An essential role for vicinal dithiol groups in the catalytic activity of the human placental Na(+)-H+ exchanger

Functional role and analysis of cysteine residues of the salt tolerance protein Sod2

Sod2 is the major salt tolerance plasma membrane protein of Schizosaccharomyces pombe. It functions to remove excess intracellular sodium (or lithium) in exchange for protons. We investigated the role of cysteine residues and created a cysteine-free Sod2 protein. Each cysteine residue of the ten present was individually mutated to serine and the different proteins expressed and characterized in S. pombe. Western blotting revealed that all the individual mutant proteins were expressed. We examined the ability of the mutant proteins to confer salt tolerance to S. pombe with the endogenous Sod2 protein deleted. Only proteins with C26S and C374S mutations were partially reduced in their ability to confer salt tolerance. Additionally, they showed a change in conformation in comparison to the wild-type protein, indicated by differential sensitivity to trypsin. Deletion of all the cysteine residues of Sod2 resulted in a functional protein that was expressed in S. pombe at levels similar to the wild type and also conferred salt tolerance. The conformation of the cysteine-free Sod2 protein was not altered relative to the wild-type protein. We examined the accessibility of amino acids of the cysteineless protein present on putative extracellular loop 2. A cysteine placed at position Ala119 was accessible to externally applied [2-(trimethylammonium)ethyl] methane thiosulfonate bromide. The results demonstrate that cysteines in the Sod2 protein can be changed to serine residues resulting in an expressed, functional protein. The utility of the cysteine-free Sod2 protein for determination of topology and amino acid accessibility is demonstrated.

Chronic alcoholism alters systemic and pulmonary glutathione redox status

RATIONALE

Previous studies have linked the development and severity of acute respiratory distress syndrome with a history of alcohol abuse. In clinical studies, this association has been centered on depletion of pulmonary glutathione and subsequent chronic oxidant stress.

OBJECTIVES

The impact on redox potential of the plasma or pulmonary pools, however, has never been reported.

METHODS

Plasma and bronchoalveolar lavage fluid were collected from otherwise healthy alcohol-dependent subjects and control subjects matched by age, sex, and smoking history.

MEASUREMENTS AND MAIN RESULTS

Redox potential was calculated from measured reduced and oxidized glutathione in plasma and lavage. Among subjects who did and did not smoke, lavage fluid glutathione redox potential was more oxidized in alcohol abusers by approximately 40 mV, which was not altered by dilution. This oxidation of the airway lining fluid associated with chronic alcohol abuse was independent of smoking history. A shift by 20 mV in plasma glutathione redox potential, however, was noted only in subjects who both abused alcohol and smoked.

CONCLUSIONS

Chronic alcoholism was associated with alveolar oxidation and, with smoking, systemic oxidation. However, systemic oxidation did not accurately reflect the dramatic alcohol-induced oxidant stress in the alveolar space. Although there was compensation for the oxidant stress caused by smoking in control groups, the capacity to maintain a reduced environment in the alveolar space was overwhelmed in those who abused alcohol. The significant alcohol-induced chronic oxidant stress in the alveolar space and the subsequent ramifications may be an important modulator of the increased incidence and severity of acute respiratory distress syndrome in this vulnerable population.

Chronic alcoholism alters systemic and pulmonary glutathione redox status

RATIONALE

Previous studies have linked the development and severity of acute respiratory distress syndrome with a history of alcohol abuse. In clinical studies, this association has been centered on depletion of pulmonary glutathione and subsequent chronic oxidant stress.

OBJECTIVES

The impact on redox potential of the plasma or pulmonary pools, however, has never been reported.

METHODS

Plasma and bronchoalveolar lavage fluid were collected from otherwise healthy alcohol-dependent subjects and control subjects matched by age, sex, and smoking history.

MEASUREMENTS AND MAIN RESULTS

Redox potential was calculated from measured reduced and oxidized glutathione in plasma and lavage. Among subjects who did and did not smoke, lavage fluid glutathione redox potential was more oxidized in alcohol abusers by approximately 40 mV, which was not altered by dilution. This oxidation of the airway lining fluid associated with chronic alcohol abuse was independent of smoking history. A shift by 20 mV in plasma glutathione redox potential, however, was noted only in subjects who both abused alcohol and smoked.

