Regulation of glucose-6-phosphate dehydrogenase activity in sea urchin eggs by reversible association with cell structural elements

Surfaces as frameworks for intracellular organization

A large fraction of soluble protein within the interior of living cells may reversibly associate with structural elements, including proteinaceous fibers and phospholipid membranes. In this opinion, we present theoretical and experimental evidence that many of these associations are due to nonspecific attraction between the protein and the surface of the fiber or membrane, and that such associations may lead to substantial changes in the association state of the adsorbed proteins, the biological function of the adsorbed proteins, and the distribution of these proteins between the many microenvironments existing within the cell.

Development of placental abnormalities in location and anatomy

Low‐lying placentas, placenta previa and abnormally invasive placentas are the most frequently occurring placental abnormalities in location and anatomy. These conditions can have serious consequences for mother and fetus mainly due to excessive blood loss before, during or after delivery. The incidence of such abnormalities is increasing, but treatment options and preventive strategies are limited. Therefore, it is crucial to understand the etiology of placental abnormalities in location and anatomy. Placental formation already starts at implantation and therefore disorders during implantation may cause these abnormalities. Understanding of the normal placental structure and development is essential to comprehend the etiology of placental abnormalities in location and anatomy, to diagnose the affected women and to guide future research for treatment and preventive strategies. We reviewed the literature on the structure and development of the normal placenta and the placental development resulting in low‐lying placentas, placenta previa and abnormally invasive placentas.

NADPH production by the pentose phosphate pathway in the zona fasciculata of rat adrenal gland

Biosynthesis of steroid hormones in the cortex of the adrenal gland takes place in smooth endoplasmic reticulum and mitochondria and requires NADPH. Four enzymes produce NADPH: glucose-6-phosphate dehydrogenase (G6PD), the key regulatory enzyme of the pentose phosphate pathway, phosphogluconate dehydrogenase (PGD), the third enzyme of that pathway, malate dehydrogenase (MDH), and isocitrate dehydrogenase (ICDH). However, the contribution of each enzyme to NADPH production in the cortex of adrenal gland has not been established. Therefore, activity of G6PD, PGD, MDH, and ICDH was localized and quantified in rat adrenocortical tissue using metabolic mapping, image analysis, and electron microscopy. The four enzymes have similar localization patterns in adrenal gland with highest activities in the zona fasciculata of the cortex. G6PD activity was strongest, PGD, MDH, and ICDH activity was approximately 60%, 15%, and 7% of G6PD activity, respectively. The K(m) value of G6PD for glucose-6-phosphate was two times higher than the K(m) value of PGD for phosphogluconate. As a consequence, virtual flux rates through G6PD and PGD are largely similar. It is concluded that G6PD and PGD provide the major part of NADPH in adrenocortical cells. Their activity is localized in the cytoplasm associated with free ribosomes and membranes of the smooth endoplasmic reticulum, indicating that NADPH-demanding processes related to biosynthesis of steroid hormones take place at these sites. Complete inhibition of G6PD by androsterones suggests that there is feedback regulation of steroid hormone biosynthesis via G6PD.

Apolipoprotein A-IV attenuates oxidant-induced apoptosis in mitotic competent, undifferentiated cells by modulating intracellular glutathione redox balance

Oxidant-mediated modulation of the intracellular redox state affects the apoptotic cascade by altering the balance between cellular signals for survival and suicide. Apolipoprotein A-IV (Apo A-IV) is known to possess antioxidant-like activity. In the present study, we tested 1) whether Apo A-IV could influence redox-dependent apoptosis and, if so, 2) whether such an effect could be mediated by modulation of intracellular redox balance. Mitotic competent, undifferentiated PC-12 cells were incubated with either tert-butyl hydroperoxide (TBH) or diamide with or without preincubation with human Apo A-IV. Apo A-IV significantly decreased apoptosis produced by both TBH and diamide, and washout of A-IV before incubation with TBH and diamide did not eliminate its protective effect. Apo A-I had no such protective effect. The Apo A-IV effect was not blocked by d,l-buthionine-[ S, R]-sulfoximine, but it was reversed by both dehydroisoandrosterone and transfection with an antisense oligodeoxynucleotide to glucose-6-phosphate dehydrogenase (G6PD). Apo A-IV abolished the transient, oxidant-induced rise in glutathione disulfide (GSSG) and cellular redox imbalance previously shown to initiate the apoptotic cascade. Apo A-IV had no effect on GSSG reductase activity, but it stimulated G6PD activity 10-fold. These results suggest a novel role for Apo A-IV in the regulation of intracellular glutathione redox balance and the modulation of redox-dependent apoptosis via stimulation of G6PD activity.

Calcium at fertilization and in early development

Fertilization calcium waves are introduced, and the evidence from which we can infer general mechanisms of these waves is presented. The two main classes of hypotheses put forward to explain the generation of the fertilization calcium wave are set out, and it is concluded that initiation of the fertilization calcium wave can be most generally explained in invertebrates by a mechanism in which an activating substance enters the egg from the sperm on sperm-egg fusion, activating the egg by stimulating phospholipase C activation through a src family kinase pathway and in mammals by the diffusion of a sperm-specific phospholipase C from sperm to egg on sperm-egg fusion. The fertilization calcium wave is then set into the context of cell cycle control, and the mechanism of repetitive calcium spiking in mammalian eggs is investigated. Evidence that calcium signals control cell division in early embryos is reviewed, and it is concluded that calcium signals are essential at all three stages of cell division in early embryos. Evidence that phosphoinositide signaling pathways control the resumption of meiosis during oocyte maturation is considered. It is concluded on balance that the evidence points to a need for phosphoinositide/calcium signaling during resumption of meiosis. Changes to the calcium signaling machinery occur during meiosis to enable the production of a calcium wave in the mature oocyte when it is fertilized; evidence that the shape and structure of the endoplasmic reticulum alters dynamically during maturation and after fertilization is reviewed, and the link between ER dynamics and the cytoskeleton is discussed. There is evidence that calcium signaling plays a key part in the development of patterning in early embryos. Morphogenesis in ascidian, frog, and zebrafish embryos is briefly described to provide the developmental context in which calcium signals act. Intracellular calcium waves that may play a role in axis formation in ascidian are discussed. Evidence that the Wingless/calcium signaling pathway is a strong ventralizing signal in Xenopus, mediated by phosphoinositide signaling, is adumbrated. The central role that calcium channels play in morphogenetic movements during gastrulation and in ectodermal and mesodermal gene expression during late gastrulation is demonstrated. Experiments in zebrafish provide a strong indication that calcium signals are essential for pattern formation and organogenesis.

