Calf thymus alkaline phosphatase. I. Properties of the membrane-bound enzyme

An Overview on the Mechanisms and Applications of Enzyme Inhibition-Based Methods for Determination of Organophosphate and Carbamate Pesticides

Acetylcholinesterase inactivating compounds, such as organophosphate (OP) and carbamate (CM) pesticides, are widely used in agriculture to ensure sustainable production of food and feed. As a consequence of their applications, they would result in neurotoxicity, even death. In this essence, the development of enzyme inhibition methods still shows great significance as rapid detection techniques for on-site large-scale screening of OPs and CMs. Initially, mechanisms and applications of various enzyme-inhibition-based methods and devices, including optical colorimetric assay, fluorometric assays, electrochemical biosensors, rapid test card, and microfluidic device, are highlighted in the present overview. Further, to enhance the enzyme sensitivity for detection; alternative enzyme sources or high yield enrichment methods (such as abzyme, artificial enzyme, and recombinant enzyme), as well as enzyme reactivation and identification, are also addressed in this comprehensive overview.

Enhanced Diffusion and Chemotaxis of Enzymes

Many enzymes appear to diffuse faster in the presence of substrate and to drift either up or down a concentration gradient of their substrate. Observations of these phenomena, termed enhanced enzyme diffusion (EED) and enzyme chemotaxis, respectively, lead to a novel view of enzymes as active matter. Enzyme chemotaxis and EED may be important in biology and could have practical applications in biotechnology and nanotechnology. They are also of considerable biophysical interest; indeed, their physical mechanisms are still quite uncertain. This review provides an analytic summary of experimental studies of these phenomena and of the mechanisms that have been proposed to explain them and offers a perspective on future directions for the field.

A Thermodynamic Limit on the Role of Self-Propulsion in Enhanced Enzyme Diffusion

A number of enzymes reportedly exhibit enhanced diffusion in the presence of their substrates, with a Michaelis-Menten-like concentration dependence. Although no definite explanation of this phenomenon has emerged, a physical picture of enzyme self-propulsion using energy from the catalyzed reaction has been widely considered. Here, we present a kinematic and thermodynamic analysis of enzyme self-propulsion that is independent of any specific propulsion mechanism. Using this theory, along with biophysical data compiled for all enzymes so far shown to undergo enhanced diffusion, we show that the propulsion speed required to generate experimental levels of enhanced diffusion exceeds the speeds of well-known active biomolecules, such as myosin, by several orders of magnitude. Furthermore, the minimal power dissipation required to account for enzyme enhanced diffusion by self-propulsion markedly exceeds the chemical power available from enzyme-catalyzed reactions. Alternative explanations for the observation of enhanced enzyme diffusion therefore merit stronger consideration.

Redox-Dependent Bone Alkaline Phosphatase Dysfunction Drives Part of the Complex Bone Phenotype in Mice Deficient for Memo1

Mediator of ErbB2-driven cell Motility 1 (MEMO1) is an intracellular redox protein that integrates growth factors signaling with the intracellular redox state. We have previously reported that mice lacking Memo1 displayed higher plasma calcium levels and other alterations of mineral metabolism, but the underlying mechanism was unresolved and the bone phenotype was not described. Here, we show that Cre/lox-mediated MEMO1 deletion in the whole body of C57Bl/6 mice (Memo cKO) leads to severely altered trabecular bone and lower mineralization, with preserved osteoblast and osteoclast number and activity, but altered osteoblast response to epidermal growth factor (EGF) and FGF2. More strikingly, Memo cKO mice display decreased alkaline phosphatase (ALP) activity in serum and in bone, while ALPL expression level is unchanged. Bone intracellular redox state is significantly altered in Memo cKO mice and we inferred that ALP dimerization was reduced in Memo cKO mice. Indeed, despite similar ALP oxidation, we found increased ALP sensitivity to detergent in Memo cKO bone leading to lower ALP dimerization capability. Thus, we report a severe bone phenotype and dysfunctional bone ALP with local alteration of the redox state in Memo cKO mice that partially mimics hypophosphatasia, independent of ALPL mutations. These findings reveal Memo as a key player in bone homeostasis and underline a role of bone redox state in controlling ALP activity.

