Respiratory control induced by ATP in human term placental mitochondria

Multiple functions of syncytiotrophoblast mitochondria

The human placenta plays a central role in pregnancy, and the syncytiotrophoblast cells are the main components of the placenta that support the relationship between the mother and fetus, in apart through the production of progesterone. In this review, the metabolic processes performed by syncytiotrophoblast mitochondria associated with placental steroidogenesis are described. The metabolism of cholesterol, specifically how this steroid hormone precursor reaches the mitochondria, and its transformation into progesterone are reviewed. The role of nucleotides in steroidogenesis, as well as the mechanisms associated with signal transduction through protein phosphorylation and dephosphorylation of proteins is discussed. Finally, topics that require further research are identified, including the need for new techniques to study the syncytiotrophoblast in situ using non-invasive methods.

Membrane potential regulates mitochondrial ATP-diphosphohydrolase activity but is not involved in progesterone biosynthesis in human syncytiotrophoblast cells

ATP-diphosphohydrolase is associated with human syncytiotrophoblast mitochondria. The activity of this enzyme is implicated in the stimulation of oxygen uptake and progesterone synthesis. We reported previously that: (1) the detergent-solubilized ATP-diphosphohydrolase has low substrate specificity, and (2) purine and pyrimidine nucleosides, tri- or diphosphates, are fully dephosphorylated in the presence of calcium or magnesium (Flores-Herrera 1999, 2002). In this study we show that ATP-diphosphohydrolase hydrolyzes first the nucleoside triphosphate to nucleoside diphosphate, and then to nucleotide monophosphate, in the case of all tested nucleotides. The activation energies (Ea) for ATP, GTP, UTP, and CTP were 6.06, 4.10, 6.25, and 5.26 kcal/mol, respectively; for ADP, GDP, UDP, and CDP, they were 4.67, 5.42, 5.43, and 6.22 kcal/mol, respectively. The corresponding Arrhenius plots indicated a single rate-limiting step for each hydrolyzed nucleoside, either tri- or diphosphate. In intact mitochondria, the ADP produced by ATP-diphosphohydrolase activity depolarized the membrane potential (ΔΨm) and stimulated oxygen uptake. Mitochondrial respiration showed the state-3/state-4 transition when ATP was added, suggesting that ATP-diphosphohydrolase and the F1F0-ATP synthase work in conjunction to avoid a futile cycle. Substrate selectivity of the ATP-diphosphohydrolase was modified by ΔΨm (i.e. ATP was preferred over GTP when the inner mitochondrial membrane was energized). In contrast, dissipation of ΔΨm by CCCP produced a loss of substrate specificity and so the ATP-diphosphohydrolase was able to hydrolyze ATP and GTP at the same rate. In intact mitochondria, ATP hydrolysis increased progesterone synthesis as compared with GTP. Although dissipation of ΔΨm by CCCP decreased progesterone synthesis, NADPH production restores steroidogenesis. Overall, our results suggest a novel physiological role for ΔΨm in steroidogenesis.

Oxygen metabolism in human placenta mitochondria

Due to the high metabolic demands of the placental tissue during gestation, we decide to analyzed the mitochondrial bioenergetic functions in the human term placenta. Different mitochondrial morphological parameters, membrane potential and cardiolipin content were determined by flow cytometry. Oxygen uptake, hydrogen peroxide production and cytochrome P450 content, were also measured. Some apoptotic mitochondrial proteins were also analyzed by western blot. Two isolated mitochondrial fractions were observed: large/heavy and small/light with different functional characteristics. Oxygen uptake showed a respiratory control (RC) of 3.4 ± 0.3 for the heavy mitochondria, and 1.1 ± 0.4 for light mitochondria, indicating a respiratory dysfunction in the light fraction. Good levels of polarization were detected in the heavy fraction, meanwhile the light population showed a collapsed ΔΨm. Increased levels of cytochrome P450, higher levels of hydrogen peroxide, and low cardiolipin content were described for the light fraction. Three pro-apoptotic proteins p53, Bax, and cytochrome c were found increased in the heavy mitochondrial fraction; and deficient in the light fraction. The heavy mitochondrial fraction showed an improved respiratory function. This mitochondrial fraction, being probably from cytotrophoblast cells showed higher content of proteins able to induce apoptosis, indicating that these cells can effectively execute an apoptotic program in the presence of a death stimulus. Meanwhile the light and small organelles probably from syncytiotrophoblast, with a low oxygen metabolism, low level of ΔΨm, and increased hydrogen peroxide production, may not actively perform an apoptotic process due to their deficient energetic level. This study contributes to the characterization of functional parameters of human placenta mitochondria in order to understand the oxygen metabolism during the physiological process of gestation.

