyde-3-phosphate dehydrogenase (GAPDHs) had been 'knocked out'. Here, I review some evidence against this idea. First, pure diffusion from the mitochondrion is likely to be adequate in species with smaller sperm, and it is possible that rapid ATP delivery required in larger sperm could be achieved by an adenylate kinase shuttle. Second, experience with alpha-chlorohydrin demonstrates that sperm can remain motile with normal ATP concentrations despite inhibition of GAPDHs; adverse effects only occur if glucose is added and high levels of glycolytic intermediates accumulate. These observations undermine the GAPDHs knockout mouse as evidence for an essential role of local glycolysis. Third, sperm from many species can remain motile for long periods in sugar-free media and excepting dog sperm, evidence that gluconeogenesis is a possible explanation, is weak. In most species, it is unlikely that local glycolysis is the only way that ATP can be supplied to the distal flagellum.

The impact of mitochondrial genetics on male infertility

Summary Human mitochondrial DNA (mtDNA) encodes 13 of the polypeptides associated with the process of oxidative phosphorylation (OXPHOS), the cells most important ATP generating pathway. Until recently, the effects of mtDNA rearrangements on male fertility have been largely ignored. However, it is becoming increasingly evident that both point mutations and large-scale deletions may have an impact on sperm motility and morphology. We discuss the implications of these rearrangements in the context of the clinical setting. We further discuss the possible consequences resulting from the transmission of sperm mtDNA deletions to the offspring. The role of nucleo-cytoplasmic interaction is investigated in the context of nuclear transcription and replication factors that regulate mtDNA transcription and replication.

Lipid peroxide formation in relation to membrane stability of fresh and frozen thawed stallion spermatozoa

In this study we used a new method to detect reactive oxygen species (ROS) induced damage at the level of the sperm plasma membrane in fresh and frozen-thawed stallion sperm. Lipid peroxidation (LPO) in sperm cells was assessed by a fluorescent assay involving the labeling of stallion sperm with the LPO reporter probe C11-BODIPY(581/591). The peroxidation dependent spectral emission shift of this membrane probe could be localized using inverted spectral confocal microscopy and quantified on living and deteriorated sperm cells using flow cytometry. Mass spectrometric analysis of the main endogenous lipid class, phosphatidylcholine (PC), was carried out to determine the formation of hydroxy- and hydroperoxyphosphatidylcholine in fresh sperm cells. Peroxidation as reported by the fluorescent probe corresponded with the presence of hydroxy- and hydroperoxyphosphatidylcholine in the sperm membranes, which are early stage products of LPO. This allowed us to correlate endogenous LPO with localization of this process in the living sperm cells. In absence of peroxidation inducers, only relatively little peroxidation was noted in fresh sperm cells whereas some mid-piece specific probe oxidation was noted for frozen-thawed sperm cells. After induction of peroxidation in fresh and frozen-thawed sperm cells with the 0.1 mM of lipid soluble ROS tert-butylhydrogen peroxide (t-BUT) intense probe oxidation was produced in the mid-piece, whereas the probe remained intact in the sperm head, demonstrating antioxidant activity in the head of fresh sperm cells. At higher levels of t-BUT, probe peroxidation was also noted for the sperm head followed by a loss of membranes there. Frozen-thawed sperm were more vulnerable to t-BUT than fresh sperm. The potential importance of the new assays for sperm assessments is discussed.

Bovine sperm capacitation: assessment of phosphodiesterase activity and intracellular alkalinization on capacitation-associated protein tyrosine phosphorylation

