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

In Situ Localization of Transketolase Activity in Epithelial Cells of Different Rat Tissues and Subcellularly in Liver Parenchymal Cells

Metabolic mapping of enzyme activities (enzyme histochemistry) is an important tool to understand (patho)physiological functions of enzymes. A new enzyme histochemical method has been developed to detect transketolase activity in situ in various rat tissues and its ultrastructural localization in individual cells. In situ detection of transketolase is important because this multifunctional enzyme has been related with diseases such as cancer, diabetes, Alzheimer's disease, and Wernicke-Korsakoff's syndrome. The proposed method is based on the tetrazolium salt method applied to unfixed cryostat sections in the presence of polyvinyl alcohol. The method appeared to be specific for transketolase activity when the proper control reaction is performed and showed a linear increase of the amount of final reaction product with incubation time. Transketolase activity was studied in liver, small intestine, trachea, tongue, kidney, adrenal gland, and eye. Activity was found in liver parenchyma, epithelium of small intestine, trachea, tongue, proximal tubules of kidney and cornea, and ganglion cells in medulla of adrenal gland. To demonstrate transketolase activity ultrastructurally in liver parenchymal cells, the cupper iron method was used. It was shown that transketolase activity was present in peroxisomes and at membranes of granular endoplasmic reticulum. This ultrastructural localization is similar to that of glucose-6-phosphate dehydrogenase activity, suggesting activity of the pentose phosphate pathway at these sites. It is concluded that the method developed for in situ localization of transketolase activity for light and electron microscopy is specific and allows further investigation of the role of transketolase in (proliferation of) cancer cells and other pathophysiological processes.

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.

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.

Glucose-6-phosphate dehydrogenase: the key to sex-related xenobiotic toxicity in hepatocytes of European flounder (Platichthys flesus L.)?

The role of glucose-6-phosphate dehydrogenase (G6PDH) in oxidative stress responses was investigated in isolated intact living hepatocytes of immature female and male European flounder (Platichthys flesus L.) because it is the major provider of NADPH needed as reducing power for various detoxification pathways. Hepatocytes were exposed to sublethal concentrations of effective prooxidants such as 100 microM hydrogen peroxide (H(2)O(2)), 100, 200 and 400 microM benzo[a]pyrene (B[a]p) and 50 microM nitrofurantoin (NF) during culture. Since there is evidence that 17-beta-estradiol inhibits certain pathways of xenobiotic biotransformation, we tested also the effects of different concentrations of 17-beta-estradiol (0.2, 1 and 2 microM) alone and 1 microM in combination with 100 microM B[a]p on G6PDH activity. After short-term (1 day) and long-term (9 days) exposure, G6PDH activity was quantified in intact living hepatocytes by a tetrazolium salt method using tetranitroblue tetrazolium salt (TNBT). Hepatocytes obtained from male fish generally showed higher G6PDH activity than those of females. We observed significant inhibition of G6PDH activity by all oxidative stressors and 17-beta-estradiol in both sexes of fish independently of culture conditions, but inhibition was stronger in cells of females than in cells of males. A cumulative effect of the steroid and B[a]p was not found. Our results indicate a sex-dependent inhibitory effect of all stressors and 17-beta-estradiol on G6PDH activity in flounder hepatocytes independent of prooxidant activity of the specific compound. Consequently, NADPH supply for xenobiotic detoxification and other cellular antioxidative defence mechanisms may be different in livers of female and male flounder. The strongly decreased supply of NADPH in hepatocytes of females may explain the reduced and/or delayed NADPH-dependent activity of xenobiotic biotransformation systems such as cytochrome P450 (CYP450) and a lower capacity of non-enzymatic defence systems such as reduced glutathione that is particularly observed in female flounder. Moreover, the strong inhibition of G6PDH in livers of female flounder may explain higher susceptibility for xenobiotic toxicity and, therefore, potentially a higher risk to develop liver cancer.

Quantitative cytochemical analysis of glucose-6-phosphate dehydrogenase activity in living isolated hepatocytes of European flounder for rapid analysis of xenobiotic effects

There is a great need for rapid but reliable assays to determine quantitatively effects of xenobiotics on biological systems in environmental research. Hepatocytes of European flounder are sensitive to low-dose toxic stress. Glucose-6-phosphate dehydrogenase (G6PDH) is the major source of NADPH in cells and is therefore of major importance for NADPH-dependent xenobiotic biotransformation and defense against toxic injury. These facts prompted us to develop a sensitive cytochemical method to detect G6PDH activity in living isolated flounder hepatocytes using the tetrazolium salt method. The intact plasma membrane did not appear to be a barrier for substrate, co-enzyme, and dye molecules because the intracellular enzyme reaction started immediately when incubation medium was added and could be monitored in real time per individual cell using image analysis. The reaction was effectively stopped for end point measurements by using 4% formaldehyde in 0.1 M phosphate buffer (pH 5.3). The final reaction product, formazan, was stable in hepatocytes for at least 12 days at 4C. This is the first time that a chromogenic histochemical assay is applied to living cells. This approach provides an easy tool for large-scale screening of xenobiotic metabolism and cellular stress defense.

