A shuttle system for the transfer of reducing equivalents in mouse sperm mitochondria

Pyruvate modulation of redox potential controls mouse sperm motility

Sperm gain fertilization competence in the female reproductive tract through a series of biochemical changes and a requisite switch from linear progressive to hyperactive motility. Despite being essential for fertilization, regulation of sperm energy transduction is poorly understood. This knowledge gap confounds interpretation of interspecies variation and limits progress in optimizing sperm selection for assisted reproduction. Here, we developed a model of mouse sperm bioenergetics using metabolic phenotyping data, quantitative microscopy, and spectral flow cytometry. The results define a mechanism of motility regulation by microenvironmental pyruvate. Rather than being consumed as a mitochondrial fuel source, pyruvate stimulates hyperactivation by repressing lactate oxidation and activating glycolysis in the flagellum through provision of nicotinamide adenine dinucleotide (NAD)+. These findings provide evidence that the transitions in motility requisite for sperm competence are governed by changes in the metabolic microenvironment, highlighting the unexplored potential of using catabolite combination to optimize sperm selection for fertilization.

mtDNA polymorphism and metabolic inhibition affect sperm performance in conplastic mice

Whereas a broad link exists between nucleotide substitutions in the mitochondrial genome (mtDNA) and a range of metabolic pathologies, exploration of the effect of specific mtDNA genotypes is on-going. Mitochondrial DNA mutations are of particular relevance for reproductive traits, since they are expected to have profound effects on male specific processes as a result of the strict maternal inheritance of mtDNA. Sperm motility is crucially dependent on ATP in most systems studied. However, the importance of mitochondrial function in the production of the ATP necessary for sperm function remains uncertain. In this study, we test the effect of mtDNA polymorphisms upon mouse sperm performance and bioenergetics by using five conplastic inbred strains that share the same nuclear background while differing in their mitochondrial genomes. We found that, while genetic polymorphisms across distinct mtDNA haplotypes are associated with modification in sperm progressive velocity, this effect is not related to ATP production. Furthermore, there is no association between the number of mtDNA polymorphisms and either (a) the magnitude of sperm performance decrease, or (b) performance response to specific inhibition of the main sperm metabolic pathways. The observed variability between strains may be explained in terms of additive effects of single nucleotide substitutions on mtDNA coding sequences, which have been stabilized through genetic drift in the different laboratory strains. Alternatively, the decreased sperm performance might have arisen from the disruption of the nuclear DNA/mtDNA interactions that have coevolved during the radiation of Mus musculus subspecies.

Selective active site inhibitors of human lactate dehydrogenases A4, B4, and C4

Human lactate dehydrogenases (LDH-A4, -B4, and -C4) are highly homologous with 84-89% sequence similarities and 69-75% amino acid identities. Active site residues are especially conserved. Gossypol, a natural product from cotton seed, is a non-selective competitive inhibitor of NADH binding to LDH, with K(i) values of 1.9, 1.4, and 4.2 microM for LDH-A4, -B4, and -C4, respectively. However, derivatives of gossypol and structural analogs of gossypol in the substituted 2,3-dihydroxy-1-naphthoic acid family exhibited markedly greater selectivity and, in many cases, greater potency. For gossypol derivatives, greater than 35-fold selectivity was observed. For dihydroxynaphthoic acids with substituents at the 4- and 7-positions, greater than 200-fold selectivity was observed. Inhibition was consistently competitive with the binding of NADH, with dissociation constants as low as 30 nM. By comparison, a series of N-substituted oxamic acids, which are competitive inhibitors of the binding of pyruvate to LDH, exhibited very modest selectivity. These results suggest that substituted dihydroxynaphthoic acids are good lead compounds for the development of selective LDH inhibitors. Selective inhibitors of LDH-C4 targeted to the dinucleotide fold may hold promise as male antifertility drugs. Selective inhibitors of LDH-A4 and -B4 may be useful for studies of lactic acidemia associated with ischemic events. More broadly, the results raise the question of the general utility of drug design targeted at the dinucleotide binding sites of dehydrogenases/reductases.