Mammalian sperm capacitation is the obligatory maturational process leading to the development of the fertilization-competent state. Heparin is known to be a unique species-specific inducer of bovine sperm capacitation in vitro and glucose a unique inhibitor of this induction. Heparin-induced capacitation of bovine sperm has been shown to correlate with protein kinase A (PKA)-dependent protein tyrosine phosphorylation driven by an increase in intracellular cAMP. This study examines the possible roles of cyclic nucleotide phosphodiesterase (PDE) activity and intracellular alkalinization on bovine sperm capacitation and the protein tyrosine phosphorylation associated with it. Measurement of whole cell PDE kinetics during capacitation reveals neither a substantial change with heparin nor one with glucose: PDE activity is effectively constitutive in maintaining intracellular cAMP levels during capacitation. In contrast to a transient increase in intracellular pH, a sustained increase in medium pH by switching from 5% CO(2)/95% air incubation to 1% CO(2)/99% air incubation over 4 hr in the absence of heparin resulted in an increase in protein tyrosine phosphorylation and in the extent of induced acrosome reaction comparable to that observed following heparin-induced capacitation in 5% CO(2). These results suggest that increased bicarbonate-dependent adenylyl cyclase activity, driven by alkalinization, increases intracellular cAMP and so increases PKA activity mediating protein tyrosine phosphorylation. Quantitative analysis of the lactic acid production rate by bovine sperm glycolysis accounts fully for intracellular acidification sufficient to offset heparin-induced alkalinization, thus inhibiting capacitation. The mechanism by which heparin uniquely induces intracellular alkalinization in bovine sperm leading to capacitation remains obscure, inviting future investigation.

Unresolved Issues in Mammalian Fertilization

This review considers the role of the sperm in fertilization, addressing areas of misunderstanding and unfounded assumptions and taking particular advantage of the large body of data resulting from work with rodent species in vitro. Considerable attention is given to the appropriate use and interpretation of assays for capacitation, acrosomal exocytosis, hyperactivation, and sperm protein phosphorylation, as well as tests for sperm-zona and sperm-oocyte membrane interactions. The lack of general agreement on the means of sperm adhesion to and penetration of the zona pellucida is addressed, and the need for new approaches to this problem is pointed out. Some molecular advances in our understanding of specific steps in the process of fertilization are discussed in the context of intact cell-matrix and cell-cell interaction. This review should provide practical information for researchers just beginning the study of fertilization and interesting but not widely known observations to stimulate new ideas in experienced scientists.

Functional relationships between capacitation-dependent cell signaling and compartmentalized metabolic pathways in murine spermatozoa

Spermatozoa are highly polarized cells with specific metabolic pathways compartmentalized in different regions. Previously, we hypothesized that glycolysis is organized in the fibrous sheath of the flagellum to provide ATP to dynein ATPases that generate motility and to protein kinases that regulate motility. Although a recent report suggested that glucose is not essential for murine sperm capacitation, we demonstrated that glucose (but not lactate or pyruvate) was necessary and sufficient to support the protein tyrosine phosphorylation events associated with capacitation. The effect of glucose on this signaling pathway was downstream of cAMP, and appeared to arise indirectly as a consequence of metabolism as opposed to a direct signaling effect. Moreover, the phosphorylation events were not affected by uncouplers of oxidative respiration, inhibitors of electron transfer, or by a lack of substrates for oxidative respiration in the medium. Further experiments aimed at identifying potential regulators of sperm glycolysis focused on a germ cell-specific isoform of hexokinase, HK1-SC, which localizes to the fibrous sheath. HK1-SC activity and biochemical localization did not change during sperm capacitation, suggesting that glycolysis in sperm is regulated either at the level of substrate availability or by downstream enzymes. These data support the hypothesis that ATP specifically produced by a compartmentalized glycolytic pathway in the principal piece of the flagellum, as opposed to ATP generated by mitochondria in the mid-piece, is strictly required for protein tyrosine phosphorylation events that take place during sperm capacitation. The relationship between these pathways suggests that spermatozoa offer a model system for the study of integration of compartmentalized metabolic and signaling pathways.

Synthesis of ether lipids and phosphatidylethanolamine by ejaculated human spermatozoa

