C. J. F. Van Noorden

The Role of Exogenous Electron Carriers in NAD(P)-dependent Dehydrogenase Cytochemistry Studied In Vitro and with a Model System of Polyacrylamide Films

The applicability of phenazine methosulfate, 1-methoxyphenazine methosulfate, menadione, and meldola blue as exogenous electron carriers for the cytochemical staining of nicotinamide adenine dinucleotide (phosphate) (NAD(P))-dependent dehydrogenases has been studied quantitatively with tetranitro BT in vitro and with a model system of polyacrylamide films incorporating either purified glucose-6-phosphate dehydrogenase or intact rat liver parenchymal cells. It was found that every assay in which a tetrazolium salt is used, whether or not an electron carrier is present, has to be carried out in darkness. Menadione did not appear to be useful, because electrons were not found to be transferred directly from reduced nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) to this compound. Phenazine methosulfate at higher concentrations and meldola blue at concentrations optimal for carrying electrons to tetrazolium salts yielded a high level of nothing dehydrogenase activity in cell-containing films, but no inhibition of enzymatic activity was found. Factors involved in the interference of oxygen with tetrazolium salt reduction are discussed. 1-Methoxyphenazine methosulfate did not stain cellular compounds and caused only a very low nothing dehydrogenase activity. The cytochemical demonstration of dehydrogenase activity was shown to be independent on the concentration of 1-methoxyphenazine methosulfate used (50-1000 microM). It is concluded that 1-methoxyphenazine methosulfate is the exogenous electron carrier of choice.

Localization of xanthine oxidoreductase activity using the tissue protectant polyvinyl alcohol and final electron acceptor Tetranitro BT.

We have detected xanthine oxidoreductase activity in unfixed cryostat sections of rat and chicken liver, rat duodenum, and bovine mammary gland using the tissue protectant polyvinyl alcohol, the electron carrier 1-methoxyphenazine methosulfate, the final electron acceptor Tetranitro BT, and hypoxanthine as a substrate. Enzyme activity was localized in rat duodenum at lateral membranes and brush borders of enterocytes and in goblet cells and mucus. Hepatocytes in pericentral areas and especially sinusoidal cells showed high activity in rat liver. Xanthine oxidoreductase was also detected in epithelial cells and milk lipid globules of lactating bovine mammary gland, which is known to contain large quantities of the oxidase form of the enzyme. Chicken liver, which contains an inconvertible dehydrogenase form, also showed high activity in sinusoidal cells. Therefore, we conclude that the tetrazolium reaction demonstrates both the dehydrogenase and the oxidase form of xanthine oxidoreductase. Control activity, in the absence of hypoxanthine or in the presence of the competitive inhibitor allopurinol, was low in all tissues studied. Addition of O2 or NAD to the incubation medium did not change the specific reaction in bovine mammary gland or chicken liver, implying that the dehydrogenase and the oxidase form are not dependent on their natural electron acceptors in this tetrazolium salt reaction. We conclude that the present light microscopic method gives specific and precise localization of xanthine oxidoreductase activity in situ.

Histochemical localization of NADP-dependent dehydrogenase activity with four different tetrazolium salts.

The properties of the four most commonly used tetrazolium salts, neotetrazolium, nitro blue tetrazolium (nitro-BT), tetranitro-BT, and 2-(2-benzothiazolyl-3-(4-phthalhydrazidyl)-5-styryl-te trazolium (BPST), have been compared for their effects on the localization of nicotinamide adenine dinucleotide phosphate (NADP)-dependent dehydrogenases under optimal incubation conditions in cryostat sections of rat liver. Glucose-6-phosphate dehydrogenase has been selected as an example of these dehydrogenases. It was found that the use of nitro-BT and tetranitro-BT, unlike neotetrazolium and BPST, in combination with an exogeneous electron carrier and azide, resulted in localization patterns in agreement with the sites of activity as determined by microchemical techniques. In the absence of an intermediate carrier the localization was very similar to that of NADPH cytochrome c (P450) reductase as demonstrated immunocytochemically. BPST did not properly localize dehydrogenase activity, most probably because of the redistribution of formazan, due to its lack of firm substantivity. Neotetrazolium reduction in nitrogen gave the localization pattern, both in the presence and absence of carrier, of the reductase, suggesting that the transfer of reducing equivalents from the exogenous electron carrier to neotetrazolium proceeds via cellular electron transport systems. The reduction of nitro-BT and tetranitro-BT via intermediate carriers was oxygen sensitive in parenchymal cells, but not in the non-parenchymal liver cells. This oxygen sensitivity could be blocked by azide. With neotetrazolium, oxygen inhibited both carrier-mediated and carrier-independent reactions, effects that were not reversed with azide. Possible mechanisms of action between oxygen, reduced carriers, and tetrazolium salts are discussed.

