Ionic permeability of isolated muscle sarcoplasmic reticulum and liver endoplasmic reticulum vesicles

Guanosine 5'-[gamma-thio]triphosphate-mediated activation of cytosol phospholipase C caused lysosomal destabilization

Lysosomal disintegration is critical for the organelle functions and cellular viability. In this study, we established that guanosine 5'-[gamma-thio]triphosphate (GTP-gamma-S)-activated cytosol of rat hepatocytes could increase lysosomal permeability to both potassium ions and protons and osmotically destabilize the lysosomes via K(+)/H(+) exchange. These results were obtained through measurements of lysosomal beta-hexosaminidase-free activity, membrane potential and intralysosomal pH. Assays of phospholipase C (PLC) activity show that cytosolic PLC was activated upon addition of GTP-gamma-S to the cytosol. The effects of cytosol on the lysosomes could be abolished by D609, an inhibitor of PLC, but not by the inhibitors of phospholipase A(2). The cytosol-treated lysosomes disintegrated markedly in hypotonic sucrose medium, reflecting that the lysosomal osmotic sensitivity increased. Microscopic observations showed that the lysosomes became more swollen in hypotonic sucrose medium. This indicates that the cytosol treatment induced osmotic shock to the lysosomes and an influx of water into the organelle.

Translocon pores in the endoplasmic reticulum are permeable to small anions

Contribution of translocon peptide channels to the permeation of low molecular mass anions was investigated in rat liver microsomes. Puromycin, which purges translocon pores of nascent polypeptides, creating additional empty pores, raised the microsomal uptake of radiolabeled UDP-glucuronic acid, while it did not increase the uptake of glucose-6-phosphate or glutathione. The role of translocon pores in the transport of small anions was also investigated by measuring the effect of puromycin on the activity of microsomal enzymes with intraluminal active sites. The mannose-6-phosphatase activity of glucose-6-phosphatase and the activity of UDP-glucuronosyltransferase were elevated upon addition of puromycin, but glucose-6-phosphatase and beta-glucuronidase activities were not changed. The increase in enzyme activities was due to a better access of the substrates to the luminal compartment rather than to activation of the enzymes. Antibody against Sec61 translocon component decreased the activity of UDP-glucuronosyltransferase and antagonized the effect of puromycin. Similarly, the addition of the puromycin antagonist anisomycin or treatments of microsomes, resulting in the release of attached ribosomes, prevented the puromycin-dependent increase in the activity. Mannose-6-phosphatase and UDP-glucuronosyltransferase activities of smooth microsomal vesicles showed higher basal latencies that were not affected by puromycin. In conclusion, translationally inactive, ribosome-bound translocons allow small anions to cross the endoplasmic reticulum membrane. This pathway can contribute to the nonspecific substrate supply of enzymes with intraluminal active centers.

Evidence for the transport of glutathione through ryanodine receptor channel type 1

In the present study, we have investigated the role of RyR1 (ryanodine receptor calcium channel type 1) in glutathione (GSH) transport through the sarcoplasmic reticulum (SR) membrane of skeletal muscles. Lanthanum chloride, a prototypic blocker of cation channels, inhibited the influx and efflux of GSH in SR vesicles. Using a rapid-filtration-based assay and lanthanum chloride as a transport blocker, an uptake of radiolabelled GSH into SR vesicles was observed. Pretreatment of SR vesicles with the RyR1 antagonists Ruthenium Red and ryanodine as well as with lanthanum chloride blocked the GSH uptake. An SR-like GSH uptake appeared in microsomes obtained from an HEK-293 (human embryonic kidney 293) cell line after transfection of RyR1. These observations strongly suggest that RyR1 mediates GSH transport through the SR membranes of skeletal muscles.

The endoplasmic reticulum membrane is permeable to small molecules

The lumen of the endoplasmic reticulum (ER) differs from the cytosol in its content of ions and other small molecules, but it is unclear whether the ER membrane is as impermeable as other membranes in the cell. Here, we have tested the permeability of the ER membrane to small, nonphysiological molecules. We report that isolated ER vesicles allow different chemical modification reagents to pass from the outside into the lumen with little hindrance. In permeabilized cells, the ER membrane allows the passage of a small, charged modification reagent that is unable to cross the plasma membrane or the lysosomal and trans-Golgi membranes. A larger polar reagent of approximately 5 kDa is unable to pass through the ER membrane. Permeation of the small molecules is passive because it occurs at low temperature in the absence of energy. These data indicate that the ER membrane is significantly more leaky than other cellular membranes, a property that may be required for protein folding and other functions of the ER.