Improved localization of glucose-6-phosphate dehydrogenase activity in cells with 5-cyano-2,3-ditolyl-tetrazolium chloride as fluorescent redox dye reveals its cell cycle-dependent regulation

Since the introduction of cyano-ditolyl-tetrazolium chloride (CTC), a tetrazolium salt that gives rise to a fluorescent formazan after reduction, it has been applied to quantify activity of dehydrogenases in individual cells using flow cytometry. Confocal laser scanning microscopy (CLSM) showed that the fluorescent formazan was exclusively localized at the surface of individual cells and not at intracellular sites of enzyme activity. In the present study, the technique has been optimized to localize activity of glucose-6-phosphate dehydrogenase (G6PD) intracellularly in individual cells. Activity was demonstrated in cultured fibrosarcoma cells in different stages of the cell cycle. Cells were incubated for the detection of G6PD activity using a medium containing 6% (w/v) polyvinyl alcohol, 5 mM CTC, magnesium chloride, sodium azide, the electron carrier methoxyphenazine methosulphate, NADP, and glucose-6-phosphate. Before incubation, cells were permeabilized with 0.025% glutaraldehyde. Fluorescent formazan was localized exclusively in the cytoplasm of fibrosarcoma cells. The amount of fluorescent formazan in cells increased linearly with incubation time when measured with flow cytometry and CLSM. When combining the Hoechst staining for DNA with the CTC method for the demonstration of G6PD activity, flow cytometry showed that G6PD activity of cells in S phase and G2/M phase is 27 +/- 4% and 43 +/- 4% higher, respectively, than that of cells in G1 phase. CLSM revealed that cells in all phases of mitosis as well as during apoptosis contained considerably lower G6PD activity than cells in interphase. It is concluded that posttranslational regulation of G6PD is responsible for this cell cycle-dependent activity.

The need for metabolic mapping in living cells and tissues

The ultimate activity of an enzyme depends on many regulatory steps from transcription of the gene up to complex formation of the enzyme. Therefore, gene expression (mRNA levels) or protein expression (protein levels) are not reliable parameters to predict the functional activity of an enzyme. Activity measurements in cell homogenates or in frozen or fixed (and thus dead) cell preparations are not appropriate either because post-translational regulation mechanisms that exist in living cells may be lost by homogenization or freezing or chemical fixation of cells. Therefore, metabolic mapping in living cells or, in other words, visualization and quantification using microscopy and image analysis of enzyme reactions in living cells is the approach of choice to understand the functional role of enzymes in vivo as is demonstrated here with a number of examples in recent literature.

Clonal xenobiotic resistance during pollution-induced toxic injury and hepatocellular carcinogenesis in liver of female flounder (Platichthys flesus (L.))

Juvenile and adult female flounder (Platichthys flesus (L.)) were caught either in the estuary of the most polluted European river, the Elbe, or as controls in a reference site to study pollution-induced xenobiotic resistance in their livers in relation to pathological alterations. In juvenile fish, livers displayed reversible and irreversible degenerative toxipathic lesion types but never showed (pre)neoplastic changes. Tumour frequencies up to 70% were found macroscopically in livers of adult female flounder which had progressed to adenomas and carcinomas in the most polluted site. Because male adult flounder show only up to 50% of livers containing early preneoplastic foci but never malignancies, we focussed our study on female individuals. (Pre)neoplastic changes ranged from early eosinophilic foci to basophilic foci, adenomas and hepatocellular carcinomas. Adenomas were generally eosinophilic whereas carcinomas were mainly basophilic. These phenotypical sequential changes strongly resemble those found in chemically-induced liver carcinogenesis in mammals. Characteristic mutations known from mammalian cancers have not been found so far in these flounder livers. Therefore, we investigated whether epigenetic events had induced a metabolic "resistant phenotype" of (pre)malignant cancer cells during hepatocellular carcinogenesis. With a quantitative immunohistochemical approach, we studied expression of P-glycoprotein (P-gp)-mediated multixenobiotic resistance (MXR), cytochrome P4501A1, glutathione-S-transferase-A which are key proteins in xenobiotic metabolism and elimination. Glucose-6-phosphate dehydrogenase (G6PDH) activity, the major source of the reducing power NADPH which is needed for biotransformation, oxyradical scavenging and biosynthesis, was detected as well. We observed upregulation of G6PDH activity already in early preneoplastic eosinophilic foci and subsequent further upregulation in basophilic foci and carcinomas. P-gp started to become overexpressed in basophilic foci and was overexpressed even more strongly in carcinomas and their invasively-growing protrusions (satellites). In carcinomas, P-gp protein was predominantly present in membranes of lysosomes which are the intracellular sites of deposition of xenobiotics. CYP450 was reduced whereas GST-A was increased in these carcinomas. Progression towards malignancy was positively correlated with levels of mitogenic organochlorines in these livers which are "fingerprint contaminants" of the river Elbe. We conclude that (pre)neoplastic hepatocytes in female flounder acquire growth advantages over normal hepatocytes by epigenetic metabolic adaptations during liver carcinogenesis as a result of chronic exposure to (pro)carcinogens in the polluted habitat.

Increased myocardial dysfunction after ischemia-reperfusion in mice lacking glucose-6-phosphate dehydrogenase

BACKGROUND

Free radical injury contributes to cardiac dysfunction during ischemia-reperfusion. Detoxification of free radicals requires maintenance of reduced glutathione (GSH) by NADPH. The principal mechanism responsible for generating NADPH and maintaining GSH during periods of myocardial ischemia-reperfusion remains unknown. Glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme in the pentose phosphate pathway, generates NADPH in a reaction linked to the de novo production of ribose. We therefore hypothesized that G6PD is essential for maintaining GSH levels and protecting the heart during ischemia-reperfusion injury.