Effect of exercise training on eNOS expression, NO production and oxygen metabolism in human placenta

Objective

To determine the effects of combined aerobic and resistance exercise training during the second half of pregnancy on endothelial NOS expression (eNOS), nitric oxide (NO) production and oxygen metabolism in human placenta.

Methods

The study included 20 nulliparous in gestational week 16–20, attending prenatal care at three tertiary hospitals in Colombia who were randomly assigned into one of two groups: The exercise group (n = 10) took part in an exercise session three times a week for 12 weeks which consisted of: aerobic exercise at an intensity of 55–75% of their maximum heart rate for 60 min and 25 mins. Resistance exercise included 5 exercise groups circuit training (50 repetitions of each) using barbells (1–3 kg/exercise) and low-to-medium resistance bands. The control group (n = 10) undertook their usual physical activity. Mitochondrial and cytosol fractions were isolated from human placental tissue by differential centrifugation. A spectrophotometric assay was used to measure NO production in cytosolic samples from placental tissue and Western Blot technique to determine eNOS expression. Mitochondrial superoxide levels and hydrogen peroxide were measured to determine oxygen metabolism.

Results

Combined aerobic and resistance exercise training during pregnancy leads to a 2-fold increase in eNOS expression and 4-fold increase in NO production in placental cytosol (p = 0.05). Mitochondrial superoxide levels and hydrogen peroxide production rate were decreased by 8% and 37% respectively in the placental mitochondria of exercising women (p = 0.05).

Conclusion

Regular exercise training during the second half of pregnancy increases eNOS expression and NO production and decreases reactive oxygen species generation in human placenta. Collectively, these data demonstrate that chronic exercise increases eNOS/NO production, presumably by increasing endothelial shear stress. This adaptation may contribute to the beneficial effects of exercise on the vascular and antioxidant system and in turn reduce the risk of preeclampsia, diabetes or hypertension during pregnancy.

5'-p-Fluorosulfonylbenzoyl adenosine inhibits progesterone synthesis in human placental mitochondria

The human placental mitochondria have an ATP-diphosphohydrolase (apyrase) activity. In this paper we characterized the effect of 5'-p-fluorosulfonylbenzoyl adenosine (FSBA) on placental apyrase, and its repercussion on progesterone synthesis and oxygen consumption. Apyrase activity was inhibited by FSBA. Nucleosides tri- and diphosphates protected against FSBA inactivation, but divalent cations did not, indicating that FSBA attaches itself to an ATP-binding site of apyrase. In mitochondria, the inactivation of apyrase by FSBA was associated with inhibition of progesterone synthesis. Also, the oxygen consumption induced by ATP but not by ADP, was inhibited, clearly showing that FSBA exclusively inactivated the apyrase in human placental mitochondria. It is concluded that the apyrase activity is closely related to progesterone synthesis, probably associated with the cholesterol transport between mitochondrial membranes.