T cell receptor can be recruited to a subset of plasma membrane rafts, independently of cell signaling and attendantly to raft clustering

The constitutive/inducible association of the T cell receptor (TCR) with isolated detergent-resistant, lipid raft-derived membranes has been studied in Jurkat T lymphocytes. Membranes resistant to 1% Triton X-100 contained virtually no CD3epsilon, part of the TCR complex, irrespective of cell stimulation. On the other hand, membranes resistant either to a lower Triton X-100 concentration (i.e. 0.2%) or to the less hydrophobic detergent Brij 58 (1%) contained (i) a low CD3epsilon amount (approximate 2.7% of total) in resting cells and (ii) a several times higher amount of the TCR component, after T cell stimulation with either antigen-presenting cells or with phytohemagglutinin. It appeared that CD3/TCR was constitutively associated with and recruited to a raft-derived membrane subset because (i) all three membrane preparations contained a similar amount of the raft marker tyrosine kinase Lck but no detectable amounts of the conventional membrane markers, CD45 phosphatase and transferrin receptor; (ii) a larger amount of particulate membranes were resistant to solubilization with 0.2% Triton X-100 and Brij 58 than to solubilization with 1% Triton X-100; and (iii) higher cholesterol levels were present in membranes resistant to either the lower Triton X-100 concentration or to Brij 58, as compared with those resistant to 1% Triton X-100. The recruitment of CD3 to the raft-derived membrane subset appeared (i) to occur independently of cell signaling events, such as protein-tyrosine phosphorylation and Ca(2+) mobilization/influx, and (ii) to be associated with clustering of plasma membrane rafts induced by multiple cross-linking of either TCR or the raft component, ganglioside GM(1). We suggest that during T cell stimulation a lateral reorganization of rafts into polarized larger domains can determine the recruitment of TCR into these domains, which favors a polarization of the signaling cascade.

Glutathione transport in the endo/sarcoplasmic reticulum

Glutathione transport through the endo/sarcoplasmic reticulum (ER/SR) membrane might play a role in the maintenance of the thiol redox potential difference between the lumen and the cytosol. The transport of glutathione (both GSH and glutathione disulfide, GSSG) is entirely different in the ER and SR membranes. The transport measurements based on either rapid filtration or light scattering techniques revealed that the SR membrane transports glutathione much faster than the hepatic ER membrane or microsomal membranes prepared from heart or brain. The fastest transport has been measured in the membrane of muscle terminal cisternae, which is enriched in ryanodine receptor type 1 (RyR1). All the studied membranes have been found to be equally impermeable to various hydrophilic substances of similar size to glutathione, thus the glutathione transport in muscle microsomes and terminal cysternae as well as the correlation between the rate of glutathione transport and the abundance of RyR1 are specific. In both muscle microsomes and terminal cysternae, glutathione influx can be either inhibited or activated by antagonists and agonists of the ryanodine receptor, respectively, while these agents do not influence the transport of other small permeant molecules. These findings strongly suggest that the ryanodine receptor channel activity is directly associated with glutathione transport activity in the skeletal muscle sarcoplasmic reticulum membrane.

Rat liver glucose-6-phosphatase system: light scattering and chemical characterization

Glucose-6-phosphatase is a multicomponent system located in the endoplasmic reticulum, involving both a catalytic subunit (G6PC) and several substrate and product carriers. The glucose-6-phosphate carrier is called G6PT1. Using light scattering, we determined K(D) values for phosphate and glucose transport in rat liver microsomes (45 and 33mM, respectively), G6PT1 K(D) being too low to be estimated by this technique. We provide evidence that phosphate transport may be carried out by an allosteric multisubunit translocase or by two distinct proteins. Using chemical modifications by sulfhydryl reagents with different solubility properties, we conclude that in G6PT1, one thiol group important for activity is facing the cytosol and could be Cys(121) or Cys(362). Moreover, a different glucose-6-phosphate translocase, representing 20% of total glucose-6-phosphate transport and insensitive to N-ethylmaleimide modification, could coexist with liver G6PT1. In the G6PC protein, an accessible thiol group is facing the cytosol and, according to structural predictions, could be Cys(284).

Histone 2A stimulates glucose-6-phosphatase activity by permeabilization of liver microsomes

Histone 2A increases glucose-6-phosphatase activity in liver microsomes. The effect has been attributed either to the conformational change of the enzyme, or to the permeabilization of microsomal membrane that allows the free access of substrate to the intraluminal glucose-6-phosphatase catalytic site. The aim of the present study was the critical reinvestigation of the mechanism of action of histone 2A. It has been found that the dose-effect curve of histone 2A is different from that of detergents and resembles that of the pore-forming alamethicin. Inhibitory effects of EGTA on glucose-6-phosphatase activity previously reported in histone 2A-treated microsomes have been also found in alamethicin-permeabilized vesicles. The effect of EGTA cannot therefore simply be an antagonization of the effect of histone 2A. Histone 2A stimulates the activity of another latent microsomal enzyme, UDP-glucuronosyltransferase, which has an intraluminal catalytic site. Finally, histone 2A renders microsomal vesicles permeable to non-permeant compounds. Taken together, the results demonstrate that histone 2A stimulates glucose-6-phosphatase activity by permeabilizing the microsomal membrane.