METHODS AND RESULTS

Susceptibility to myocardial ischemia-reperfusion injury was determined in Langendorff-perfused hearts isolated from wild-type mice (WT) and mice lacking G6PD (G6PD(def)) (20% of WT myocardial G6PD activity). During global zero-flow ischemia, cardiac function was similar between WT and G6PD(def) hearts. On reperfusion, however, cardiac relaxation and contractile performance were greatly impaired in G6PD(def) myocardium, as demonstrated by elevated end-diastolic pressures and decreased percent recovery of developed pressure relative to WT hearts. Contractile dysfunction in G6PD(def) hearts was associated with depletion of total glutathione stores and impaired generation of GSH from its oxidized form. Increased ischemia-reperfusion injury in G6PD(def) hearts was reversed by treatment with the antioxidant MnTMPyP but unaffected by supplementation of ribose stores.

CONCLUSIONS

These results demonstrate that G6PD is an essential myocardial antioxidant enzyme, required for maintaining cellular glutathione levels and protecting against oxidative stress-induced cardiac dysfunction during ischemia-reperfusion.

Fluorogenic substrate [Ala-Pro]2-cresyl violet but not Ala-Pro-rhodamine 110 is cleaved specifically by DPPIV activity: a study in living Jurkat cells and CD26/DPPIV-transfected Jurkat cells

Fluorogenic substrates [Ala-Pro](2)-cresyl violet and Ala-Pro-rhodamine 110 have been tested for microscopic detection of protease activity of dipeptidyl peptidase IV (DPPIV) in living cells. DPPIV activity is one of the many functions of the multifunctional or moonlighting protein CD26/DPPIV. As a model we used Jurkat cells, which are T-cells that lack CD26/DPPIV expression, and CD26/DPPIV-transfected Jurkat cells. Ala-Pro-rhodamine 110 is not fluorescent, but after proteolytic cleavage rhodamine 110 fluoresces. [Ala-Pro](2)-cresyl violet is fluorescent by itself but proteolytic cleavage into cresyl violet induces a shift to longer wavelengths. This phenomenon enables the simultaneous determination of local (intracellular) substrate and product concentrations, which is important for analysis of kinetics of the cleavage reaction. [Ala-Pro](2)-cresyl violet, but not Ala-Pro-rhodamine 110, appeared to be specific for DPPIV. When microscopic analysis is performed on living cells during the first minutes of the enzyme reaction, DPPIV activity can be precisely localized in cells with the use of [Ala-Pro](2)-cresyl violet. Fluorescent product is rapidly internalized into submembrane granules in transfected Jurkat cells and is redistributed intracellularly via internalization pathways that have been described for CD26/DPPIV. We conclude that [Ala-Pro](2)-cresyl violet is a good fluorogenic substrate to localize DPPIV activity in living cells when the correct wavelengths are used for excitation and emission and images are captured in the early stages of the enzyme reaction.

Up-regulation of glucose metabolism during male pronucleus formation determines the early onset of the s phase in bovine zygotes

After in vitro fertilization with spermatozoa from bulls with high in vitro fertility, a beneficial paternal effect is manifested during the G1 phase of the first cell cycle. This benefit determines an earlier onset of the first S phase, and then a successful morula-blastocyst transition 7 days later. We hypothesized that the origin of the paternal effect could be a shift of the metabolism of the fertilized oocyte, because in mice, sperm decondensation is responsible for a dramatic increase in glucose metabolism. In this study we investigated the interaction between both pronuclei and compared glycolysis and pentose phosphate pathway (PPP) activities in bovine oocytes fertilized with spermatozoa from bulls of high or low fertility. Here we demonstrate that male pronucleus formation is necessary for the onset of the S phase in the female pronucleus, and that the component promoting an early S phase in both pronuclei is metabolic and linked to an up-regulation of the PPP during the male pronucleus formation. This long-lasting paternal effect is more evidence of the important role of epigenetic control during early embryo development.

Peroxynitrite protects neurons against nitric oxide-mediated apoptosis. A key role for glucose-6-phosphate dehydrogenase activity in neuroprotection

Peroxynitrite is thought to be a nitric oxide-derived neurotoxic effector molecule involved in the disruption of key energy-related metabolic targets. To assess the consequences of such interference in cellular glucose metabolism and viability, we studied the possible modulatory role played by peroxynitrite in glucose oxidation in neurons and astrocytes in primary culture. Here, we report that peroxynitrite triggered rapid stimulation of pentose phosphate pathway (PPP) activity and the accumulation of NADPH, an essential cofactor for glutathione regeneration. In contrast to peroxynitrite, nitric oxide elicited NADPH depletion, glutathione oxidation, and apoptotic cell death in neurons, but not in astrocytes. These events were noticeably counteracted by pretreatment of neurons with peroxynitrite. In an attempt to elucidate the mechanism responsible for this PPP stimulation and neuroprotection, we found evidence consistent with both exogenous and endogenous peroxynitrite-mediated activation of glucose-6-phosphate dehydrogenase (G6PD), an enzyme that catalyzes the first rate-limiting step in the PPP. Moreover, functional overexpression of the G6PD gene in stably transformed PC12 cells induced NADPH accumulation and offered remarkable resistance against nitric oxide-mediated apoptosis, whereas G6PD gene-targeted antisense inhibition depleted NADPH levels and exacerbated cellular vulnerability. In light of these results, we suggest that G6PD activation represents a novel role for peroxynitrite in neuroprotection against nitric oxide-mediated apoptosis.

Post-translational regulation of glucose-6-phosphate dehydrogenase activity in (pre)neoplastic lesions in rat liver

Glucose-6-phosphate dehydrogenase (G6PD; EC 1.1.1.49) is the key regulatory enzyme of the pentose phosphate pathway and produces NADPH and riboses. In this study, the kinetic properties of G6PD activity were determined in situ in chemically induced hepatocellular carcinomas, and extralesional and control parenchyma in rat livers and were directly compared with those of the second NADPH-producing enzyme of the pentose phosphate pathway, phosphogluconate dehydrogenase (PGD). Distribution patterns of G6PD activity, protein, and mRNA levels were also compared to establish the regulation mechanisms of G6PD activity. In (pre)neoplastic lesions, the V(max) of G6PD was 150-fold higher and the K(m) for G6P was 10-fold higher than in control liver parenchyma, whereas in extralesional parenchyma, the V(max) was similar to that in normal parenchyma but the K(m) was fivefold lower. This means that virtual fluxes at physiological substrate concentrations are 20-fold higher in lesions and twofold higher in extralesional parenchyma than in normal parenchyma. The V(max) of PGD was fivefold higher in lesions than in normal and extralesional liver parenchyma, whereas the K(m) was not affected. Amounts of G6PD protein and mRNA were similar in lesions and in extralesional liver parenchyma. These results demonstrate that G6PD is strongly activated post-translationally in (pre)neoplastic lesions to produce NADPH.