Differential effects of magnesium on the hydrolysis of ADP and ATP in human term placenta. Effect of substrates and potassium

This study evaluates the effect of Mg2+ on the extramitochondrial hydrolysis of ATP and ADP by human term placental mitochondria (HPM) and submitochondrial particle (SMP). Extramitochondrial ATPase and ADPase activities were evaluated in the presence or absence of K+, and different oxidizable substrates. Mg2+ increased both ATP and ADP hydrolysis according to the experimental conditions, and this stimulation was related to the mitochondrial intactness. The ADPase activity in intact mitochondria is 100-fold higher in presence of K+, succinate and 1mM Mg2+ while this activity is only increased by two-fold on the SMP when compared to the sample without Mg2+. It is clearly demonstrated that up-regulation of these enzyme activities occur in intact mitochondria and not on the enzyme itself. The results suggest that the regulation of ATP and ADP hydrolysis is complex, and Mg2+ plays an important role in the modulation of the extramitochondrial ATPase and ADPase activities in HPM

A trypsin-sensitive protein is required for utilization of exogenous cholesterol for pregnenolone synthesis by placental mitochondria

The utilization of cholesterol for steroid hormone synthesis by human placental mitochondria is poorly understood. The human placenta does not express the steroidogenic acute regulator protein, which is critical for cholesterol delivery to the cholesterol side chain cleavage system in adrenal and gonadal mitochondria. We explored the mechanism underlying cholesterol transport in human placental mitochondria by measuring its transformation into pregnenolone. Mitochondria of syncytiotrophoblast from human term placenta were isolated by centrifugation through a sucrose gradient. The synthesis of pregnenolone in the presence of exogenous cholesterol was increased two-fold in syncytiotrophoblast mitochondria. Treatment of mitochondria with trypsin prevented the increase in the synthesis of pregnenolone in the presence of exogenous cholesterol. However, when 22-OH cholesterol, a substrate that readily crosses membranes, was added, the trypsin-treated mitochondria synthesized increased amounts of pregnenolone. The trypsin-treated mitochondria were intact, since oxygen consumption, succinate dehydrogenase and the adenine nucleotide translocase activities were not significantly different from in untreated mitochondria. However, activity of NADH cytochrome c oxidoreductase, an outer mitochondrial membrane enzyme, was reduced in the trypsin-treated mitochondria, reflecting the selective degradation of proteins. In addition, SDS-PAGE analysis revealed the loss of a prominent 34 kDa band which proved to be a novel porin-like protein that binds to cholesterol. These results support our previous assumption that human placental mitochondria employ a novel protein(s)-mediated the mechanism to take up cholesterol for steroidogenesis.

A novel ATP-diphosphohydrolase from human term placental mitochondria

This report describes an ATP-diphosphohydrolase activity associated with the inner membrane of human term placental mitochondria. An enriched fraction containing 30 per cent of the total protein and 80 per cent of the total ATP-diphosphohydrolase activity was obtained from submitochondrial particles. ATP-diphosphohydrolase activity was characterized in this fraction. The enzyme had a pH optimum of 8 and catalysed the hydrolysis of triphospho- and diphosphonucleosides other than ATP or ADP. Pyrophosphate was also hydrolysed, but AMP or other monoester phosphates were not. The activity of ATP-diphosphohydrolase was dependent on Mg(2 + ), Ca(2 + )or Mn(2 + )and the enzyme substrate was the cation-nucleotide complex. An excess of free cation produced inhibition.ATP-diphosphohydrolase activity was stimulated at micromolar concentrations of calcium or magnesium in the presence of La-PPi. Negative cooperativity kinetics was observed with all substrates tested. The V(max)ranged from 150 to 300nmol of Pi released/mg/min. The [S](0.5)for nucleotides was 1-10m m and 182m m for PPi. The enzyme was inhibited by orthovanadate, but not by l -phenylalanine, oligomycin, sodium azide, P(1),P(5)-di(adenosine-5')pentaphosphate or sodium fluoride.The experimental evidence showing absence of inhibition by sodium azide and sodium fluoride, hydrolysis of pyrophosphate but not of monoester phosphates, and negative cooperativity suggested that this enzyme was a novel ATP-diphosphohydrolase.