The glucose-6-phosphatase system

Glucose-6-phosphatase (G6Pase), an enzyme found mainly in the liver and the kidneys, plays the important role of providing glucose during starvation. Unlike most phosphatases acting on water-soluble compounds, it is a membrane-bound enzyme, being associated with the endoplasmic reticulum. In 1975, W. Arion and co-workers proposed a model according to which G6Pase was thought to be a rather unspecific phosphatase, with its catalytic site oriented towards the lumen of the endoplasmic reticulum [Arion, Wallin, Lange and Ballas (1975) Mol. Cell. Biochem. 6, 75--83]. Substrate would be provided to this enzyme by a translocase that is specific for glucose 6-phosphate, thereby accounting for the specificity of the phosphatase for glucose 6-phosphate in intact microsomes. Distinct transporters would allow inorganic phosphate and glucose to leave the vesicles. At variance with this substrate-transport model, other models propose that conformational changes play an important role in the properties of G6Pase. The last 10 years have witnessed important progress in our knowledge of the glucose 6-phosphate hydrolysis system. The genes encoding G6Pase and the glucose 6-phosphate translocase have been cloned and shown to be mutated in glycogen storage disease type Ia and type Ib respectively. The gene encoding a G6Pase-related protein, expressed specifically in pancreatic islets, has also been cloned. Specific potent inhibitors of G6Pase and of the glucose 6-phosphate translocase have been synthesized or isolated from micro-organisms. These as well as other findings support the model initially proposed by Arion. Much progress has also been made with regard to the regulation of the expression of G6Pase by insulin, glucocorticoids, cAMP and glucose.

Ryanodine receptor channel-dependent glutathione transport in the sarcoplasmic reticulum of skeletal muscle

We found that glutathione transport across endo/sarcoplasmic reticulum membranes correlates with the abundance of ryanodine receptor type 1 (RyR1). The transport was the fastest in muscle terminal cisternae, fast in muscle microsomes and slow in liver, heart, and brain microsomes. Glutathione influx could be inhibited by RyR1 blockers and the inhibitory effect was counteracted by RyR1 agonists. The effect of blockers was specific to glutathione, as the transport of other small molecules was not hindered. Therefore, the glutathione transport activity seems to be associated with RyR1 in sarcoplasmic reticulum.

Transmembrane redox sensor of ryanodine receptor complex

Inositol 1,4,5-trisphosphate receptors (IP(3)R) and ryanodine receptors (RyR) mediate the release of endoplasmic and sarcoplasmic reticulum (ER/SR) Ca(2+) stores and regulate Ca(2+) entry through voltage-dependent or ligand-gated channels of the plasma membrane. A prominent property of ER/SR Ca(2+) channels is exquisite sensitivity to sulfhydryl-modifying reagents. A plausible role for sulfhydryl chemistry in physiologic regulation of Ca(2+) release channels and the fidelity of Ca(2+) release from ER/SR is lacking. This study reveals the existence of a transmembrane redox sensor within the RyR1 channel complex that confers tight regulation of channel activity in response to changes in transmembrane redox potential produced by cytoplasmic and luminal glutathione. A transporter selective for glutathione is co-localized with RyR1 within the SR membrane to maintain local redox potential gradients consistent with redox regulation of ER/SR Ca(2+) release. Hyperreactive sulfhydryls previously shown to reside within the RyR1 complex (Liu, G., and Pessah, I. N. (1994) J. Biol. Chem. 269, 33028-33034) are an essential biochemical component of a transmembrane redox sensor. Transmembrane redox sensing may represent a fundamental mechanism by which ER/SR Ca(2+) channels respond to localized changes in transmembrane glutathione redox potential produced by physiologic and pathophysiologic modulators of Ca(2+) release from stores.

N-Bromoacetylethanolamine phosphate as a probe for the identification of a liver microsomal glucose-6-phosphate transporter peptide in rats and Ehrlich ascites tumor-bearing mice

Hepatic microsomal glucose-6-phosphatase is a multicomponent system composed of substrate/product translocases and a catalytic subunit. Previously we (Foster et al. (1996) Biochim. Biophys. Acta 12, 244-254) demonstrated that N-bromoacetylethanolamine phosphate (BAEP) is a time-dependent, irreversible inhibitor of glucose-6-phosphate hydrolysis in intact but not disrupted microsomes. We proposed that BAEP manifests its inhibitory effect by binding with a glucose-6-phosphate translocase protein of the glucose-6-phosphatase system. Here we provide additional evidence that BAEP inhibits glucose-6-phosphate transport in microsomal vesicles and utilize [(32)P]BAEP as an affinity label in the identification of a glucose-6-phosphate transport protein. In this study, we identify 51-kDa rat and mouse liver microsomal proteins involved in glucose-6-phosphate transport into and out of microsomal vesicles by utilizing (1) an Ehrlich ascites tumor-bearing mouse model, which displays a decreased sensitivity to the time-dependent inhibitory effect of BAEP, and (2) another glucose-6-phosphate translocase inhibitor, tosyl-lysine chloromethyl ketone, in conjunction with [(32)P]BAEP as an affinity label.