Distribution patterns of ornithine decarboxylase in cells and tissues: facts, problems, and postulates

Ornithine decarboxylase (ODC) is a key enzyme in polyamine biosynthesis. Increased polyamine levels are required for growth, differentiation, and transformation of cells. In situ detection of ODC in cells and tissues has been performed with biochemical, enzyme cytochemical, immunocytochemical, and in situ hybridization techniques. Different localization patterns at the cellular level have been described, depending on the type of cells or tissues studied. These patterns varied from exclusively cytoplasmic to both cytoplasmic and nuclear. These discrepancies can be partially explained by the (lack of) sensitivity and/or specificity of the methods used, but it is more likely that (sub)cellular localization of ODC is cell type-specific and/or depends on the physiological status (growth, differentiation, malignant transformation, apoptosis) of cells. Intracellular translocation of ODC may be a prerequisite for its regulation and function.

Enzyme cytochemical techniques for metabolic mapping in living cells, with special reference to proteolysis

Specific enzymes play key roles in many pathophysiological processes and therefore are targets for therapeutic strategies. The activity of most enzymes is largely determined by many factors at the post-translational level. Therefore, it is essential to study the activity of target enzymes in living cells and tissues in a quantitative manner in relation to pathophysiological processes to understand its relevance and the potential impact of its targeting by drugs. Proteases, in particular, are crucial in every aspect of life and death of an organism and are therefore important targets. Enzyme activity in living cells can be studied with various tools. These can be endogenous fluorescent metabolites or synthetic chromogenic or fluorogenic substrates. The use of endogenous metabolites is rather limited and nonspecific because they are involved in many biological processes, but novel chromogenic and fluorogenic substrates have been developed to monitor activity of enzymes, and particularly proteases, in living cells and tissues. This review discusses these substrates and the methods in which they are applied, as well as their advantages and disadvantages for metabolic mapping in living cells.

Localization of glucose-6-phosphate dehydrogenase activity on ribosomes of granular endoplasmic reticulum, in peroxisomes and peripheral cytoplasm of rat liver parenchymal cells

Glucose-6-phosphate dehydrogenase activity has been localized ultrastructurally in fixed tissues. Activity was found in particular in association with ribosomes of granular endoplasmatic reticulum. Biochemical studies indicated that glucose-6-phosphate dehydrogenase activity is also present in the cytoplasm and in peroxisomes. Fixation may be held responsible for selective inactivation of part of glucose-6-phosphate dehydrogenase activity. In the present study, we applied the ferricyanide method for the demonstration of glucose-6-phosphate dehydrogenase activity in unfixed cryostat sections of rat liver in combination with the semipermeable membrane technique and in isolated rat liver parenchymal cells. Isolated liver parenchymal cells were permeabilized with 0.025% glutaraldehyde after NADP+ protection of the active site of glucose-6-phosphate dehydrogenase. This treatment resulted in only slight inactivation of glucose-6-phosphate dehydrogenase activity. The composition of the incubation medium was optimized on the basis of rapid light microscopical analysis of the formation of reddish-brown final reaction product in sections. With the optimized method, electron dense reaction product was observed in cryostat sections on granular endoplasmic reticulum, in mitochondria and at the cell border. However, the ultrastructural morphology was rather poor. In contrast, the morphology of incubated isolated cells was preserved much better. Electron dense precipitate was found on ribosomes of the granular endoplasmic reticulum, in peroxisomes and the cytoplasm, particularly at the periphery of cells. In conclusion, our ultrastructural study clearly demonstrates that it is essential to use mildly-fixed cells to allow detection of glucose-6-phosphate dehydrogenase activity in all cellular compartments where activity is present.

Heterogeneous distribution of glucose-6-phosphate dehydrogenase in lingual epithelium

Lingual epithelium undergoes oxidative stress and apoptosis with consequent renewal of superficial keratinized cells by proliferation and differentation of the stem cells of the basal germinative layer. In 3 distinct areas of lingual epithelium of rat and rabbit, the anterior third, central third and posterior third, we determined the activity of hexose monophosphate shunt enzymes and antioxidant enzymes, which are essential for support of cell proliferation and differentation. Enzymatic assays of the epithelium showed that glucose-6-phosphate dehydrogenase (G6PD) activity was highest in the anterior third, whereas activity of glutathione peroxidase, 6-phosphogluconate dehydrogenase, glutathione reductase, superoxide dismutase and catalase was similar over all areas. Histochemical localization of activity and immunohistochemical localization of protein of G6PD showed that all types of papillae had a similar G6PD content; moreover, the presence of different G6PD isoforms in the 3 areas was excluded by electrophoretic analysis. We conclude that the higher G6PD activity in the anterior part of the epithelium is due only to the anatomical organization of the epithelial surface of this area, in which many filiform and fungiform papillae are arranged in a compact manner, which corresponds with a higher number of proliferating and differentiating cells. These processes need products of G6PD activity. This study indicates that G6PD is a good marker for the number of differentiating cells in tongue epithelium.

Posttranslational regulation of glucose-6-phosphate dehydrogenase activity in tongue epithelium

Expression of glucose-6-phosphate dehydrogenase (G6PD) activity is high in tongue epithelium, but its exact function is still unknown. It may be related either to the high proliferation rate of this tissue or to protection against oxidative stress. To elucidate its exact role, we localized quantitatively G6PD activity, protein and mRNA using image analysis in tongue epithelium of rat and rabbit, two species with different diets. Distribution patterns of G6PD activity were largely similar in rat and rabbit but the activities were twofold lower in rabbit. Activity was two to three times higher in upper cell layers of epithelium than in basal cell layers, whereas basal layers, where proliferation takes place, contained twice as much G6PD protein and 40% more mRNA than upper layers. Our findings show that G6PD is synthetized mainly in basal cell layers of tongue epithelium and that it is posttranslationally activated when cells move to upper layers. Therefore, we conclude that the major function of G6PD activity in tongue epithelium is the formation of NADPH for protection against oxidative stress and that diet affects enzyme expression in this tissue.