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.

Lobular distribution pattern of lactate dehydrogenase and 6-phosphogluconate dehydrogenase activity in rat liver

Lactate dehydrogenase (LDH) and 6-phosphogluconate dehydrogenase (6-PGDH) activities were measured in lobular areas expanding between 3 portal tracts and an efferent central vein in the livers of male Wistar rats, using a Lowry technique. The maximum of LDH activity was found in a nearly uniform broad area in the lobular periphery. From that area values decreased along periportal/septal-->perivenous gradients, but only slightly within that area along the periportal-->septal axis of the vascular septum. Maximum values of 6-PGDH activity were present in an intermediate area close to the central vein demonstrating a rather inhomogeneous distribution pattern without a clear definition of zonal limits. Our data on the distribution pattern of LDH are in agreement with the concept of the metabolic lobulus and are supported by a recent evaluation of the vascular architecture in rat liver. The lobular distribution pattern of 6-PGDH cannot be interpreted without doubt in accordance with that concept.

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.

Kinetic parameters of lactate dehydrogenase in liver and gastrocnemius determined by three quantitative histochemical methods

We determined the Michaelis constant (K(m)) and maximal velocity (Vmax) of lactate dehydrogenase (LDH) in periportal hepatocytes and skeletal muscle fibers by three different histochemical assay methods. Unfixed sections of mouse liver and gastrocnemius were incubated at 37C either on substrate (L-lactate)-containing agarose gel films or in aqueous assay media with and without 18% polyvinyl alcohol (PVA) as a tissue protectant. The absorbances of the formazan final reaction products were continuously measured at 584 nm in the cytoplasm of individual cells as a function of incubation time, using an image analysis system. The kinetic parameters of purified rabbit skeletal muscle LDH incorporated into polyacrylamide gel sections were similarly determined. The intrinsic initial velocities (vi) of LDH, corrected for "nothing dehydrogenase," were determined as described in the previous article. The Km and Vmax were calculated from Hanes plots of s/vi on L-lactate concentration (s). The Km values obtained with three assay methods were similar and in the range of 21.1-21.9 mM for pure LDH, 8.62-13.5 mM for LDH in mouse periportal hepatocytes, and 13.3-17.9 mM for LDH in mouse skeletal muscle fibers. The Vmax values determined on agarose gel substrate films and in aqueous assay media without PVA were in good agreement but were 53-65% lower when 18% PVA was included in the medium. These results indicate that catalytic center activity kcat of LDH is retarded by the high viscosity of PVA media because PVA hardly inhibited the enzyme. The K(m) values of LDH determined histochemically in skeletal muscle fibers and periportal hepatocytes were respectively three to five times and two to three times higher than those determined biochemically. These differences may be due to interactions of LDH with intracellular components.

Glucose-6-phosphate dehydrogenase activity is higher in the olfactory bulb than in other brain areas

The activity of antioxidant enzymes was measured in the olfactory bulb (OB) of rat and compared with cortex, hippocampus, striatum and septum. Glutathione reductase, glutathione peroxidase, catalase and superoxide dismutase were not significantly different in the five brain areas, while glucose-6-phosphate dehydrogenase (G6PD) and 6-phosphogluconate dehydrogenase activities were four times higher in the OB than in the other areas. This picture prompted us to explore the reasons of the marked increase of G6PD, since it is the enzyme that regulates the operation of the hexose monophosphate shunt. A first approach was to analyze the G6PD electrophoretic pattern. The analysis revealed that the high G6PD activity of the bulb was neither due to new isoenzymes nor to a modification of the equilibrium between the G6PD dimers. We secondly hypothesized an induction of G6PD activity in the OB by oxidant stress. The assay of markers of the oxidant stress, such as thiobarbituric acid reactive substances, oxidized and reduced glutathione, did not confirm this hypothesis. A third approach was the cytochemical analysis of cryostat sections of OB. By this method we identified a particular cell type which was very rich in G6PD and located at the border of the glomerular layer. Thus, we attributed the high G6PD activity of the OB to the consistent presence of periglomerular cells, that probably need a high G6PD activity for their regulatory function in the neurochemical transmission.