Adsorption and separation of proteins by a synthetic hydrotalcite

In this study, the potential use of a synthetic Mg/Al hydrotalcite (layered double hydroxide) as a novel chromatography material for protein purification was investigated. The hydrotalcite is present in its carbonate form and is characterized by an Al/Mg-ratio of 1.85. Zetapotential measurements confirm a positive surface potential up to pH 10 suggesting applicability as anion exchanger. The binding of model proteins covering a broad range of isoelectric points and molecular weights was performed at different pH-values under batch conditions to evaluate the binding behaviour of the hydrotalcite. Furthermore, static binding capacities were exemplarily determined for hemoglobin and human serum albumin. Additionally, the adsorption and elution of hemoglobin was studied under dynamic conditions. The binding behaviour of the hydrotalcite was compared to commercially available anion exchangers and was found to be a function of pH, depending on the model protein. Variant adsorption behaviour is explained by further interactions like hydrogen bonds and by an unequal charge distribution over the protein surfaces. The hydrotalcite reveals high adsorption capacities under static (260 mg/g) as well as under dynamic conditions (88 mg/g at 34 cm/h; 61 mg/g at 340 cm/h). With appropriate buffers like 500 mM carbonate (pH 10) the adsorbed proteins can be nearly completely desorbed making regeneration possible. Due to the binding and elution properties it is concluded, that the hydrotalcite can serve anion exchange material for chromatographic protein separations.

A high-resolution, fluorescence-based method for localization of endogenous alkaline phosphatase activity

We describe a high-resolution, fluorescence-based method for localizing endogenous alkaline phosphatase in tissues and cultured cells. This method utilizes ELF (Enzyme-Labeled Fluorescence)-97 phosphate, which yields an intensely fluorescent yellow-green precipitate at the site of enzymatic activity. We compared zebrafish intestine, ovary, and kidney cryosections stained for endogenous alkaline phosphatase using four histochemical techniques: ELF-97 phosphate, Gomori method, BCIP/NBT, and naphthol AS-MX phosphate coupled with Fast Blue BB (colored) and Fast Red TR (fluorescent) diazonium salts. Each method localized endogenous alkaline phosphatase to the same specific sample regions. However, we found that sections labeled using ELF-97 phosphate exhibited significantly better resolution than the other samples. The enzymatic product remained highly localized to the site of enzymatic activity, whereas signals generated using the other methods diffused. We found that the ELF-97 precipitate was more photostable than the Fast Red TR azo dye adduct. Using ELF-97 phosphate in cultured cells, we detected an intracellular activity that was only weakly labeled with the other methods, but co-localized with an antibody against alkaline phosphatase, suggesting that the ELF-97 phosphate provided greater sensitivity. Finally, we found that detecting endogenous alkaline phosphatase with ELF-97 phosphate was compatible with the use of antibodies and lectins. (J Histochem Cytochem 47:1443-1455, 1999)

Effect of calcium ions on rat osseous plate alkaline phosphatase activity

Rat osseous plate alkaline phosphatase is a metalloenzyme with two binding sites for Zn2+ (sites I and III) and one for Mg2+ (site II). This enzyme is stimulated synergistically by Zn2+ and Mg2+ (Ciancaglini et al., 1992) and also by Mn2+ (Leone et al., 1995) and Co2+ (Ciancaglini et al., 1995). This study was aimed to investigate the modulation of enzyme activity by Ca2+. In the absence of Zn2+ and Mg2+, Ca2+ had no effects on the activity of Chelex-treated, Polidocanol-solubilized enzyme. However, in the presence of 10 microM MgCl2, increasing concentration of Ca2+ were inhibitory, suggesting the displacement of Mg2+ from the magnesium-reconstituted enzyme. For calcium-reconstituted enzyme, Zn2+ concentrations up to 0.1 microM were stimulatory, increasing specific activity from 130 U/mg to about 240 U/mg with a K0.5 = 8.5 nM. Above 0.1 microM Zn2+ exerted a strong inhibitory effect and concentrations of Ca2+ up to 1 mM were not enough to counteract this inhibition, indicating that Ca2+ was easily displaced by Zn2+. At fixed concentrations of Ca2+, increasing concentrations of Mg2+ increased the enzyme specific activity from 472 U/mg to about 547 U/mg, but K0.5 values were significantly affected (from 4.4 microM to 38.0 microM). The synergistic effects observed for the activity of Ca2+ plus magnesium-reconstituted enzyme, suggested that these two ions bind to the different sites. A model to explain the effect of Ca2+ on the activity of the enzyme is presented.

Hepatitis B virus binding to leucocyte plasma membranes utilizes a different region of the preS1 domain to the hepatocyte receptor binding site and does not require receptors for opsonins

A quantitative assay of hepatitis B virus (HBV) binding to hepatocyte plasma membranes was adapted to show that leucocyte plasma membranes bind serum-derived HBV saturably, and that this binding is inhibited using synthetic peptides representative of the large envelope protein of HBV. Using a panel of ligand-blocking monoclonal antibodies (mAb) to opsonin receptors, it was shown that the three classes of Fc gamma R and CR3 are not major receptors for HBV on leucocytes or hepatocytes. It was also shown that HBV does not utilize the receptor for IgA, Fc alpha R, for attachment to leucocytes, despite reported sequence homology between the large envelope protein of HBV and the Fc portion of human IgA. Evidence is presented that the receptor for HBV on leucocytes may differ from the hepatocyte receptor(s), based on synthetic peptide inhibition assays of HBV binding. Furthermore, it was observed that glycosaminoglycans influence the HBV-liver and leucocyte interactions, providing evidence that HBV attachment may be a multi-stage process.