The roles of changes in NADPH and pH during fertilization and artificial activation of the sea urchin egg

Incubating unfertilized sea urchin eggs in weak bases activates nuclear centering, DNA synthesis, and chromosome cycles. These effects were initially attributed to raising the intracellular pH (pH(i)), but later experiments indicated that these weak bases also lead to increases in reduced pyridine nucleotides. These findings raised the question whether the activation of the nucleus was due to increased pH(i) or to increased NAD(P)H or possibly other effects. This report attempts to clarify how ammonia activates eggs by independently altering NADPH and pH(i). To increase the pH(i), unfertilized eggs were injected with zwitterionic buffers. This stimulated pronuclear centering, DNA synthesis, and nuclear envelope breakdown; there appeared to be a threshold corresponding to the fertilized pH(i). However, like incubation in ammonia, injection of base also increased NAD(P)H. The NAD(P)H rise caused by directly raising the pH(i) occurred in the presence of intracellular calcium chelators, indicating that calcium is not required. Increasing NAD(P)H alone did not activate nuclear centering, DNA synthesis, or nuclear envelope breakdown. Although these experiments cannot eliminate a role for the NADPH increase in initiating events leading to nuclear centering and entry into mitosis, they provide additional and strong evidence that increasing the pH(i) may be a primary signal.

Adaptational changes in kinetic parameters of G6PDH but not of PGDH during contamination-induced carcinogenesis in livers of North Sea flatfish

Kinetic parameters of glucose-6-phosphate dehydrogenase (G6PDH) and phosphogluconate dehydrogenase (PGDH) were determined in situ in livers of marine flatfish flounder that were caught in unpolluted areas in the open sea and in the highly polluted river Elbe (Germany). Analysis was performed quantitatively in liver sections using valid enzyme histochemical methods and image analysis. G6PDH but not PGDH was strongly affected by contaminant exposure and subsequent carcinogenesis. G6PDH showed a gradual decrease in Vmax and Km for glucose-6-phosphate in extralesional normal-looking liver tissue. Hepatocellular carcinomas also showed a low Km, whereas the Vmax was upregulated. These findings are interpreted as follows: prolonged challenges of the livers by pollutants inhibit or inactivate G6PDH and this is compensated for by reduction in Km. In carcinomas, G6PDH levels are upregulated but the low Km values are kept to increase the NADPH production capacity required in cancer cells showing that posttranslational regulation processes are important to control cellular metabolism under various environmental conditions.

The dynamics of local kinetic parameters of glutamate dehydrogenase in rat liver

Kinetic parameters of glutamate dehydrogenase (GDH, EC 1.4.1.2) for glutamate were determined in periportal and pericentral zones of adult male and female rat liver lobules under normal fed conditions and after starvation for 24 h. GDH activity was measured as formazan production over time against a range of glutamate concentrations in serial cryostat sections using image analysis. Captured gray value images were transformed to absorbance images and local initial velocities (Vini) were calculated. A hyperbolic function was used to describe the relationship between substrate concentration and local Vini. Under fed conditions, Vmax values were similar in male and female rats (8 +/- 2 and 16 +/- 2 mumol min-1 cm-3 liver tissue in periportal and pericentral zones, respectively). Starvation increased Vmax, especially in pericentral zones of females (to 27 +/- 1 mumol min-1 cm-3 liver tissue). Under fed conditions, the affinity of GDH for glutamate was similar in male and female rats (2.5 +/- 0.5 mM and 3.5 +/- 0.8 mM in periportal and pericentral zones, respectively). Starvation had no effect on K(m) values in male rats, but in female rats affinity for glutamate decreased significantly in both zones (K(m) values of 4.0 +/- 0.1 mM and 8.6 +/- 0.8 mM, respectively). These local changes in the kinetic parameters of GDH indicate that conversion of glutamate to alpha-oxoglutarate cannot be predicted on the basis of GDH concentrations or zero-order activity in the different zones of liver lobules alone.

Kupffer cells and pit cells are not effective in the defense against experimentally induced colon carcinoma metastasis in rat liver

The present study was performed to investigate processes involved in circumvention of the immune system by advanced stages of tumor growth in the liver. The efficacy of Kupffer cells and pit cells against cancer cells was tested in vivo in an experimental model of colon carcinoma metastasis in rat liver. Liver tumors were induced by administration of CC531 colon cancer cells into the vena portae. After 3 weeks, livers were obtained and partly fixed for electron microscopic procedures or frozen in liquid nitrogen for enzyme and immunohistochemistry at the light microscope level. The activation status of Kupffer cells was studied by expression of Ia-antigen (MHC class II) and by measurement of glucose-6-phosphate dehydrogenase (G6PDH) activity in the cells in situ as a measure of production of reactive oxygen species. Large numbers of Kupffer cells were found in liver parenchyma surrounding colon carcinomas when compared with levels in control livers, but these cells were not activated. Large numbers of activated monocytes and macrophages, cytotoxic T cells but only a few pit cells were found to be recruited to the boundary between liver parenchyma and tumors or their stroma. In those areas where cancer cells invaded liver parenchyma, only newly recruited macrophages and some Kupffer cells were present but few cytotoxic T cells or pit cells were found. The low activation status of Kupffer cells both in terms of production of reactive oxygen species and Ia-antigen expression and the absence of significant numbers of pit cells at tumor sites suggest that Kupffer cells and pit cells do not play a significant role in advanced stages of tumor growth. High levels of prostaglandin E2 were detected in the parenchyma of livers containing tumors and transforming growth factor beta was detected in the stroma of the tumors, therefore suggest that cytotoxicity of newly recruited monocytes, macrophages and cytotoxic T cells may be limited in these stages because of local production of these immunosuppressive factors.