Image analysis and image processing as tools to measure initial rates of enzyme reactions in sections: distribution patterns of glutamate dehydrogenase activity in rat liver lobules.

To analyze regional differences in the activity of glutamate dehydrogenase in rat liver in situ, we developed an image recording and processing system for monitoring the formation of a colored final reaction product in time. All absorbance measurements of test and control reactions in time in consecutive sections were used to fit the data to a quadratic curve, with the derivative at t = 0 representing the initial velocity of formazan formation. The images of sections incubated for test and control reactions were topographically matched with an affine transformation using the positions of vessels as fiducials. Specific enzyme activity was calculated by subtracting the coefficients representing the initial velocity at corresponding locations in the test and control reactions and appeared to be 8 and 4 mumoles glutamate converted per min per cm3 of tissue at 20 degrees C in pericentral and periportal zones of fasted female rats, respectively. Those values are in agreement with biochemical data. The ability to construct two-dimensional images of cellular distribution patterns of enzyme activity in liver lobules is particularly useful for the study of metabolic zonation in this organ.

A quantitative histochemical study of xanthine oxidase activity in rat liver using the cerium capture method in the presence of polyvinyl alcohol.

A recently developed histochemical technique to demonstrate xanthine oxidase activity in milk globules of bovine mammary gland and in epithelial cells of rat small intestine using cerium ions and a semipermeable membrane was slightly modified. The semipermeable membrane method was replaced by the addition of 10% (w/v) polyvinyl alcohol to the incubation medium. This technically more simple procedure enabled detection of xanthine oxidase activity in unfixed cryostat sections of rat liver. Both methods gave qualitatively and quantitatively similar results. Activity was found in sinusoidal cells and in liver parenchymal cells, with 50% higher activity in pericentral than in periportal areas. The specificity of the reaction was proven by the generation of only small amounts of final reaction product on incubation either in the absence of the substrates hypoxanthine or oxygen or in the presence of hypoxanthine and allopurinol. Allopurinol is a specific inhibitor of xanthine oxidase activity. The amount of final reaction product, as measured cytophotometrically in rat liver, increased linearly with incubation time (15-90 min) and with section thickness (up to 12 microns). By varying the hypoxanthine concentrations, a Km value of 0.05 mM was found. Addition of dithiothreitol to the incubation medium reduced the amount of final reaction product by 85%, which was caused by conversion of reversible xanthine oxidase into xanthine dehydrogenase. This histochemical method can be used for quantitative analysis of in situ xanthine oxidase activity.

Quantitative histochemical analysis of glucose-6-phosphatase activity in rat liver using an optimized cerium-diaminobenzidine method.

We have optimized a cerium-diaminobenzidine-based method for histochemical analysis of glucose-6-phosphatase (G6Pase) activity and have determined quantitative data on the zonal distribution pattern in the liver acinus of fasted male rats. In the cerium-diaminobenzidine technique, cerium instead of lead ions is used as capturing reagent for the enzymatically liberated phosphate. For light microscopy, the primary reaction product, cerium phosphate, is then visualized by conversion into cerium perhydroxide using hydrogen peroxide and subsequent oxidative polymerization of diaminobenzidine to diaminobenzidine brown as the final reaction product. Variation of the substrate (glucose-6-phosphate) concentration in the incubation medium yielded in periportal zones a KM value of 2.3 +/- 0.7 mM and a Vmax value of 0.96 +/- 0.18 (expressed as mean integrated absorbance). In perivenous zones a KM value of 1.1 +/- 0.4 mM and a Vmax value of 0.51 +/- 0.08 were calculated. The cytophotometric analysis performed in this study demonstrated for the first time that a functional difference of G6Pase, the key enzyme for gluconeogenesis, exists in the periportal and perivenous zones of the liver acinus. Periportal zones contain twice as many enzyme molecules (high Vmax) as perivenous zones, but the affinity for the substrate is twice as low. This may have important implications for the concept of metabolic zonation of the liver and also for glucose homeostasis in the blood.

