Linearity in dehydrogenase reaction rate studies in tissue sections is affected by loss of endogenous substrates during the reaction

Advantages and Limitations of Salmon-Gal/Tetrazolium Salt Histochemistry for the Detection of LacZ Reporter Gene Activity in Murine Epithelial Tissue

The Escherichia coli LacZ gene is a widely used reporter for gene regulation studies in transgenic mice. It encodes bacterial β-galactosidase (Bact β-Gal), which causes insoluble precipitates when exposed to chromogenic homologues of galactose. We and others have recently reported that Bact β-Gal detection with Salmon-Gal (S-Gal) in combination with nitro blue tetrazolium chloride (NBT) is very sensitive and not prone to interference by acidic endogenous β-galactosidases. Unfortunately, as we show here, the method appears to be inadequate for evaluation of Bact β-Gal expression in keratinized epithelial appendages but not in other keratinized epithelia. NBT in the reaction mixture, just as other tetrazolium salts, inevitably causes unwanted staining artifacts in lingual filiform papillae, penile spines, and hair fibers by interacting with keratin sulfhydryl-rich regions. The methodological limitation can be overcome in part by pretreating the tissues before the S-Gal/NBT staining with an iodine-potassium iodide solution. Alternatively, the use of iodonitrotetrazolium chloride instead of NBT in the S-Gal reaction mixture provides enough color resolution to distinguish the specific Bact β-Gal staining in orange from the artifact staining in dark red. In summary, we provide evidence that S-Gal/NBT histochemistry has limitations, when staining keratinized epithelial appendages.

Can you trust your cryostat? Reproducibility of cryostat section thickness

Reproducibility of cryostat section thickness is required for valid quantitative microscopy. This is generally pursued by motorized sectioning using a low but constant speed. The purpose of our study was to compare variation in section thickness between motorized and manual cryostat sectioning. Serial sections were cut from a frozen block of homogenized tissue on different days. Lactate dehydrogenase activity was histochemically detected and calibrated absorbance measurements were taken. The coefficients of variation of measurements was 9.7% for motorized sectioning and 3.3% for manual sectioning. In conclusion, section thickness is similarly reproducible after manual sectioning compared with motorized sectioning, if not better.

Estimating the initial reaction velocity of a soluble dehydrogenase in situ

The initial reaction velocities (vi) of lactate dehydrogenase in single hepatocytes were determined, by microdensitometry or computer-assisted image analysis, in sections of unfixed mouse liver incubated at 37 degrees C on substrate-containing agarose gel films. They were found to fit the equations vi = 2.82 degrees A and vi = vi + 2 degrees A, where vi and degrees A are, respectively, the gradients (or steady-state linear velocities) and the intercepts on the absorbance axis of the linear regression lines of the absorbance (A) on incubation time plots for incubation times between 1 and 3 min. Both equations were independent of section thickness between 4 and 14 microns. The observed and calculated values of vi agreed within 11.5% (n = 71). The validity of the equations for vi was confirmed by showing that the calculated vi was proportional to the thickness of the section and hence the amount of enzyme present. Thus, vi can be determined from measurements of either degrees A alone or vi and degrees A.

Initial reaction kinetics of succinate dehydrogenase in mouse liver studied with a real-time image analyser system

The initial reaction kinetics of succinate dehydrogenase in situ were investigated in sections of mouse unfixed liver using an ARGUS-100 image analyser system. The sections were incubated on substrate-containing agarose gel films. Images of a section, illuminated with monochromatic light (584 nm), were captured with the image analyser in real time at intervals of 10 s during the incubation. The absorbances of selected hepatocytes in the successive images were determined as a function of time. In every cell, the absorbance increased nonlinearly after the first minute of incubation. The initial velocity of the dehydrogenase was calculated from the linear activities during the first 20 s of incubation. Hanes plots of the initial velocities and succinate concentration yielded the following mean kinetic constants. For periportal hepatocytes, the apparent Km = 1.2 +/- 0.8 mM and Vmax = 29 +/- 2 mumol hydrogen equivalents formed/cm3 hepatocyte cytoplasm per min. For pericentral hepatocytes, Km = 1.4 +/- 1.0 mM and Vmax = 21 +/- 2 mumol hydrogen equivalents/cm3 per min. The Km values are very similar to those determined previously from biochemical assays. These results, and the observed dependence of the initial velocity on the enzyme concentration, suggest that the technique reported here is valid for the histochemical assay of succinate dehydrogenase.

Enzyme reaction rate studies in electromotor neurons of the weakly electric fish Apteronotus leptorhynchus

A histochemical analysis of reaction rates of a series of enzymes was performed in electromotor neurons of the weakly electric fish Apteronotus leptorhynchus. These neurons were selected because of their functional homogeneity. The high metabolic activity of these cells as well as their large size facilitate cytophotometric analysis in cryostat sections. Sections were incubated for the activity of hexokinase, glucose-6-phosphate dehydrogenase, succinate dehydrogenase, NADPH dehydrogenase, NADPH ferrihaemoprotein reductase and beta-hydroxybutyrate dehydrogenase. All media contained polyvinyl alcohol as tissue stabilizer and Nitro BT as final electron acceptor. Measurements were performed with a Vickers M85a cytophotometer. Linear relationships between the specific formation of formazan (test minus control reaction) and incubation time were obtained for all enzymes although some reactions showed an initial lag phase or an intercept with the ordinate. The relatively high activities of hexokinase, succinate dehydrogenase and the extremely low activity of hydroxybutyrate dehydrogenase indicate that energy is mainly supplied by glycolysis. Glucose-6-phosphate dehydrogenase showed a high activity whereas NADPH reductase and dehydrogenase activity were low in electromotor neurons, indicating that the NADPH generated is largely used for biosynthesis. Despite their synchronous firing pattern activity, electromotor neurons showed a considerable heterogeneity with respect to their metabolic activity.