High-performance liquid chromatographic system for separating sugar phosphates and other intermediary metabolites

A simple method for separating intermediates of carbohydrate metabolism, including the difficult-to-resolve sugar phosphates, using anion-exchange high-performance liquid chromatography is described. A gradient of decreasing borate concentration (10 to 0 mM) and increasing ionic strength (0 to 150 mM NH4Cl) separates phosphorylated sugars based on the strength of the ester complex that they form with borate anion. Lyophillized samples are reconstituted in a low ionic strength aqueous medium (5 mM triethanolamine-HCl, pH 8.1) and chromatographed on a commercially available anion-exchange column (Hamilton PRP-X100). The process requires only 3 h and permits nanomole detection of the phosphorylated intermediates of the glycolytic and pentose shunt pathways.

Heterogeneity of kinetic parameters of enzymes in situ in rat liver lobules

In the present review, metabolic compartmentation in liver lobules is discussed as being dynamic and more complex than thus far assumed on the basis of numbers of mRNA or protein molecules or the capacity (zero-order activity) of enzymes. Isoenzyme distribution patterns and local kinetic parameters of enzymes may vary over the different zones of liver lobules. As a consequence, metabolic fluxes in vivo at physiological substrate concentrations may be completely different from those that are assumed on the basis of the number of molecules or the capacity of enzymes present in zones of liver lobules. For a more correct estimation of the levels of metabolic processes in the different compartments of liver tissue, local kinetic parameters and substrate concentrations have to be determined to calculate local metabolic fluxes. Direct measurements of metabolic fluxes in vivo with the use of noninvasive techniques is a promising alternative and the techniques will become increasingly important in future metabolic research.

Analysis of enzyme reactions in situ

Estimations of metabolic rates in cells and tissues and their regulation on the basis of kinetic properties of enzymes in diluted solutions may not be applicable to intact living cells or tissues. Enzymes often behave differently in living cells because of the high cellular protein content that can lead to homologous and heterologous associations of protein molecules. These associations often change the kinetics of enzymes as part of post-translational regulation mechanisms. An overview is given of these interactions between enzyme molecules or between enzyme molecules and structural elements in the cell, such as the cytoskeleton. Biochemical and histochemical methods are discussed that have been developed for in vivo and in situ analyses of enzyme reactions, particularly for the study of effects of molecular interactions. Quantitative (histochemical) analysis of local enzyme reactions or fluxes of metabolites has become increasingly important. At present, it is possible to calculate local concentrations of substrates in cells or tissue compartments and to express local kinetic parameters in units that are directly comparable with those obtained by biochemical assays of enzymes in suspensions. In situ analysis of the activities of a number of enzymes have revealed variations in their kinetic properties (Km and Vmax) in different tissue compartments. This stresses the importance of in vivo or in situ analyses of cellular metabolism. Finally, histochemical determinations of enzyme activity in parallel with immunohistochemistry for the detection of the total number of enzyme molecules and in situ hybridization of its messenger RNA allow the analysis of regulation mechanisms at all levels between transcription of the gene and post-translational activity modulation.

Effects of partial hepatectomy, phenobarbital and 3-methylcholanthrene on kinetic parameters of glucose-6-phosphate and phosphogluconate dehydrogenase in situ in periportal, intermediate and pericentral zones of rat liver lobules

Glucose-6-phosphate dehydrogenase (G6PDH) and phosphogluconate dehydrogenase (PGDH) are heterogeneously distributed in liver lobules of female rats. The maximum activity of both enzymes is approximately twice higher in intermediate and pericentral zones than in periportal zones. Enzyme activities and their distribution patterns were manipulated by partial hepatectomy and treatment with phenobarbital (PB) or 3-methylcholanthrene (3-MC). Vmax values of G6PDH for glucose-6-phosphate decreased mainly in intermediate and pericentral zones after partial hepatectomy, whereas they increased after PB treatment. Vmax values of PGDH for phosphogluconate decreased after partial hepatectomy in both zones, whereas other treatments did not have any effect. The affinity of G6PDH for glucose-6-phosphate was similar in all zones and it was decreased 2-3 fold by PB and 3-MC treatment. The affinity of PGDH for phosphogluconate was 1.4-2.3 times lower in intermediate and pericentral zones than in periportal zones of all livers tested and was not affected by treatment. From these data it can be concluded that not only the maximum activity of enzymes may differ in periportal, intermediate and pericentral zones of the liver lobule but also the affinity of enzymes for their substrates. The implication of these findings is that metabolic flux rates as they occur in vivo in these different metabolic compartments may be significantly different from predictions on the basis of maximum enzyme activities as detected immunohistochemically, microchemically or cytophotometrically.

The diverse Michaelis constants and maximum velocities of lactate dehydrogenase in situ in various types of cell

The kinetics of lactate dehydrogenase in mouse cardiac muscle fibres, skeletal muscle fibres, gastric parietal cells, parotid gland ductal and acinar cells, oocytes and mouse and human hepatocytes were studied as a function of substrate concentration in sections of unfixed mouse and human tissues incubated at 37 degrees C on lactate agarose gel films. The absorbances of the final reaction products deposited in single cells of various types were measured continuously as a function of incubation time using an image analysis system. The initial velocities (vi) of the dehydrogenase were calculated from two equations deduced previously by us, vi = a1 zero A (equation 1) and vi = v + a2 zero A (equation 2), where v and zero A are, respectively, the gradient (steady-state velocity) and intercept of the linear regression line of absorbance on time for incubation times between 1 and 3 min, and a1 and a2 are constants characteristic for each cell type. Hanes plots using vi calculated from equation 2 gave more consistent estimates of the Michaelis constant (Km) and the maximum reaction velocity (Vmax) than those employing either steady-state velocity measurements or vi calculated from equation 1. The Km thus found for mouse skeletal muscle fibres (10.4-12.5 mM) and hepatocytes (14.3-16.7 mM) agreed well with values determined previously in biochemical assays. However, the Km for cardiac muscle fibres (13.4 mM) was higher. The Km of the enzyme in gastric parietal cells, parotid gland cells and oocytes was in the range 7.6-9.7 mM.(ABSTRACT TRUNCATED AT 250 WORDS)

Mitogenic action of insulin-like growth factor-I on human osteosarcoma MG-63 cells and rat osteoblasts maintained in situ: the role of glucose-6-phosphate dehydrogenase