Presence of two enzymes, different from the F1F0-ATPase, hydrolyzing nucleotides in human term placental mitochondria

The hydrolysis of ATP, ADP or GTP was characterized in mitochondria and submitochondrial particles since a tightly-bound ATPase associated with the inner mitochondrial membrane from the human placenta has been described. Submitochondrial particles, which are basically inner membranes, were used to define the location of this enzyme. Mitochondria treated with trypsin and specific inhibitors were also used. The oxygen consumption stimulated by ATP or ADP was 100% inhibited in intact mitochondria by low concentrations of oligomycin (0.5 microgram/mg) or venturicidine (0.1 microgram/mg), while the hydrolysis of ATP or ADP was insensitive to higher concentrations of these inhibitors but it was inhibited by vanadate. Oligomycin or venturicidine showed a different inhibition pattern in intact mitochondria in relation to the hydrolysis of ATP, ADP or GTP. When submitochondrial particles were isolated from mitochondria incubated with oligomycin or venturicidine, no further inhibition of the nucleotide hydrolysis was observed, contrasting with the partial inhibition observed in the control. By incubating the placental mitochondria with trypsin, a large fraction of the hydrolysis of nucleotides was eliminated. In submitochondrial particles obtained from mitochondria treated with trypsin or trypsin plus oligomycin, the hydrolysis of ATP was 100% sensitive to oligomycin at low concentrations, resembling the oxygen consumption; however, this preparation still showed some ADP hydrolysis. Native gel electrophoresis showed two bands hydrolyzing ADP, suggesting at least two enzymes involved in the hydrolysis of nucleotides, besides the F1F0-ATPase. It is concluded that human placental mitochondria possesses ADPase and ATP-diphosphohydrolase activities (247).

Characterization of the F1F0-ATPase and the tightly-bound ATPase activities in submitochondrial particles from human term placenta

In a previous study we demonstrated the existence of a tightly-bound ATPase in the human placental mitochondria (Martínez et al., 1993). The current study characterizes the ATP hydrolysis produced by the F1F0-ATPase and the tightly-bound ATPase in submitochondrial particles from the human term placenta. Both enzymes were not differentiated by pH. Inhibitors were necessary to distinguish the activity of each enzyme. The kinetic of the total ATP hydrolysis fitted into a model of two enzymes. During the characterization, it was observed that the tightly-bound ATPase activity was partially inhibited by vanadate and Mg2+, whereas the F1F0-ATPase was totally inhibited by Mg2+. Different nucleotides were hydrolyzed by the tightly-bound ATPase; the F1F0-ATPase hydrolyzed exclusively ATP. Glucose-6-phosphate, p-nitrophenylphosphate, or pyrophosphate were not hydrolyzed by the F1F0-ATPase, although some hydrolysis was observed with the tightly-bound ATPase. It is concluded that the tightly-bound ATPase activity corresponded to a 5'-nucelotidase, and that the human placental mitochondria could participate in the metabolism of nucleotides.

The effect of osmolarity on human placental mitochondria function

Human placental explants survive large changes in osmolarity, but the mechanism for this property is unknown. The goal of this work was to examine the effect of osmolarity on human placental mitochondria. Mitochondria from human term placenta were isolated by differential centrifugation, and incubated in the presence of different concentrations of sucrose or KCl, to modify the osmolarity of the media. Rat liver mitochondria were used as control. The parameters studied were: respiration rate, adenine nucleotide hydrolysis, calcium transport, membrane potential, and mitochondrial morphology. Stimulation of the mitochondrial respiration rate and an increase in Ca2+ transport was observed in the presence of K+. With sucrose, Ca2+ transport showed a complex kinetic behavior, whereas the respiratory control was slightly diminished. Although the ATPase activity was enhanced in the absence of a respiratory substrate, no change in ATP hydrolysis due to osmolarity was observed. ADP hydrolysis was inhibited by a high K+ concentration, but not by sucrose. The membrane potential was not modified by osmolarity, even in the absence of sucrose or K+ in the medium. Mitochondria isolated with KCl showed aggregation, whereas dispersed mitochondria were observed with sucrose. This study showed that sucrose-induced changes in osmolarity, does not modify metabolic and transport properties of human placental mitochondria, whereas KCl-induced osmolarity changes does affect these functions.