Selective inhibition of the sperm-specific lactate dehydrogenase isozyme-C4 by N-isopropyl oxamate

In the present study, we demonstrated that the attachment of the nonpolar isopropylic carbon chain in the nitrogen of oxamate, converted this competitive inhibitor of LDH isozymes into a powerful selective inhibitor of mouse LDH-C4. The comparative study of the inhibitory effect of oxamate and N-isopropyl oxamate on mouse LDH isozymes pointed out that the isopropylic carbon chain conferred upon N-isopropyl oxamate a high affinity for LDH-C4 and a marked decrease in the affinity for the other isozymes since oxamate showed more inhibitory effect on LDH-1 (Ki = 0.06 mM) and LDH-5 (Ki = 0.08 mM), and less inhibitory effect on LDH-C4 (Ki = 0.25 mM). On the other hand, N-isopropyl oxamate showed the highest inhibitory effect on LDH-C4 (Ki = 0.014 mM) and poor inhibitory effect on LDH-1 (Ki = 0.4 mM) and LDH-5 (Ki = 0.8 mM). Apparently, the enzymatic inactivation proceeded through a reversible binding of N-isopropyl oxamate, facilitated by nonpolar interactions with a hydrophobic region present only in the active site of mouse LDH-C4, resulting in a selective inhibition of this isozyme in comparison with the other LDH isozymes. N-isopropyl oxamate was also a powerful competitive inhibitor of LDH-C4 (Ki = 0.015 mM) compared with oxamate (Ki = 0.35 mM), using alpha-ketoisocaproate as a substrate.

Inhibition kinetics of NAD-linked enzymes by gossypol acetic acid

Effects of gossypol on human spermatozoal enzymes were investigated. This compound was found to be a potent inhibitor of the NAD-linked enzymes, glucelaldehyde-3-phosphate dehydrogenase (GA3PDH), lactate dehydrogenase (LDH), malate dehydrogenase (MDH), and isocitrate dehydrogenase (ICDH). MDH was inhibited by gossypol when the reaction was carried out in malate-oxalacetate (direct) or oxalacetate-malate (reverse) directions. The I50 of gossypol for the direct reaction was 2.9 microM, whereas that for the reverse reaction was 1.2 microM. Reciprocal plots due to Lineweaver-Burk showed that MDH is inhibited in a noncompetitive manner with respect to both reactions. LDH was also inhibited by this compound when pyruvate or alpha-ketobutyrate was used as a substrate. Gossypol was a noncompetitive inhibitor for LDH. The I50 of gossypol for LDH were 9.8 microM and 11.3 microM, when using pyruvate and alpha-ketobutyrate, respectively as substrates. The I50 of gossypol for GA3PDH and ICDH were 110 microM and 2.7 microM, respectively.

Subcellular localization of branched-chain amino acid aminotransferase and lactate dehydrogenase C4 in rat and mouse spermatozoa

Spermatozoa isolated from rat and mouse epididymes show a relatively high branched-chain amino acid aminotransferase (leucine aminotransferase, EC 2.6.1.6) activity. There is a significant reduction of leucine aminotransferase and of the isoenzyme C4 of lactate dehydrogenase (EC 1.1.1.27) in the gametes during their epididymal transit. Studies of patterns of liberation of the leucine aminotransferase and of the lactate dehydrogenase C4 from intact spermatozoa, treated with increasing concentrations of digitonin, indicate that both enzymes have the same dual subcellular location, i.e. in the cytosol and in the mitochondria.

Subcellular localization of leucine aminotransferase and alpha-hydroxyacid dehydrogenase in Trypanosoma cruzi

Homogenates of Trypanosoma cruzi epimastigotes (Tulahuén strain) show L-leucine aminotransferase activity (EC 2.6.1.6). Subcellular distribution of this enzyme and of alpha-hydroxyacid dehydrogenase, enzymes which share a common substrate/product (alpha-ketoisocaproate), has been studied by means of differential centrifugation, digitonin treatment of entire parasites, isopycnic centrifugation and determination of latency of enzymes in the large granule fraction. The results indicate that both enzymes have a dual localization, in the cytosol and in the mitochondrion, probably in the matrix. On the basis of this location, it is proposed that they operate in a shuttle system transferring reducing equivalents between the cytosol and the mitochondrion.