The use of unfixed cryostat sections for electron microscopic study of D-amino acid oxidase activity in rat liver.

Unfixed cryostat sections of rat liver were incubated to demonstrate D-amino acid oxidase activity at the ultrastructural level. Incubation was performed by mounting the sections on a semipermeable membrane which was stretched over a gelled incubation medium containing D-proline as substrate and cerium ions as capture reagent for hydrogen peroxide. After an incubation period of 30 min, ultrastructural morphology was retained to such an extent that the final reaction product could be localized in peroxisomes, whereas the crystalline core remained unstained. Control incubations were performed in the absence of substrate; the lack of final reaction product in peroxisomes indicated the specificity of the reaction. We conclude that the semipermeable membrane technique opens new perspectives for localization of enzyme activities at the ultrastructural level without prior tissue fixation, thus enabling localization of the activity of soluble and/or labile enzymes.

Flow Cytofluorometric Analysis of Enzyme Reactions Based on Quenching of Fluorescence by the Final Reaction Product: Detection of Glucose-6-Phosphate Dehydrogenase Deficiency in Human Erythrocytes

We developed a method for accurate cytofluorometric analysis of the final reaction product of enzyme reactions in individual cells. Glucose-6-phosphate dehydrogenase (G6PD) activity in human erythrocytes was demonstrated cytochemically, and the amount of final reaction product (formazan) per cell was detected indirectly by quenching of autofluorescence generated by glutaraldehyde fixation. Formazan quenches fluorescence in a dose-dependent manner. The method has been used for detection of G6PD deficiency. Heterozygous and homo(hemi)zygous deficiency could easily be established, even in cases of extreme Lyonization where microscopic inspection failed to discriminate between either normal individuals and heterozygously deficient patients or heterozygously and homozygously deficient patients. The principle of quenching of fluorescence by final reaction products of enzymes can be applied to flow cytofluorometric analysis of enzyme activity in individual cells in general.

Gender-dependent regulation of glutamate dehydrogenase expression in periportal and pericentral zones of rat liver lobules.

We studied the level(s) at which glutamate dehydrogenase (GDH; EC 1.4.1.2) expression is regulated in the livers of fed male and female rats. The cellular content of GDH mRNA, protein, and enzyme activity was determined quantitatively using image analysis for measurement of the absorbance in consecutive serial sections that were processed for in situ hybridization, immunohistochemistry, and enzyme histochemistry. In both males and females, GDH protein and activity patterns were similar, with pericentral values being twice as high as periportal values. GDH mRNA distribution patterns in female liver lobules reflected those of GDH protein and activity, but GDH mRNA distribution patterns in male rat livers were found to be homogeneous owing to a more than twofold lower cellular mRNA content in pericentral zones than in female rats. We conclude that gender affects GDH expression selectively in pericentral zones at posttranscriptional and pretranslational levels.

A quantitative histochemical study of D-amino acid oxidase activity in rat liver in relationship with feeding conditions

The histochemical method for the demonstration of D-amino acid oxidase activity in rat liver, based on the use of cerium ions and the diaminobenzidine-cobalt-hydrogen peroxide procedure, was improved by the application of unfixed cryostat sections and a semipermeable membrane interposed between section and gelled incubation medium. The amount of final reaction product precipitated in a granular form was about four times higher with this technique in comparison with conventional procedures using fixed sections and aqueous incubation media. The specificity of the reaction was proven by the 70% reduction of the amount of final reaction product when incubating in the presence of substrate and D,L-beta-hydroxybutyrate, a specific inhibitor of D-amino acid oxidase activity. Cytophotometric analysis of liver sections revealed that the specific test minus control reaction was linear with incubation time and section thickness. The Km value of the enzyme of 10.3 +/- 2.7 mM, as determined in periportal areas, is about five times the value found with biochemical methods in liver cell homogenates. The enzyme activity in periportal areas is about five times the activity in pericentral areas. Fasting (24 and 48 hr) induced a significant decrease in D-amino acid activity in periportal and pericentral areas. The possible physiological role of the enzyme in liver is discussed.