CONCLUSIONS

Chronic alcoholism was associated with alveolar oxidation and, with smoking, systemic oxidation. However, systemic oxidation did not accurately reflect the dramatic alcohol-induced oxidant stress in the alveolar space. Although there was compensation for the oxidant stress caused by smoking in control groups, the capacity to maintain a reduced environment in the alveolar space was overwhelmed in those who abused alcohol. The significant alcohol-induced chronic oxidant stress in the alveolar space and the subsequent ramifications may be an important modulator of the increased incidence and severity of acute respiratory distress syndrome in this vulnerable population.

Glutathione availability modulates alveolar macrophage function in the chronic ethanol-fed rat

We have previously demonstrated that chronic alcohol exposure decreases glutathione in the alveolar space. Although alcohol use is associated with decreased alveolar macrophage function, the mechanism by which alcohol impairs macrophage phagocytosis is unknown. In the current study, we examined the possibility that ethanol-induced alveolar macrophage dysfunction was secondary to decreased glutathione and subsequent chronic oxidative stress in the alveolar space. After 6 wk of ethanol ingestion, oxidant stress in the alveolar macrophages was evidenced by a 30-mV oxidation of the GSH/GSSG redox potential (P <or= 0.05). For control macrophages, approximately 80% internalized fluorescent Staphylococcus aureus were added in vitro. In contrast, only 20% of the macrophages from the ethanol-fed rats were able to bind and internalize fluorescent S. aureus. This ethanol-induced decreased capacity for phagocytosis was paralleled by increased apoptosis. When added to the ethanol diet, the glutathione precursors procysteine or N-acetyl cysteine normalized glutathione and oxidant stress in the epithelial lining fluid as well as the alveolar macrophages to control values. This attenuation of oxidant stress was associated with normalization of macrophage phagocytosis and viability. These results suggested that decreased glutathione availability in the alcoholic lung contribute to alveolar macrophage dysfunction via oxidative stress, resulting in not only decreased function but decreased viability.

DNA binding activity of the aryl hydrocarbon receptor is sensitive to redox changes in intact cells

The potential involvement of vicinal dithiols in the transformation of the aryl hydrocarbon (Ah) receptor from its ligand binding to DNA binding form in Hepa-1 cells was explored through the use of diamide and phenylarsine oxide (PAO), which have been shown to specifically form a stable ring complex with vicinal sulfhydryl groups in selected proteins. Pretreatment with diamide and PAO rapidly prevented the inducer-dependent formation of the Ah receptor/xenobiotic response element complex detected by electrophoretic mobility shift assays and suppressed Ah receptor-mediated transcription. Diamide and PAO also inhibited DNA binding activity of the nuclear Ah receptor subsequent to its translocation to the nucleus but to a lesser extent than that observed with pretreatment conditions. The Ah receptor exhibited much higher sensitivity to cellular redox changes than Sp1, a transcription factor previously shown to be very sensitive to redox regulation. Diamide added to nuclear extracts inhibited Ah receptor DNA binding more than when it was added in intact cells. In contrast, Ah receptor DNA binding activity was more sensitive to PAO when it was added to intact cells than when it was added to nuclear extracts. Finally, dithiol 2,3-dimercaptopropanol was over 100 times more effective than monothiol 2-mercaptoethanol in reversing the PAO-dependent inhibition of Ah receptor DNA binding activity. This suggests that vicinal sulfhydryl residues may be involved in DNA binding of the Ah receptor.

Functional role of cysteine residues in the Na+/H+ exchanger effects of mutation of cysteine residues on targeting and activity of the Na+/H+ exchanger

We investigated the role of cysteine residues in activity and localization of the NHE1 isoform of the Na+/H+ exchanger. Each of the nine cysteine residues was mutated to serine or arginine. Mutation of the first serine (amino acid number 9) and serine number six (amino acid number 477) resulted in dramatic decreases in detectable activity of the Na+/H+ exchanger when transfected into AP-1 cells. Some other mutations resulted in minor decreases in activity of the protein. Confocal and light microscopy of mutant cells with decreased activity showed that the antiporter protein was mostly retained in an intracellular compartment which colocalized with the medial-Golgi cisternae. Smaller amounts of active protein still remained targeted to the plasma membrane in these mutants. Treatment of wild-type cells with DTT also caused the retention of the Na+/H+ exchanger to the same intracellular compartment. The results suggest that cysteines play an important role in intracellular folding and trafficking of the Na+/H+ exchanger.