Beta-glucuronidase latency in isolated murine hepatocytes

The physiological function of microsomal beta-glucuronidase is unclear. Substrates may be either glucuronides produced in the lumen of endoplasmic reticulum (ER) or those taken up by hepatocytes. In the latter case, efficient inward transport of glucuronides at the plasma membrane and the ER membrane would be required. Therefore, the potential role of beta-glucuronidase in ER was investigated. Isolated mouse hepatocytes and mouse and rat liver microsomal vesicles were used in the experiments. Selective permeabilization of the plasma membrane of isolated hepatocytes with saponin or digitonin resulted in an almost 4-fold elevation in the rate of beta-nitrophenol glucuronide hydrolysis, while the permeabilization of plasma membrane plus ER membrane by Triton X-100 caused a further 2-fold elevation. In microsomal vesicles, the p-nitrophenol glucuronide or phenolphthalein glucuronide beta-glucuronidase activity showed about 50% latency as revealed by alamethicin or Triton X-100 treatment. A light-scattering study indicated that the microsomes are relatively impermeable to both glucuronides and to glucuronate. On the basis of our results, the role of liver microsomal beta-glucuronidase in the deconjugation of glucuronides taken up by the liver seems unlikely. Hydrolysis of the glucuronides produced in the ER lumen may play a role in substrate supply for ascorbate synthesis or in "proofreading" of glucuronidation.

Preferential transport of glutathione versus glutathione disulfide in rat liver microsomal vesicles

A bi-directional, saturable transport of glutathione (GSH) was found in rat liver microsomal vesicles. GSH transport could be inhibited by the anion transport blockers flufenamic acid and 4, 4'-diisothiocyanostilbene-2,2'-disulfonic acid. A part of GSH taken up by the vesicles was metabolized to glutathione disulfide (GSSG) in the lumen. Microsomal membrane was virtually nonpermeable toward GSSG; accordingly, GSSG generated in the microsomal lumen could hardly exit. Therefore, GSH transport, contrary to previous assumptions, is preferred in the endoplasmic reticulum, and GSSG entrapped and accumulated in the lumen creates the oxidized state of its redox buffer.

Conformational change of the catalytic subunit of glucose-6-phosphatase in rat liver during the fetal-to-neonatal transition

The glucose-6-phosphatase system was investigated in fetal rat liver microsomal vesicles. Several observations indicate that the orientation of the catalytic subunit is different in the fetal liver in comparison with the adult form: (i) the phosphohydrolase activity was not latent using glucose-6-phosphate as substrate, and in the case of other phosphoesters it was less latent; (ii) the intravesicular accumulation of glucose upon glucose-6-phosphate hydrolysis was lower; (iii) the size of the intravesicular glucose-6-phosphate pool was independent of the glucose-6-phosphatase activities; (iv) antibody against the loop containing the proposed catalytic site of the enzyme inhibited the phosphohydrolase activity in fetal but not in adult rat liver microsomes. Glucose-6-phosphate, phosphate, and glucose uptake could be detected by both light scattering and/or rapid filtration method in fetal liver microsomes; however, the intravesicular glucose-6-phosphate and glucose accessible spaces were proportionally smaller than in adult rat liver microsomes. These data demonstrate that the components of the glucose-6-phosphatase system are already present, although to a lower extent, in fetal liver, but they are functionally uncoupled by the extravesicular orientation of the catalytic subunit.

Heterogeneity of glucose transport in rat liver microsomal vesicles

Glucose transport across the membrane of rat liver microsomal vesicles was studied by a rapid filtration method in three different experimental systems: (i) inward transport in the presence of extravesicular glucose, (ii) efflux from passively preloaded vesicles, and (iii) efflux of glucose generated intravesicularly by glucose-6-phosphatase upon addition of glucose 6-phosphate were investigated. The apparent intravesicular glucose space estimated with the rapid filtration method was lower than the total microsomal glucose accessible space both the in the steady-state phase of uptake and at the starting point of efflux: 0.5 versus 2.3 microl/mg protein. The initial rate of influx/efflux was dependent on the extravesicular/intravesicular glucose concentration and was much lower than the rate of influx estimated previously by the light-scattering technique. Both influx and efflux could be inhibited by N-ethylmaleimide and possibly became saturable at high (>100 mM) glucose concentration. Known inhibitors of GLUT transporters (genistein, cytochalasin B, phloretin, and hexoses) did not affect glucose influx. The time course of glucose efflux from vesicles preincubated in the presence of glucose 6-phosphate was similar to that from glucose-loaded vesicles. These data together with that obtained previously (by a light-scattering technique; Marcolongo, P., Fulceri, R., Giunti, R., Burchell, A., and Benedetti, A. (1996) Biochem. Biophys. Res. Commun. 219, 916-922) indicate that microsomal vesicles are heterogeneous regarding their glucose-transporting properties and that glucose transport is bidirectional and its feature meets the requirements of a facilitative transport.