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.

Reaction rate measurements of proteases and glycosidases with chromogenic methods

Simultaneous azo-coupling and indigogenic methods were evaluated for the quantitative histochemical assay of the plasma membrane proteases gamma-glutamyl transpeptidase (EC 2.3.2.2) and dipeptidyl peptidase IV (EC 3.4.14.5) and the glycosidases maltase-glucoamylase and glucoamylase (EC 3.2.1.20) in decidual cells, jejunal enterocytes and renal proximal tubulocytes. Using kinetic (continuous) microdensitometry, a linear increase in the final reaction product was found from 3 up to 10 min, depending on the substrate concentration and the plasma membrane glycosidase or protease under investigation. Combined continuous and end point (static) microdensitometry revealed a linear relationship between the section thickness (enzyme concentration) and final reaction product up to 12 microns for the proteases and up to 16 microns for the glycosidases. Apparent Km and Vmax values were calculated with a computerized version of the direct linear plot and compared with the results obtained with the linear transformations according to Lineweaver-Burk, Eadie-Hofstee and Hanes. Apparent Km and Vmax values for the proteases were calculated separately for each animal and were 1.82 mM and 1.02 mM and 2.43 arbitrary units (a.u.) and 1.67 a.u. (gamma-glutamyl transpeptidase, decidua) and 0.42 mM and 0.38 mM and 0.29 and 0.26 a.u. (dipeptidyl peptidase IV, decidua). For the alpha-D-glucosidases, the corresponding values were 0.23 mM and 0.15 a.u. (kidney) and 0.55 mM and 0.20 a.u. (jejunum). The results show the suitability of the indigogenic methods for quantitative histochemical measurements of plasma membrane alpha-D-glucosidases, whereas the simultaneous azo-coupling procedures seemed to be less suitable for the quantification of surface membrane proteases, due to, for example, interactions of diazonium salts with amino acid or peptide substrates, reaction products and peptide activators.

Quantitative histochemistry of creatine kinase in rat myocardium and skeletal muscle

Creatine kinase (EC 2.7.3.2) activity was demonstrated in rat myocardium using a polyvinyl alcohol-containing incubation medium and auxiliary enzymes. The activity was quantified by microdensitometry using both endpoint measurements and kinetic measurements. Control reactions were performed in the absence of creatine phosphate and ADP. The linear regression lines of the absorbances of reduced Nitro BT at the isobestic wavelength (585 nm) on incubation time were highly significant for both endpoint and kinetic measurements. The activity obtained from endpoint measurements was about 40% lower. This was caused by loss of the formazan reaction product from the tissue sections when the incubation medium was removed at the end of the reaction. The relationship between creatine kinase activity (test minus control reaction) and section thickness was not linear for either myocardium or skeletal muscle; control reactions, however, showed linear relationships with section thickness for both tissues. Limited penetration of auxiliary enzymes into the sections may be responsible for this disporportionality. Therefore, care should be taken in the interpretation of quantitative data obtained with different tissues. In conclusion, multi-step enzyme reactions can be used for quantitative histochemical purposes provided it is taken into account that the reactivity is not proportional to section thickness.

On the role of oxygen in dehydrogenase reactions using tetrazolium salts

Fundamental aspects of the reduction of tetrazolium salts were investigated and, in particular, the role of oxygen in the reduction. It was found that oxygen had a competitive inhibitory effect on the reduction of (Tetra)Nitro BT mediated by NADH and phenazine methosulphate. This competitive effect, under aerobic conditions, could be reversed by using tetrazolium concentrations of 5 mM. Oxygen did not have a significant effect on BPST reduction, whereas the inhibitory effect of oxygen on the reduction of Neotetrazolium was not reversed by increasing the tetrazolium concentration. The oxygen effect on Nitro BT reduction was considerably less when macromolecular substances such as albumin or polyvinyl alcohol were added to the medium. This may be due to increased Nitro BT concentrations being built up at the surface of macromolecules due to the nonpolar components of the Nitro BT molecule. When demonstrating glucose-6-phosphate dehydrogenase activity in vitro or in tissue sections with the use of Nitro BT, oxygen also had a direct inhibitory effect, even when azide was added to the medium for the inhibition of flavoprotein-mediated electron transfer to oxygen. Again, this direct inhibition of Nitro BT reduction by oxygen could be excluded by using a high Nitro BT concentration. Macromolecules present in the incubation medium or in tissue sections counteracted the oxygen effect. It is concluded that the maximum reaction rate and optimum localization of dehydrogenases is obtained when histochemical media are used containing 5 mM (Tetra)Nitro BT and 20% polyvinyl alcohol.