Sodium-dependent high-affinity binding of carnitine to human placental brush border membranes

The interaction of carnitine with human placental brush-border membrane vesicles was investigated. Carnitine was found to associate with the membrane vesicles in a Na(+)-dependent manner. The time course of this association did not exhibit an overshoot, which is typical of a Na+ gradient-driven transport process. The absolute requirement for Na+ was noticeable whether the association of carnitine with the vesicles was measured with a short time incubation or under equilibrium conditions, indicating Na(+)-dependent binding of carnitine to the human placental brush-border membranes. The binding was saturable and was of a high-affinity type with a dissociation constant of 1.37 +/- 0.03 microM. Anions had little or no influence on the binding process. The binding process was specific for carnitine and its acyl derivatives. Betaine also competed for the binding process, but other structurally related compounds did not. Kinetic analyses revealed that Na+ increased the affinity of the binding process for carnitine and the Na+/carnitine coupling ratio for the binding process was 1. The dissociation constant for the interaction of Na+ with the binding of carnitine was 24 +/- 4 mM. This constitutes the first report on the identification of Na(+)-dependent high-affinity carnitine binding in the plasma membrane of a mammalian cell. Studies with purified rat renal brush-border membrane vesicles demonstrated the presence of Na+ gradient-driven carnitine transport but no Na(+)-dependent carnitine binding in these membrane vesicles. In contrast, purified intestinal brush-border membrane vesicles posses neither Na+ gradient-driven carnitine transport nor Na(+)-dependent carnitine binding.

Effects of the protein tyrosine phosphatase inhibitor phenylarsine oxide on excision-activated calcium channels in Lymnaea neurons

Most calcium channels are tightly regulated, closed in the resting cell, and open only in response to specific physiological signals such as depolarization or binding of a particular ligand. In addition, calcium permeable channels which can be activated experimentally by excising a small patch of plasma membrane from the cell have been described in several preparations, including neurons and cardiac muscle. Little is known about possible physiological regulators of these channels. We examined an excision-activated calcium channel from neurons of the pond snail Lymnaea stagnalis. This channel, the 'HP channel', is divalent selective and voltage-independent. In this report, we show that excision activation can occur very rapidly (within 200-400 ms after patch excision), and that this activity can be at least partially inhibited by 'cramming' the isolated membrane patch back into the cell's cytoplasm. We also show that excision activation is inhibited in cells which have been pretreated with inhibitors of protein tyrosine phosphatases, either pervanadate (0.5 mM) or phenylarsine oxide (1-7 microM). The effect of phenylarsine oxide is not seen in cells which have been pretreated with tyrosine kinase inhibitors (genistein or herbimycin A). The results suggest that tyrosine phosphorylation signalling pathways may play a role in the physiological regulation of these channels.

Characterization of the placental brush border membrane Na+/H+ exchanger: identification of thiol-dependent transitions in apparent molecular size

We examined the protein and mRNA encoding the amiloride-sensitive Na+/H+ exchanger from human placenta. Reverse transcriptase PCR of human placental RNA and a human choriocarcinoma cell line showed that the message for the amiloride-sensitive Na+/H+ exchanger from human placenta. Reverse transcriptase PCR of human placental RNA and a human choriocarcinoma cell line showed that the message for the amiloride-sensitive Na+/H+ exchanger is present in the placenta and its derived cell line. Northern blot analysis showed only one species of Na+/H+ exchanger mRNA, of about 5 kb in size. To examine the Na+/H+ exchanger protein two different affinity-purified antibodies were produced against the C-terminal cytoplasmic region of the Na+/H+ exchanger. The antibodies both identified a 105 kDa protein in human placental brush border membrane vesicles. Under non-reducing conditions the amount of 105 kDa protein was greatly decreased, while a 205 kDa protein became apparent. This is probably a dimer of the 105 kDa protein. The monomer-to-dimer transition was dependent on the concentration of beta-mercaptoethanol. The results show that the amiloride-sensitive Na+/H+ exchanger is relatively abundant in human placenta and that it can exist as a larger 205 kDa protein linked by disulphide bonds.