Three thiol groups are important for the activity of the liver microsomal glucose-6-phosphatase system. Unusual behavior of one thiol located in the glucose-6-phosphate translocase

Liver microsomal glucose-6-phosphatase (Glc-6-Pase) is a multicomponent system involving both substrate and product carriers and a catalytic subunit. We have investigated the inhibitory effect of N-ethylmaleimide (NEM), a rather specific sulfhydryl reagent, on rat liver Glc-6-Pase activity. Three thiol groups are important for Glc-6-Pase system activity. Two of them are located in the glucose-6-phosphate (Glc-6-P) translocase, and one is located in the catalytic subunit. The other transporters (phosphate and glucose) are not affected by NEM treatment. The NEM alkylation of the catalytic subunit sulfhydryl residue is prevented by preincubating the disrupted microsomes with saturating concentrations of substrate or product. This suggests either that the modified cysteine is located in the protein active site or that substrate binding hides the thiol group via a conformational change in the enzyme structure. Two other thiols important for the Glc-6-Pase system activity are located in the Glc-6-P translocase and are more reactive than the one located in the catalytic subunit. The study of the NEM inhibition of the translocase has provided evidence of the existence of two distinct areas in the protein that can behave independently, with conformational changes occurring during Glc-6-P binding to the transporter. The recent cloning of a human putative Glc-6-P carrier exhibiting homologies with bacterial phosphoester transporters, such as Escherichia coli UhpT (a Glc-6-P translocase), is compatible with the fact that two cysteine residues are important for the bacterial Glc-6-P transport.

Gulonolactone oxidase activity-dependent intravesicular glutathione oxidation in rat liver microsomes

The orientation of gulonolactone oxidase activity was investigated in rat liver microsomes. Ascorbate formation upon gulonolactone addition resulted in higher intravesicular than extravesicular ascorbate concentrations in native microsomal vesicles. The intraluminal ascorbate accumulation could be prevented or the accumulated ascorbate could be released by permeabilising the vesicles with the pore-forming alamethicin. The formation of the other product of the enzyme, hydrogen peroxide caused the preferential oxidation of intraluminal glutathione in glutathione-loaded microsomes. In conclusion, these results suggest that the orientation of the active site of gulonolactone oxidase is intraluminal and/or the enzyme releases its products towards the lumen of the endoplasmic reticulum.

Tungstate: a potent inhibitor of multifunctional glucose-6-phosphatase

The insulin-like action of tungstate in diabetic rats (A. Barberà et al., 1994, J. Biol. Chem. 269, 20047-20053) prompted us to examine the effects of tungstate on the glucose-6-phosphatase system. Our results indicate that tungstate is a potent inhibitor of glucose-6-phosphatase, with a Ki in the 10-25 microM range determined with native microsomes and in the 1-7 microM range determined with detergent-treated microsomes. With both preparations, simple linear competitive inhibition was observed versus glucose 6-phosphate (glucose-6-P) as substrate with the glucose-6-P phosphohydrolase activity of the enzyme. Tungstate was a simple linear competitive inhibitor versus carbamyl phosphate (carbamyl-P) and a linear noncompetitive inhibitor versus glucose with the carbamyl-P:glucose phosphotransferase activity of the glucose-6-phosphatase system. These findings, in addition to the observation that tungstate protected the enzyme against thermal inactivation, indicate that tungstate binds with high affinity and competes at the active site of the enzyme where the substrates glucose-6-P and carbamyl-P bind prior to catalysis. Our results suggest that potent inhibition of glucose-6-P hydrolysis by tungstate is likely responsible, at least in part, for the normalization of glycemia and the rebound in hepatic glucose-6-P levels observed in earlier studies in which tungstate exhibited insulin-like action in diabetic rats.

Dehydroascorbate and ascorbate transport in rat liver microsomal vesicles

Ascorbate and dehydroascorbate transport was investigated in rat liver microsomal vesicles using radiolabeled compounds and a rapid filtration method. The uptake of both compounds was time- and temperature-dependent, and saturable. Ascorbate uptake did not reach complete equilibrium, it had low affinity and high capacity. Ascorbate influx could not be inhibited by glucose, dehydroascorbate, or glucose transport inhibitors (phloretin, cytochalasin B) but it was reduced by the anion transport inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and by the alkylating agent N-ethylmaleimide. Ascorbate uptake could be stimulated by ferric iron and could be diminished by reducing agents (dithiothreitol, reduced glutathione). In contrast, dehydroascorbate uptake exceeded the level of passive equilibrium, it had high affinity and low capacity. Glucose cis inhibited and trans stimulated the uptake. Glucose transport inhibitors were also effective. The presence of intravesicular reducing compounds increased, while extravesicular reducing environment decreased dehydroascorbate influx. Our results suggest that dehydroascorbate transport is preferred in hepatic endoplasmic reticulum and it is mediated by a GLUT-type transporter. The intravesicular reduction of dehydroascorbate leads to the accumulation of ascorbate and contributes to the low intraluminal reduced/oxidized glutathione ratio.

Liver microsomal transport of glucose-6-phosphate, glucose, and phosphate in type 1 glycogen storage disease

The transport of glucose-6-phosphate (G6P), glucose, and orthophosphate into liver microsomes, isolated from six patients with various subtypes of type 1 glycogen storage disease (GSD), was measured using a light-scattering method. We found that G6P, glucose, and phosphate could all cross the microsomal membrane, in four cases of type 1a GSD. In contrast, liver microsomal transport of G6P and phosphate was deficient in the GSD 1b and 1c patients, respectively. These results support the involvement of multiple proteins (and genes) in GSD type 1. The results obtained with the light-scattering method are in accordance with conventional kinetic analysis of the microsomal glucose-6-phosphatase system. Therefore, this technique could be used to directly diagnose type 1b and 1c GSD.