The mechanisms involved in the mitogenic actions of insulin-like growth factor-I (IGF-I) on skeletal cells are at present unclear. We have investigated the role of glucose-6-phosphate dehydrogenase (G6PD) in this mechanism and provide strong evidence that stimulation of G6PD activity is required for the growth promoting activities of IGF-I. IGF-I (10 ng/ml) significantly elevated G6PD activity in MG-63 human osteosarcoma cells within 30 min which preceded the IGF-I induced DNA synthesis in these cells. Inhibition of G6PD activity by epiandrosterone decreased DNA synthesis in IGF-I stimulated MG-63 cells but this was partly overcome by the addition of a combination of the four deoxyribonucleosides. IGF-I did not cause a general increase in cell metabolism as succinate dehydrogenase and iso-citrate dehydrogenase activity were not altered. Although IGF-I caused a significant increase in lactate dehydrogenase activity this was not inhibited by epiandrosterone. The culture of metatarsals of 4-week-old rats with IGF-I (10 ng/ml) also stimulated G6PD activity in osteoblasts lining the metaphyseal trabeculae.

Isolation and the complete amino acid sequence of lumenal endoplasmic reticulum glucose-6-phosphate dehydrogenase

I have isolated glucose-6-phosphate dehydrogenase from rabbit liver microsomes and determined its complete amino acid sequence. Sequence determination was achieved by automated Edman degradation of peptides generated by chemical and enzymatic cleavages. The microsomal enzyme consists of 763 residues and is quite dissimilar from the previously characterized cytosolic enzymes. The N terminus of the microsomal enzyme is blocked by a pyroglutamyl residue. Carbohydrate is attached at Asn-138 and Asn-263, implying that the bulk of the protein is oriented on the lumenal side of the endoplasmic membrane. The amino acid sequence of the microsomal protein shows limited homology to the extensively sequenced cytosolic glucose-6-phosphate dehydrogenases. Clusters of up to six identical residues can be identified in four regions: peptide segments at residues 10-21, 154-163, and 173-261. In addition, another array of identical residues, requiring a 100-residue deletion in the sequence of the microsomal enzyme, spans residues 436-462 and corresponds to residues 348-373 of the cytosolic protein. Two segments with a Gly-Xaa-Gly-Xaa-Xaa-Gly motif, related to a coenzyme binding fold, were identified at Gly-399 and Gly-491. In the cytosolic enzymes, a variation of this sequence motif occurs at Gly-37 and Gly-241. The 300-residue C-terminal segment of the microsomal enzyme is unique and has no counterpart in the cytosolic or the bacterial enzymes. An unexpected finding with regard to the microsomal enzyme is that it lacks an identifiable membrane-spanning region or the lumenal-protein C-terminal consensus sequences Lys-Asp-Glu or His-Ile/Thr-Glu-Leu. Thus, the mode of transport and retention of this protein in the lumen of endoplasmic reticulum remains to be determined.

In situ glucose-6-phosphate dehydrogenase activity during development of pre-implantation mouse embryos

Glucose-6-phosphate dehydrogenase activity was analysed cytophotometrically in oocytes and pre-implantation embryos of mice. A bimodal distribution pattern was not found. Therefore, female and male embryos could not be discriminated on the basis of linkage of the enzyme with the X-chromosome during the pre-implantation period. The dehydrogenase activity in ovulated eggs and pre-implantation embryos up to the 8-cell stage was 65% of that present in follicular oocytes. In morulae and blastulae, the activity was further decreased to a level that was only 10-20% of the activity present in oocytes. The dramatic decrease in dehydrogenase activity could not be explained by modulation of the enzyme molecules, because KM values did not vary strongly. It is unlikely that the abundant activity of glucose-6-phosphate dehydrogenase in oocytes is due to high activity of the pentose phosphate pathway because of the low activity of 6-phosphogluconate dehydrogenase, the next step in this pathway. It is concluded that high activity of glucose-6-phosphate dehydrogenase in oocytes is needed for keeping oocytes viable, and for generation of NADPH which is important for the fertilization process.

The use of caged substrates to assess the activity of 6-phosphogluconate dehydrogenase in living sea urchin eggs

As part of our inquiries into the regulation of the hexose monophosphate shunt in the early development of sea urchin eggs and embryos, we have developed a novel method to assess the in vivo activity of the enzyme 6-phosphogluconate dehydrogenase (6PGDH) before and after fertilization. Our measurements show that the intracellular level of 6-phosphogluconate (6PG) in eggs decreases 60% after fertilization, which is consistent with the increase in the activity of 6PGDH previously reported using irreversibly permeabilized cell assays (Swezey and Epel, Proc. Natl. Acad. Sci USA 85, 812-816, 1988). The in vivo turnover of the 6PG pool was assessed using a new radioisotopic technique. 1-14C-labeled 6PG was chemically modified such that it was not metabolized by cellular 6PGDH and could be rapidly converted back to 6PG by photolysis. This "caged" 6PG was introduced into unfertilized sea urchin eggs using a transient permeabilization procedure, and then the oxidation of [1-14C]6PG in vivo upon irradiation was followed. Oxidation of 6PG was complete within 7-11 s of irradiation, indicating an extremely rapid turnover of this pool in sea urchin eggs. Based on the 6PG pool sizes and the kinetic properties of 6PGDH, determined here, along with the activity levels seen in permeabilized cells, the half-time for the label in the 6PG pool in sea urchin eggs is calculated to be 26 s. This is inconsistent with the in vivo turnover rates seen in these studies, indicating that the permeabilized cell assays overestimate the degree of inhibition of 6PGDH before fertilization. These results suggest that caution should be exercised in extrapolating data obtained from permeabilized cells to the situation in vivo.

The control of oxidant stress at fertilization

Metazoan eggs alter their coats after fertilization to protect the early embryo. In sea urchins, this modification consists of a rapid, coordinated set of noncovalent macromolecular assembly steps that are stabilized by protein cross-linking. The sea urchin egg uses an oxidative cross-linking reaction that requires hydrogen peroxide and a secreted peroxidase and thus faces the challenge of oxidant stress at the beginning of its development. Protection from the deleterious effects of this oxidative mechanism is afforded by regulation of the production and utilization of oxidizing species. This regulation requires a specific protein kinase C-activated oxidase and ovothiol, an intracellular antioxidant.