Properties of the branched-chain 2-hydroxy acid/2-oxo acid shuttle in mouse spermatozoa

Operation of the branched-chain 2-hydroxy acid/2-oxo acid shuttle for the transfer of reducing equivalents in mitochondria of mouse spermatozoa was studied in vitro in reconstituted systems. Results show that the branched-chain 2-oxo acids within the mitochondria are offered several metabolic pathways. (a) Decarboxylation: mouse sperm mitochondria possess high branched-chain 2-oxo acid decarboxylase activity. (b) Recycling to the cytosol by using a transport system which can be inhibited by alpha-cyano-3-hydroxycinnamate and pH 6.8. (c) Transamination to the corresponding amino acids: experiments presented indicate that leucine formed from 4-methyl-2-oxopentanoate may pass to the external phase, re-initiating the cycle. These two last possibilities would allow autocatalytic operation of the shuttle. The branched-chain 2-hydroxy acids apparently do not utilize the monocarboxylate carrier to penetrate the mitochondria.

In vitro inhibition by gossypol of oxidoreductases from human tissues

The effect of gossypol, a polyphenolic compound with antifertility action on human males, has been investigated on the following oxidoreductases purified from human tissues: lactate dehydrogenase (EC 1.1.1.27) isozymes 1 or B4 from heart, 5 or A4 from liver and X or C4 from spermatozoa; malate dehydrogenase (EC 1.1.1.37) mitochondrial and "soluble" isozymes from heart and NADP-glutamate dehydrogenase (EC 1.4.1.4) from liver. Gossypol proved to be a powerful inhibitor of the six enzymes studied. For all of them, inhibition was of the competitive type with respect to the coenzyme and non-competitive in relation to substrate. The lowest ki values were shown for lactate dehydrogenase isozyme 1 or B4 and for the two isozymes of malate dehydrogenase. Results did not show selectivity of gossypol for the sperm-specific isozyme X or C4 of lactate dehydrogenase.

Changes in porcine sperm lactate dehydrogenase isoenzymes during sperm maturation

The changes in sperm lactate dehydrogenase (LDH-lactate: NAD oxidoreductase, EC 1.1.1.27) activity and the relative occurrence of LDH isoenzymes during sperm maturation were studied using pubertal German improved Landrace boars. The LDH of spermatozoa liberated from the testis, caput epididymis, proximal and distal corpus epididymidis and cauda epididymidis was spectrophotometrically quantified while the LDH isoenzymes were separated on fine cellulose acetate membrane strips with the Sartorius Sartophor system. The LDH content of sperm dropped drastically as they moved from the testis to the caput epididymidis. Thereafter, only little and insignificant changes were observed. Testicular sperm was composed more of the fastest anodically-migrating isoenzyme (LDH1) while with sperm maturation, the least or slowest migrating isoenzymes (LDH4 and 5) became progressively more dominant. This loss in LDH content in sperm and the shifts in the LDH isoenzyme patterns indicate that the development of sperm during maturation is dependent on a delicate balance between lactate and pyruvate, such that the cathodic isoenzymes involved in the anaerobic energy-supplying metabolic processes are sufficiently available for sperm activity and survival.

Lactate dehydrogenase X, malate dehydrogenase and total protein in rat spermatozoa during epididymal transit

Lactate dehydrogenase isozyme X (LDH X), malate dehydrogenase (MDH) and total soluble protein have been determined in lysates of spermatozoa isolated from caput, corpus and cauda of rat epididymis. Transit of spermatozoa through epididymis is accompanied by a reduction of LDH X, MDH and total protein per cell in sexually rested animals. The profiles of reduction along epididymal segments are different for the three variables studied. Mating with receptive females during the 5 days prior to determinations increases significantly the levels of MDH in spermatozoa from all sections of epididymis and produces increase of total soluble protein in the cells contained in cauda.