Demonstration of a metabolically active glucose-6-phosphate pool in the lumen of liver microsomal vesicles

Glucose-6-phosphate transport was investigated in rat or human liver microsomal vesicles using rapid filtration and light-scattering methods. Upon addition of glucose-6-phosphate, rat liver microsomes accumulated the radioactive tracer, reaching a steady-state level of uptake. In this phase, the majority of the accumulated tracer was glucose, but a significant intraluminal glucose-6-phosphate pool could also be observed. The extent of the intravesicular glucose pool was proportional with glucose-6-phosphatase activity. The relative size of the intravesicular glucose-6-phosphate pool (irrespective of the concentration of the extravesicular concentration of added glucose-6-phosphate) expressed as the apparent intravesicular space of the hexose phosphate was inversely dependent on glucose-6-phosphatase activity. The increase of hydrolysis by elevating the extravesicular glucose-6-phosphate concentration or temperature resulted in lower apparent intravesicular glucose-6-phosphate spaces and, thus, in a higher transmembrane gradient of glucose-6-phosphate concentrations. In contrast, inhibition of glucose-6-phosphate hydrolysis by vanadate, inactivation of glucose-6-phosphatase by acidic pH, or genetically determined low or absent glucose-6-phosphatase activity in human hepatic microsomes of patients suffering from glycogen storage disease type 1a led to relatively high intravesicular glucose-6-phosphate levels. Glucose-6-phosphate transport investigated by light-scattering technique resulted in similar traces in control and vanadate-treated rat microsomes as well as in microsomes from human patients with glycogen storage disease type 1a. It is concluded that liver microsomes take up glucose-6-phosphate, constituting a pool directly accessible to intraluminal glucose-6-phosphatase activity. In addition, normal glucose-6-phosphate uptake can take place in the absence of the glucose-6-phosphatase enzyme protein, confirming the existence of separate transport proteins.

Loss of lysosomal integrity caused by the decrease of proton translocation in methylene blue-mediated photosensitization

Loss of lysosomal integrity is a critical event for killing tumor cells in the photodynamic therapy of cancers. To elucidate the mechanism of photodamage induced lysosomal disintegration, we investigated the role of losing lysosomal proton translocation in latency loss of photosensitized lysosomes. Isolated rat liver lysosomes were light exposed in the presence of Methylene blue. Through monitoring lysosomal delta pH with Acridine orange and measuring its membrane potential with 3,3'-dipropylthiadicarbocyanine iodide, loss of Mg-ATP dependent proton translocation and decrease in electrogenicity of the proton pump were observed after lysosomes were photosensitized. When normal lysosomes were incubated for 60 min in K+ contained medium, percentage free activity of lysosomal enzyme beta-galactosidase increased, i.e. lysosomal latency decreased. In the presence of Mg-ATP, the latency loss of incubated lysosomes reduced. Addition of n-ethylmaleimide, a potent inhibitor of lysosomal H(+)-ATPase, abolished the effect of Mg-ATP on lysosomal latency. It suggests a role of proton translocation in protecting lysosomal integrity. Under the same conditions, Methylene blue photosensitized lysosomes increasingly lost latency of beta-hexosaminidase and beta-galactosidase with light exposure, presumably due to the photodamage induced loss of proton pumping. In contrast, the photosensitization did not decrease lysosomal latency in the absence of Mg-ATP, implying that lysosomal integrity might not be impaired via other photodamage effects under the conditions of this study. These results indicate that lysosomal integrity can be photodestructed via the loss of proton translocation.

Evidence for an UDP-glucuronic acid/phenol glucuronide antiport in rat liver microsomal vesicles

The transport of glucuronides synthesized in the luminal compartment of the endoplasmic reticulum by UDP-glucuronosyltransferase isoenzymes was studied in rat liver microsomal vesicles. Microsomal vesicles were loaded with p-nitrophenol glucuronide (5 mM), phenolphthalein glucuronide or UDP-glucuronic acid, by a freeze-thawing method. In was shown that: (i) the loading procedure resulted in millimolar intravesicular concentrations of the different loading compounds; (ii) addition of UDP-glucuronic acid (5 mM) to the vesicles released both intravesicular glucuronides within 1 min; (iii) glucuronides stimulated the release of UDP-glucuronic acid from UDP acid-loaded microsomal vesicles; (iv) trans-stimulation of UDP-glucuronic acid entry by loading of microsomal vesicles with p-nitrophenol glucuronide, phenolphthalein glucuronide, UDP-glucuronic acid and UDP-N-acetyl-glucosamine almost completely abolished the latency of UDP-glucuronosyltransferase, although mannose 6-phosphatase latency remained unaltered; (v) the loading compounds by themselves did not stimulate UDP-glucuronosyltransferase activity. This study indicates that glucuronides synthesized in the lumen of endoplasmic reticulum can leave by an antiport, which concurrently transports USP-glucuronic acid into the lumen of the endoplasmic reticulum.