Ether lipids are the 1-O-alkyl derivatives of phospholipids. In contrast to nongerminal tissues where the plasma membrane content of ether lipids is low, over 40% of the phospholipids present in sperm plasma membranes are ether lipids. This study was undertaken to determine whether ejaculated human serm could synthesize ether lipids either through reacylation of 1-alkyl-sn-2-lysophosphatidylcholine or through direct incorporation of 1-hexadecanol into diacyl phosphatidylcholine or diacyl phosphatidylethanolamine. The ability of sperm to reacylate 1-acyl-sn-2-lysophosphatidylethanolamine was also assessed. In these experiments, freshly ejaculated sperm were unable to reacylate a phosphocholine lyso ether lipid with either palmitic (16:0) or docosahexaenoic (22:6) acids. In contrast, sperm readily incorporated both 16:0 and 22:6 into 1-acyl lysophosphatidylethanolamine. Similarly, sperm freely incorporated 1-hexadecanol into diacyl phosphatidylethanolamine thus forming a 1-alkyl phosphoethanolamine ether lipid. Diacyl phosphatidylcholine could not serve as a substrate in this reaction. It is apparent, based on these data, that human spermatozoa can directly synthesize phosphoethanolamine ether lipids that may subsequently undergo exhaustive methylation to form phosphocholine ether lipids.

Phosphorylant capacity study and lactate mitochondrial oxidation in frozen bovine sperm

Frozen-stored bovine sperm-pellets of proven fertility were used, and the response to respiratory chain effectors was studied, thus demonstrating the energy conservation capacity. It was further observed that the assayed suspensions used lactate oxidatively, which proves the LDH-X mitochondrial activity (the presence of oxidative substrates is fundamental in capacitation and acrosome reaction processes). The suspensions were treated with 10mM phosphate buffer hypotonic medium to eliminate plasmalema and cytoplasmic content. Lactate respiration was sensitive to respiratory chain effectors, such as oligomycin and antimycin. To evaluate the LDH-X contribution to mitochondrial respiration, lipoate dehydrogenase was inhibited through 5-methoxyindole-2-carboxylic acid (MICA) in the presence of pyruvate-malate and citrate-malate, obtaining with the addition of lactate, oxygen uptakes of 18% and 51% with respect to respiration with the mentioned substrates. In the MICA dose-effect curve, a major sensitivity to inhibitor in active state mitochondrial respiration is obtained when pyruvate-malate is used. Lactate competence with pyruvate by mitochondrial LDH-X was observed. The results obtained would allow the thorough study of the necessity of oxidative energy in the capacitation and fertilization processes, and of the LDH-X role in frozen-stored bovine sperm.

Phosphatidylcholine synthesis in human spermatozoa

To clarify the mechanism of phospholipid synthesis in spermatozoa, fresh human spermatozoa were incubated with labeled fatty acids and 1-acyl-lysophosphatidyl choline (LPC) in the presence or absence of coenzyme A (CoASH). Both docosahexaenoic acid and palmitic acid were incorporated into phosphatidylcholine; however, this reaction was absolutely dependent upon the presence of CoASH in the incubation medium. The rate of incorporation of docosahexaenoic acid was 2.7-fold higher than that of palmitic acid, but more palmitic acid was incorporated into phosphatidylcholine in the absence of LPC. These data provide direct evidence for acyl transferase activity in human spermatozoa and may furnish a mechanism for phospholipid remodeling in sperm membranes. The different incorporation rates of these fatty acids into phosphatidylcholine may be due to the kinetics of the activation step, long chain fatty acid:CoASH ligase (AMP), or the substrate specificity of the acyl transferase.

Kinetics of human spermatozoa long-chain fatty acid: CoASH ligase

The kinetics of long-chain saturated fatty acid activation were studied in the supernatant obtained from Triton-treated human spermatozoa. Sperm long-chain fatty acid: CoASH ligase (AMP) (E.C. 6.2.1.3) was able to activate myristic, palmitic, and stearic acids, but was incapable of utilizing lauric, arachidic, and behenic acids. Peak activity was obtained with palmitic acid. Although the Kms for each fatty acid were similar (4.3 to 5.0 microM), the Vmax was several-fold higher for palmitate. In contrast, ligase from liver homogenates assayed under identical conditions activated lauric through stearic acids, with maximal rates being noted with myristic acid. When compared with nongerminal tissues, sperm ligase appears unique because of its narrower acyl substrate specificity.