CTP:cholinephosphate cytidylyltransferase in human and rat lung: association in vitro with cytoskeletal actin

CTP:cholinephosphate cytidylyltransferase activities were compared in saline homogenates of immature fetal (15-16 weeks gestation) and adult human lung. There were no differences in subcellular enzyme distribution, in Vmax activity, or in the phosphatidylglycerol-mediated stimulation of soluble enzyme activity. These results provide no support for a developmental translocation of cytidylyltransferase from a cytosolic to a microsomal location in human lung, such as that proposed to accompany the maturation of pulmonary surfactant phosphatidylcholine biosynthesis in rat. Soluble cytidylyltransferase activity from human but not rat lung was increased after manipulation in vitro. Resolution of human H form (greater than 10(3) kDa) and L form (200 kDa) enzyme by gel filtration led to an activity increase of 200%. Incubation at 37 degrees C for 2 h increased soluble enzyme recovery, although prior centrifugal removal of generated actin-rich aggregates was necessary in adult lung fractions. In contrast, 85% of soluble rat lung cytidylyltransferase was actin aggregate-associated after incubation. The apparent heteroassociation of rat and human lung enzyme with actin in the presence of poly(ethylene glycol) at 4 degrees C strongly suggested close in vitro and potential in vivo linkage. A partial co-purification of adult human lung cytidylyltransferase with actin was also consistent with this idea. We propose that some reported cytidylyltransferase translocation phenomena may be mediated by cytoskeletal interactions in vitro.

Stable, resealable pores formed in sea urchin eggs by electric discharge (electroporation) permit substrate loading for assay of enzymes in vivo

We describe a simple electroporation procedure for loading suspensions of unfertilized sea urchin eggs with impermeant small molecules under conditions that allow close to 90% successful fertilization and development. Poration is carried out in a low-Ca2+ medium that mimicks the intracellular milieu. The induced pores remain open for several minutes in this medium, allowing loading of the cells; resealing is achieved by adding back millimolar calcium ions to the medium. While the pores are open, an influx of exogenous molecules and efflux of endogenous metabolites takes place, and the eggs can lose up to 40% of their ATP content and still survive. Introduced metabolites are utilized by the cells, e.g., introduced 3H-thymidine is incorporated into DNA. This procedure will be useful for loading impermeant substrates into eggs, permitting in vivo assessment of metabolism, and also for introducing other interesting impermeant molecules, such as inhibitors, fluorescent indicators, etc. Though the details may differ, the principle of electroporation in an intracellular-like medium may prove to be useful for loading other cell types with minimal loss of viability.

In situ kinetic parameters of glucose-6-phosphate dehydrogenase and phosphogluconate dehydrogenase in different areas of the rat liver acinus

The reaction velocity of glucose-6-phosphate dehydrogenase (G6PDH) and phosphogluconate dehydrogenase (PGDH) was quantified with a cytophotometer by continuous monitoring of the reaction product as it was formed in liver cryostat sections from normal, young mature female rats at 37 degrees C. Control incubations were performed in media lacking both substrate and coenzyme for G6PDH activity and lacking substrate for PGDH activity. All reaction rates were non-linear but test minus control reactions showed linearity with incubation time up to 5 min using Nitro BT as final electron acceptor. End point measurements after incubation for 5 min at 37 degrees C revealed that the highest specific activity of G6PDH was present in the intermediate area (Vmax = 7.79 +/- 1.76 mumol H2 cm-3 min-1) and of PGDH in the pericentral and intermediate areas (Vmax = 17.19 +/- 1.73 mumol H2 cm-3 min-1). In periportal and pericentral areas, Vmax values for G6PDH activity were 4.48 +/- 1.03 mumol H2 cm-3 min-1) and 3.47 +/- 0.78 mumol H2 cm-3 min-1), respectively. PGDH activity in periportal areas showed a Vmax of 10.84 +/- 0.33 mumol H2 cm3 min-1. Variation of the substrate concentration for G6PDH activity yielded similar KM values of 0.17 +/- 0.07 mM, 0.15 +/- 0.13 mM and 0.22 +/- 0.11 mM in periportal, pericentral and intermediate areas, respectively. KM values of 0.87 +/- 0.12 mM in periportal and of 1.36 +/- 0.10 mM in pericentral and intermediate areas were found for PGDH activity. The significant difference between KM values for PGDH in areas within the acinus support the hypothesis that PGDH is present in the cytoplasmic matrix and in the microsomes. A discrepancy existed between KM and Vmax values determined in cytochemical assays using cryostat sections and values calculated from biochemical assays using diluted homogenates. In cytochemical assays, the natural microenvironment for enzymes is kept for the demonstration of their activity and thus may give more accurate information on enzyme reactions as they take place in vivo.

Enzyme stimulation upon fertilization is revealed in electrically permeabilized sea urchin eggs

Sea urchin eggs and embryos subjected to high-voltage electric discharge in a medium mimicking the intracellular milieu retain their structural integrity and remain permeable, permitting substrates to enter the cytoplasm and thus assay of enzyme activity. At saturating concentrations of substrates, five of six enzymes assayed for more active (three to fifteen times) in permeabilized embryos than in permeabilized eggs, but no fertilization-related differences are seen in homogenates prepared from these same permeabilized cells. Furthermore, enzyme activity in homogenates always exceeds that in the permeabilized cell suspensions. This difference in enzyme reaction rates between unfertilized eggs and fertilized eggs is not due to differences in the diffusibility of substrates into the permeabilized cells. The activity of glucose-6-phosphate dehydrogenase (D-glucose-6-phosphate:NADP+ 1-oxidoreductase, EC 1.1.1.49) in permeabilized cells was studied in greater detail and has the following characteristics. (i) Regulation of activity persists during early development. (ii) This regulation is not mediated by diffusible allosteric agents. (iii) Stimulation at fertilization is initiated by a rise in intracellular calcium and is further promoted by cytoplasmic alkalinization. (iv) The microenvironment experienced by this enzyme intracellularly differs from that of the enzyme in homogenates as evidenced by markedly different pH vs. activity profiles. These results indicate that the regulatory status of enzymes is preserved in electrically permeabilized cells and suggest that this regulation depends on some cell structural feature(s) that is (are) destroyed upon homogenization.