Glucose-6-phosphate dehydrogenase activity in spinach as measured by image analysis: a new approach for plant enzyme histochemistry

A relatively low-cost computer-assisted image analysis system is described. Software has been specifically written for the continuous monitoring of absorbance readings on cryostat sections of plant tissues incubated in media to reveal enzyme activities. The equipment was tested by quantifying glucose-6-phosphate dehydrogenase activity in cryostat sections from shoot apices of spinach plants. The reaction rate of the dehydrogenase activity was monitored at two incubation temperatures, 20 degrees C and 30 degrees C. Control incubations were performed in media lacking substrate. The specific test minus control reaction at 30 degrees C was twice that at 20 degrees C. Variation of the substrate concentration at 30 degrees C yielded a Km value of 0.37 mM. These preliminary results show that our image analysis system can be used for kinetic measurements of dehydrogenase activity in frozen tissue sections and constitute a new approach for enzyme histochemistry in the shoot apical meristem.

Combined enzyme histochemical and ultrastructural study on cryostat sections of pig heart to detect early reperfusion damage after ischaemia

In cardiac surgery, recognition of peroperative myocardial infarction may improve patient selection for prolonged circulatory support. The value of enzyme histochemistry to discriminate between reversible and irreversible myocardial damage at short periods of reperfusion was studied in an in vivo model of regional ischaemia in pig hearts. The left anterior descending coronary artery (LADCA) was clamped for 45 min followed by 2 h reperfusion (group 1, n = 3). Post-mortem heart tissue showed markedly decreased activities of lactate dehydrogenase (LDH) and beta-hydroxybutyrate dehydrogenase (BDH) as demonstrated in cryostat sections, accompanied by massive leakage of LDH in the venous effluent. The depleted areas showed irreversible cell damage based on the presence of flocculent densities in mitochondria. In group 2 (n = 6), LADCA flow was reduced to 40 per cent of the base-line value followed by 2 h reperfusion. Heart tissue showed normal LDH and BDH activities, except for some cells surrounding blood vessels, which activity was minimally decreased. Flocculent densities in mitochondria were never observed. We conclude that enzyme histochemistry of LDH and BDH activity on cryostat sections is a useful tool for detecting irreversible myocardial cell damage as early as 2 h reperfusion after ischaemia of the pig heart. The technique has potential applications in the detection of peroperative infarction in human biopsies.

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.

Adaptive sex-dependent changes in the zonation of carbohydrate and lipid metabolism in rat liver lobules after partial hepatectomy

To evaluate changes in metabolic heterogeneity in rat liver lobules after partial hepatectomy, we measured parameters of carbohydrate and lipid metabolism cytophotometrically in periportal and pericentral zones of livers of mature female and male rats. Glycogen content was shown to be always higher in pericentral zones than in periportal zones. After a rapid depletion of glycogen stores during the first 8 hr after partial hepatectomy, the levels were restored to normal after 24 hr, but a significant depletion was found again at 48 hr after operation. These fluctuations were similar in female and male rat livers. The lipid content in control rat livers was low and was mainly localized in periportal zones. Partial hepatectomy caused a significant increase in lipid content after 24 to 48 hr in periportal zones only, which was distinctly higher in female than in male rat livers. Activity of NADPH-producing glucose-6-phosphate dehydrogenase was heterogeneously distributed in lobules of female control rats with highest activity in pericentral zones, whereas a lower but evenly distributed activity was found in lobules of control male rats. The activity was not affected by partial hepatectomy in male rats, whereas the activity in female rat livers decreased to levels found in male rats at 24 to 48 hr after operation. Another NADPH-producing enzyme, malate dehydrogenase, showed the highest activity pericentrally in female rats, and a low activity was evenly distributed in male rats. The activity did not change significantly after partial hepatectomy. The ketogenic enzyme beta-hydroxybutyrate dehydrogenase showed the highest activity in pericentral zones of control livers. The activity in male rat livers was almost twice as high as in female rat livers in both zones. Partial hepatectomy caused a distinct reduction in activity in both zones and both sexes, but the strongest reduction was found periportally. Alkaline phosphatase activity, which is linked with bile acid secretion by hepatocytes, was low in control male and female rats and was mainly found periportally. The activity was increased dramatically at 24 to 48 hr after partial hepatectomy in both zones and particularly in male rat livers. The index for the Krebs cycle, succinate dehydrogenase activity, was highest in periportal zones. At 24 to 48 hr after partial hepatectomy, this preferential zonation was lost, and the activity was slightly higher in pericentral zones. This reversal of zonation was found in all livers of female and male rats investigated.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantitative comparison between the gel-film and polyvinyl alcohol methods for dehydrogenase histochemistry reveals different intercellular distribution patterns of glucose-6-phosphate and lactate dehydrogenases in mouse liver