Activation of palmitic acid by human spermatozoa

Human spermatozoa were studied to determine if a long chain fatty acid, CoASH ligase (AMP) (E.C. 6.2.1.3), was present. Ligase activity was measured with a radioligand millipore filter technique and was readily detectable in spermatozoa or in the protein fraction extracted with Triton X-100, but was not present in seminal plasma. The assay was optimized for pH, protein concentration, and incubation time. Activity was dependent upon palmitic acid, ATP, coenzyme A, and a divalent cation. Sperm ligase appeared similar to the ligase characterized from other tissues by sharing a common pH optimum (approximately 8.0-8.4), and a preference for magnesium over manganese in the incubation media.

Oxygen metabolism of mammalian spermatozoa. Generation of hydrogen peroxide by rabbit epididymal spermatozoa

Rabbit spermatozoa from the cauda epididymis produced 0.7-0.8nmol of H(2)O(2)/min per 10(8) cells at cell concentrations below 10(7) cells/ml with linear dependence on cell concentration. Above 2 x 10(7) cells/ml, the rate again became linear with cell concentration but decreased to 0.1-0.2nmol/min per 10(8) cells. Spermatozoa treated with amphotericin B, which makes the plasma membrane highly permeable to low-molecular-weight compounds, showed a similar dependence of H(2)O(2) production rate on cell concentration; below 10(7) cells/ml the rate was 0.3-0.4nmol/min per 10(8) cells; above 2 x 10(7) cells/ml, the rate was 0.1-0.2nmol/min per 10(8) cells. Hypo-osmotically treated rabbit epididymal spermatozoa, a preparation useful for studying mitochondrial function in sperm [Keyhani & Storey (1973) Biochim. Biophys. Acta305, 557-565] produced 0.1-0.2nmol/min per 10(8) cells in the absence of added substrates. The dependence of rate on cell concentration was linear from 10(7) to 2.2 x 10(8) cells/ml. This endogenous rate was unaffected by rotenone, but stimulated 4-fold by antimycin A. Addition of the mitochondrial substrates lactate plus malate increased the rate of H(2)O(2) production to 0.3nmol/min per 10(8) cells. The decreased rate of H(2)O(2) production observed with intact sperm at high cell concentrations is attributed to reaction of H(2)O(2) with the cells, possibly with the plasma membrane, which is lost after hypo-osmotic treatment. Rabbit spermatozoa have glutathione peroxidase and glutathione reductase activities, but these seem to play little role in removal of H(2)O(2) generated. The rate at low cell concentration is taken to be the unperturbed rate. The sources of H(2)O(2) production in rabbit spermatozoa have been tentatively resolved into a low-molecular-weight component, lost after amphotericin treatment, a mitochondrial component and a rotenone-insensitive component that has not been identified.

Oxidative metabolism of spermatozoa from inbred and random bred mice

Epididymal spermatozoa from the random-bred CFW and from the inbred C57 BL/6 strains of mice were treated either hypotonically or with the antibiotic filipin in order to study the mitochondrial oxidative activities of the two strains in the absence of permeability barriers imposed by the plasma membrane. The percentage of motile spermatozoa from C57BL/6 mice was consistently higher than that of CFW mice, but sperm from the latter fertilized a higher percentage of eggs in vitro. In vivo, there was no apparent difference in fertilizing capacity in vivo: no significant difference between strains was observed. There is a strategy of oxidative metabolism in mouse spermatozoa which is common to the two genetic strains of the species tested, but which differs from that of rabbit and bull spermatozoa. The mitochondria of mouse spermatozoa oxidize L--3-glycerolphosphate but not glutamate in the presence of malate; both activities are present in bull spermatozoa but neither are present in rabbit spermatozoa. In common wit those of the mammalian species thus far studied, the mitochondria of mouse spermatozoa readily oxidize lactate and pyruvate in the presence of malate. They also oxidize acetyl CoA, acetyl carnitine, and long-chain acyl CoA esters directly, without the intermediacy of the carnitine esters. Mouse spermatozoa, therefore, have access to endogenous acyl CoA esters as a source of metabolic energy, which is consistent with their ability to maintain motility for 4-6 hours in the absence of added energy sources. Mouse spermatozoa are self-sufficient with regard to oxidative metabolism, which suggests that energy sources are not readily available to them in the mouse female reproductive tract.