Postischemic changes in cardiac sarcoplasmic reticulum Ca2+ channels. A possible mechanism of ischemic preconditioning

We investigated the modifications of cardiac ryanodine receptors/sarcoplasmic reticulum Ca2+ release channels occurring in ischemic preconditioning. In an isolated rat heart model, the injury produced by 30 minutes of global ischemia was reduced by preexposure to three 3-minute periods of global ischemia (preconditioning ischemia). The protection was still present 120 minutes after preconditioning ischemia but disappeared after 240 minutes. Three 1-minute periods of global ischemia did not provide any protection. In the crude homogenate obtained from ventricular myocardium, the density of [3H]ryanodine binding sites averaged 372 +/- 18 fmol/mg of protein in the control condition, decreased 5 minutes after preconditioning ischemia (290 +/- 15 fmol/mg, P < .01), was still significantly reduced after 120 minutes (298 +/- 17 fmol/mg, P < .05), and recovered after 240 minutes (341 +/- 21 fmol/mg). Three 1-minute periods of ischemia did not produce any change in ryanodine binding. The Kd for ryanodine (1.5 +/- 0.3 nmol/L) was unchanged in all cases. In parallel experiments, the crude homogenate or a microsomal fraction was passively loaded with 45Ca, and Ca(2+)-induced Ca2+ release was studied by the quick filtration technique. In both preparations, the rate constant of Ca(2+)-induced Ca2+ release decreased 5 and 120 minutes after preconditioning ischemia (homogenate values: 19.7 +/- 1.4 and 18.9 +/- 0.9 s-1 vs a control value of 25.4 +/- 1.7 s-1, P < .05 in both cases) and recovered after 240 minutes (23.0 +/- 1.9 s-1). The Ca2+ dependence of Ca(2+)-induced Ca2+ release was not affected by preconditioning ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Fatty acyl-CoA esters inhibit glucose-6-phosphatase in rat liver microsomes

In native rat liver microsomes glucose 6-phosphatase activity is dependent not only on the activity of the glucose-6-phosphatase enzyme (which is lumenal) but also on the transport of glucose-6-phosphate, phosphate and glucose through the respective translocases T1, T2 and T3. By using enzymic assay techniques, palmitoyl-CoA or CoA was found to inhibit glucose-6-phosphatase activity in intact microsomes. The effect of CoA required ATP and fatty acids to form fatty acyl esters. Increasing concentrations (2-50 microM) of CoA (plus ATP and 20 microM added palmitic acid) or of palmitoyl-CoA progressively decreased glucose-6-phosphatase activity to 50% of the control value. The inhibition lowered the Vmax without significantly changing the Km. A non-hydrolysable analogue of palmitoyl-CoA also inhibited, demonstrating that binding of palmitoyl-CoA rather than hydrolysis produces the inhibition. Light-scattering measurements of osmotically induced changes in the size of rat liver microsomal vesicles pre-equilibrated in a low-osmolality buffer demonstrated that palmitoyl-CoA alone or CoA plus ATP and palmitic acid altered the microsomal permeability to glucose 6-phosphate, but not to glucose or phosphate, indicating that T1 is the site of palmitoyl-CoA binding and inhibition of glucose-6-phosphatase activity in native microsomes. The type of inhibition found suggests that liver microsomes may comprise vesicles heterogeneous with respect to glucose-6-phosphate translocase(s), i.e. sensitive or insensitive to fatty acid ester inhibition.

Chicken skeletal muscle ryanodine receptor isoforms: ion channel properties

To define the roles of the alpha- and beta-ryanodine receptor (RyR) (sarcoplasmic reticulum Ca2+ release channel) isoforms expressed in chicken skeletal muscles, we investigated the ion channel properties of these proteins in lipid bilayers. alpha- and beta RyRs embody Ca2+ channels with similar conductances (792, 453, and 118 pS for K+, Cs+ and Ca2+) and selectivities (PCa2+/PK+ = 7.4), but the two channels have different gating properties. alpha RyR channels switch between two gating modes, which differ in the extent they are activated by Ca2+ and ATP, and inactivated by Ca2+. Either mode can be assumed in a spontaneous and stable manner. In a low activity mode, alpha RyR channels exhibit brief openings (tau o = 0.14 ms) and are minimally activated by Ca2+ in the absence of ATP. In a high activity mode, openings are longer (tau o1-3 = 0.17, 0.51, and 1.27 ms), and the channels are activated by Ca2+ in the absence of ATP and are in general less sensitive to the inactivating effects of Ca2+. beta RyR channel openings are longer (tau 01-3 = 0.34, 1.56, and 3.31 ms) than those of alpha RyR channels in either mode. beta RyR channels are activated to a greater relative extent by Ca2+ than ATP and are inactivated by millimolar Ca2+ in the absence, but not the presence, of ATP. Both alpha- and beta RyR channels are activated by caffeine, inhibited by Mg2+ and ruthenium red, inactivated by voltage (cytoplasmic side positive), and modified to a long-lived substate by ryanodine, but only alpha RyR channels are activated by perchlorate anions. The differences in gating and responses to channel modifiers may give the alpha- and beta RyRs distinct roles in muscle activation.