The precise histochemical localization and quantification of the activity of soluble dehydrogenases in unfixed cryostat sections requires the use of tissue protectants. In this study, two protectants, polyvinyl alcohol (PVA) and agarose gel, were compared for assaying the activity of lactate dehydrogenase (LDH) and glucose-6-phosphate dehydrogenase (G6PDH) in normal female mouse liver. Quantification of enzyme activity was determined cytophotometrically in periportal (PP), pericentral (PC) and midzonal (MZ) areas. No coloured reaction product was present in PVA media after the incubation period. In contrast, the agarose gels appeared to be highly coloured after incubation. As a consequence, sections incubated with gel media were less intensely stained than those incubated in PVA-containing media. The specific G6PDH reaction (test minus control) yielded approximately 75% less formazan in sections incubated by the agarose gel method than with the PVA method. Further, the amount of formazan deposits attributable to G6PDH activity was highest in the midzonal and pericentral zones of the liver lobule with PVA media, and Kupffer cells could be discriminated easily because of their high G6PDH activity. Significant zonal differences or Kupffer cells could not be observed when agarose gel films were used for the detection of G6PDH activity. The LDH localization patterns appeared to be more uniform after incubation with both methods: no significant differences in specific test minus control reactions were seen between PP, PC and MZ. However, less formazan production (33%) was detected in sections incubated with agarose gels when compared with those incubated with PVA media.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucose-6-phosphate dehydrogenase is enriched in oligodendrocytes of the rat spinal cord. Enzyme histochemical and immunocytochemical studies

Glucose-6-phosphate dehydrogenase (G6PD) was localized in rat spinal cord by catalytic enzyme histochemistry and immunocytochemistry. G6PD detected by either method was shown to be strongly enriched in cell bodies and processes of oligodendrocytes, whereas in the compact myelin G6PD was not detected. The enzyme histochemical procedure for the demonstration of G6PD was also adapted for microphotometric measurements of G6PD activity in the spinal cord white matter. There was a linear relationship between G6PD activity and section thickness up to 14 microns and between G6PD activity and reaction time up to 5-6 min as demonstrated by kinetic and end-point measurements. Significantly lower activities were measured in end-point measurements than in kinetic measurements because of formazan loss during rinsing. Methoxyphenazine methosulphate as an exogenous electron carrier and sodium azide as a blocker of the respiratory chain significantly increased the demonstrable G6PD activity. The Km was 0.62 mM and the Vmax 3 mumol glucose-6-phosphate/cm3 wet tissue and per min at 25 degrees C. It is concluded that G6PD in oligodendrocytes may be important for the generation of NADPH required for lipid biosynthesis related to myelogenesis, and reduction of glutathione required for protection of membrane sulphydryl groups.

Experimentally induced colon cancer metastases in rat liver increase the proliferation rate and capacity for purine catabolism in liver cells

Metastases in rat liver were generated experimentally by intraportal injection of colon cancer cells to investigate the effects of cancerous growth on the metabolism of surrounding liver tissue. Maximum activities (capacity) of glucose-6-phosphate dehydrogenase, phosphogluconate dehydrogenase, lactate dehydrogenase, succinate dehydrogenase, alkaline phosphatase, 5'-nucleotidase, xanthine oxidoreductase, purine nucleoside phosphorylase and adenosine triphosphatase have been determined. Two types of metastases were found, a small type surrounded by stroma and a larger type in direct contact with hepatocytes. Both types affected the adjacent tissue in a similar way suggesting that the interactions were not mediated by stroma. High capacity of the degradation pathway of extracellular purines released from dead cells of either tumours or host tissue was found in stroma and sinusoidal cells. Metastases induced both an increase in the number of Kupffer cells and proliferation of hepatocytes. The distribution pattern in the liver lobulus of most enzymes investigated did not change distinctly. However, activity of alkaline phosphatase, succinate dehydrogenase and phosphogluconate dehydrogenase was increased in hepatocytes directly surrounding metastases. These data imply that the overall metabolic zonation in liver lobuli is not dramatically disturbed by the presence of cancer cells despite the fact that various metabolic processes in liver cells are affected.

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.

Metabolic zonation of the liver: regulation and implications for liver function

Liver parenchyma shows a remarkable heterogeneity of the hepatocytes along the porto-central axis with respect to ultrastructure and enzyme activities resulting in different cellular functions within different zones of the liver lobuli. According to the concept of metabolic zonation, the spatial organization of the various metabolic pathways and functions forms the basis for the efficient adaptation of liver metabolism to the different nutritional requirements of the whole organism in different metabolic states. The present review summarizes current knowledge about this heterogeneity, its development and determination, as well as about its significance for the understanding of all aspects of liver function and pathology, especially of intermediary metabolism, biotransformation of drugs and zonal toxicity of hepatotoxins.