Phospholipids and sialic acid changes produced by vitamin D3 on intestinal mitochondria

Vitamin D3 administration to vitamin D-deficient chicks produces, 24 hours after treatment, a large increment of phospholipids in mitochondria from intestinal mucosa. Proportion of the different phospholipid classes and fatty acid composition of the organelles were not modified by that treatment. A time course study of the effects of 1,25(OH)2D3 on calcium absorption and sialic acid content of intestinal mitochondria glycoproteins showed that both effects were correlated. The results suggest that either vitamin D or the increase of calcium transfer are involved in the make up of intestinal mitochondria membranes.

Sperm maturation and storage in the male rat after acute treatment with aflatoxin B1

The effects of acute aflatoxicosis on sperm maturation in the rat were evaluated using sperm GOT, GPT and LDH activities as indices. The levels of these enzymes in sperm undergoing maturation were stable like the sperm populations in the testis and epididymis. Acute aflatoxicosis enhanced corpus epididymal sperm transaminase but not sperm transit.

Inhibition by gossypol of oxidoreductases from Trypanosoma cruzi

The effects of gossypol, a polyphenolic compound isolated from the cotton plant upon six oxidoreductases from cultured epimastigotes of Typanosoma cruzi were studied. Gossypol was a powerful inhibitor of the alpha-hydroxyacid and malate dehydrogenases, NAD-linked enzymes, and of glutamate dehydrogenase, malic enzyme and glucose-6-phosphate dehydrogenase, NADP-dependent enzymes. The drug did not have an effect on succinate dehydrogenase, a flavoprotein. The Ki values with respect to substrate were 0.73, 0.3 and 3.5 microM for alpha-hydroxyacid, malate and glutamate dehydrogenases, respectively, and 1.1, 0.19 and 7.8 microM with respect to the coenzyme. Inhibition was noncompetitive with respect to substrate and uncompetitive in relation to the coenzyme.

Catalytic properties of the sperm-specific lactate dehydrogenase (LDH X or C4) from different species

A comparative study of catalytic properties of the sperm-specific lactate dehydrogenase (EC 1.1.1.27) isozyme X or C4 from a variety of animals (boar, bull, goat, Guinea pig, man, mouse, pigeon, rabbit, and rat) is presented. Optimum concentration and Km values for pyruvate, inhibition by substrate, and activity against analog substrates (alpha-ketoacids with linear and branched chains from 4 to 6 carbon atoms) for isozyme X of different species showed significant differences. The observed properties are correlated with available evidence on the metabolic role of the enzyme.

Studies in vitro on shuttle systems of mouse spermatozoa

Observations on systems reconstituted in vitro with different starting substrates (2-hydroxy-acids, 2-oxo-acids or leucine) indicate that a branched-chain 2-hydroxy-acid/2-oxo-acid shuttle for the transfer of reducing equivalents from cytosol to mitochondria may be operational in mouse sperm. Evidence is presented suggesting that the 2-oxo-acids produced by intramitochondrial oxidation of 2-hydroxy-acids ingressed from the cytosol can recycle back into the external phase. Observations in vitro demonstrate that, in addition to the branched-chain 2-hydroxy-acid/2-oxo-acid shuttle, the malate/aspartate system is also active in mouse sperm. On the contrary, the lactate/pyruvate redox couple does not appear to function as part of a shuttle system in mouse sperm mitochondria. The glycerol 3-phosphate shuttle probably is not functionally significant in mouse spermatozoa, since the activity of the 'soluble' glycerol 3-phosphate dehydrogenase is very low.

Inhibitory action of gossypol on enzymes and growth of Trypanosoma cruzi

Gossypol, a phenolic compound isolated from the cotton plant, is a powerful inhibitor of nicotinamide adenine dinucleotide-linked enzymes (alpha-hydroxyacid dehydrogenase and malate dehydrogenase) of Trypanosoma cruzi, the parasite that causes Chagas' disease. Parasites at the epimastigote stage that were incubated for 5 minutes with 100 micromolar gossypol were completely immobilized. Concentrations of gossypol as low as 0.01 micromolar markedly reduced the growth rate of T. cruzi in culture.