Astrocytic glucose-6-phosphatase and the permeability of brain microsomes to glucose 6-phosphate

Cells from primary rat astrocyte cultures express a 36.5 kDa protein that cross-reacts with polyclonal antibodies to the catalytic subunit of rat hepatic glucose-6-phosphatase on Western blotting. Glucose-6-phosphate-hydrolysing activity of the order of 10 nmol/min per mg of total cellular protein can be demonstrated in cell homogenates. This activity shows latency, and is localized to the microsomal fraction. Kinetic analysis shows a Km of 15 mM and a Vmax. of 30 nmol/min per mg of microsomal protein in disrupted microsomes. Approx. 40% of the total phosphohydrolase activity is specific glucose-6-phosphatase, as judged by sensitivity to exposure to pH 5 at 37 degrees C. Previous reports that the brain microsomal glucose-6-phosphatase system does not distinguish glucose 6-phosphate and mannose 6-phosphate are confirmed in astrocyte microsomes. However, we demonstrate significant phosphomannose isomerase activity in brain microsomes, allowing for ready interconversion between mannose 6-phosphate and glucose 6-phosphate (Vmax. 15 nmol/min per mg of microsomal protein; apparent Km < 1 mM; pH optimum 5-6 for the two-step conversion). This finding invalidates the past inference from the failure of brain microsomes to distinguish mannose 6-phosphate and glucose 6-phosphate that the cerebral glucose-6-phosphatase system lacks a 'glucose 6-phosphate translocase' [Fishman and Karnovsky (1986) J. Neurochem. 46, 371-378]. Furthermore, light-scattering experiments confirm that a proportion of whole brain microsomes is readily permeable to glucose 6-phosphate.

Evidence for an effect of phospholamban on the regulatory role of ATP in calcium uptake by the calcium pump of the cardiac sarcoplasmic reticulum

The purpose of this study was to investigate the functional relationship between phospholamban and the nucleotide site of the calcium pump protein of the cardiac sarcoplasmic reticulum. We used control and trypsin-treated cardiac microsomes in which cleavage of the inhibitory cytoplasmic domain of phospholamban is associated with an activation of the calcium pump similar to that produced by protein kinase A catalyzed phospholamban phosphorylation. Phenylglyoxal was shown to inactivate the calcium pump in a pseudo-first-order reaction by binding to a single Arg at the nucleotide binding site. No differences upon trypsin treatment of microsomes were observed in the kinetics of phenylglyoxal inactivation or the ability of millimolar ATP to protect against inactivation. In subsequent kinetic studies, Ca-uptake rates measured at saturating Ca2+ and 5 microM-1 mM MgATP2- were increased 15-32% by trypsin treatment in each of three different microsome preparations. Double-reciprocal plots of the data showed marked downward curvature indicating an acceleratory effect associated with ligand binding to a lower affinity site. At 0.32 microM Ca2+, Ca-uptake rates were lower than at 11 microM Ca2+ but were stimulated to a greater extent by trypsin treatment; control microsomes showed reduced evidence of apparent negative cooperativity. At 0-2 microM MgATP2- and saturating Ca2+, there was a 50% increase in Vmax(app) when the Hill coefficient (N) was 1. At 0-10 microM MgATP2-, second-site binding was evident. At both 0-10 microM and 5 microM-1 mM MgATP2-, trypsin-treated microsomes showed greater activation of Ca uptake attributable to second-site binding than did control microsomes.(ABSTRACT TRUNCATED AT 250 WORDS)

Permeability of rat liver microsomal membrane to glucose 6-phosphate

Light-scattering measurements of osmotically induced changes in the size of rat liver microsomal vesicles pre-equilibrated in a low-osmolality buffer revealed the following. (1) The increase in extravesicular osmolality by addition of glucose 6-phosphate or mannose 6-phosphate (25 mM each) caused a rapid shrinking of microsomal vesicles. After shrinkage, a rapid swelling phase (t1/2 approx. 22 s) was present with glucose 6-phosphate but absent with mannose 6-phosphate, indicating that the former had entered microsomal vesicles, but the latter had not. (2) Almost identical results were obtained in the absence of any glucose 6-phosphate hydrolysis, i.e. with microsomes pre-treated with 100 microM-vanadate. (3) The anion-channel blocker 4,4'-di-isothiocyanostilbene-2,2'-disulphonic acid (DIDS) suppressed the glucose 6-phosphate-induced swelling phase. (4) The swelling phase was more prolonged as the glucose 6-phosphate concentration increased (t1/2 = 16 +/- 3, 22 +/- 3 and 35 +/- 4 s with 25 mM, 37.5 mM- and 50 mM-glucose 6-phosphate respectively). The behaviour of glucose-6-phosphatase activity of intact and disrupted microsomes measured in the presence of high concentrations (less than 30 mM) of substrate also indicated the saturation of the glucose 6-phosphate permeation system by extravesicular concentrations of glucose 6-phosphate higher than 20-30 mM. Additional experiments showed that vanadate-treated microsomes pre-equilibrated with 0.1 mM- and 1.0 mM-glucose 6-phosphate (and [1-14C]glucose 6-phosphate as a tracer) rapidly (t1/2 less than 20 s) released [1-14C]glucose 6-phosphate when diluted in a glucose 6-phosphate-free medium. The efflux of [1-14C]glucose 6-phosphate was largely prevented by DIDS, allowing an evaluation of the intravesicular space of glucose 6-phosphate of approx. 1.0 microliter/mg of microsomal protein.