Mechanism of action of cobalt ions on rat osseous plate alkaline phosphatase

Polidocanol-solubilized osseous plate alkaline phosphatase was modulated by cobalt ions in a similar way as by magnesium ions. For concentrations up to 1 microM, the Chelex-treated enzyme was stimulated by cobalt ions, showing Kd = 6.0 microM, V = 977.5 U/mg, and site-site interactions (n = 2.5). Cobalt-enzyme was highly unstable at 37 degrees C, following a biphasic inactivation process with inactivation constants of about 0.0625 and 0.0015 min-1. Cobalt ions stimulated the enzyme synergistically in the presence of magnesium ions (Kd = 5.0 microM; V = 883.0 U/mg) or in the presence of zinc ions (Kd = 75.0 microM; V = 1102 U/mg). A steady-state kinetic model for the modulation of enzyme activity by cobalt ions is proposed.

Effect of pH on the modulation of rat osseous plate alkaline phosphatase by metal ions

1. Metal ions other than zinc and magnesium were effective in modulating the activity of rat osseous plate alkaline phosphatase. 2. Increasing pH had remarkable effects on the modulation of rat osseous plate alkaline phosphatase. 3. The modulation of enzyme activity by zinc, manganese and cobalt ions was slightly affected by pH variations. 4. Zinc ions were stimulatory for the enzyme at very low concentrations (50 nM). Above 50 nM zinc ions inhibited the enzyme by displacing magnesium ions. 5. Calcium ions were inhibitors of alkaline phosphatase (Kd = 10 microM) whereas manganese (Kd = 1.3 microM) and cobalt (Kd = 0.2 microM) ions were stimulatory in the pH range 8.0-10.0.

Polyoxyethylene 9-lauryl ether-solubilized alkaline phosphatase: synergistic stimulation by zinc and magnesium ions

1. Polidocanol-solubilized apoalkaline phosphatase could be stimulated either by zinc ions (Kd = 8.5 nM) or by magnesium ions alone (Kd = 3.8 microM). 2. Zinc and magnesium ions had synergistic effects on Polidocanol-solubilized apoalkaline phosphatase, leading to a fully active enzyme (700-800 U/mg). 3. Zinc ions inhibited non-competitively the Polidocanol-solubilized apoenzyme (Ki = 7.1 microM) by displacing magnesium ions from their binding sites. 4. A model for the action of zinc and magnesium ions on the modulation of the enzyme activity is proposed.

Bound and purified alkaline phosphatase from rat duodenal and jejunal brush-border membranes: kinetics and magnesium stimulation

Brush-border membranes from rat duodenal and jejunal mucosa were prepared by differential Ca(2+)-precipitation. Kinetical properties and Mg(2+)-stimulation of alkaline phosphatase were studied for the enzyme either bound to these membranes or purified from these membranes by liquid chromatography. With p-nitrophenylphosphate as substrate, the alkaline phosphatase apparent Km was lower in jejunum (90 microM) as compared with duodenum (160 microM), and lower for the purified enzyme (jejunum: 55 microM; duodenum: 97 microM) as compared to the bound one. In the presence of 5 mM MgCl2, the substrate affinity was in all cases decreased. For the bound enzyme Vmax was 10 times greater in duodenum compared to jejunum. 5 mM MgCl2 tripled the Vmax of the duodenal bound enzyme and increased it by 50% for the jejunal one, but a seven-fold increase was recorded for the purified enzyme at both levels of intestine. The apparent affinity for Mg2+ was similar for the bound and the free enzyme, for duodenum and for jejunum (Mg0.5: +/- 40 microM).

Distribution and structure of the vacuolar H+ ATPase in endosomes and lysosomes from LLC-PK1 cells

Vacuolar proton pumps acidify several intracellular membrane compartments in the endocytic pathway. We have examined the distribution of the vacuolar H+ ATPase in LLC-PK1 cells and the structure of the biosynthetically labeled enzyme in membrane fractions enriched for endosomes or lysosomes. LLC-PK1 cells were allowed to internalize cytochrome c-coated colloidal gold as a marker for endocytic compartments. Proton pumps were identified in these cells by staining the cells with a monoclonal antibody against the vacuolar pump detected with either immunogold or immunoperoxidase techniques. H+ ATPase labeling was seen on structures resembling endosomes and lysosomes, but not on Golgi or plasma membrane. To examine the structure of the H+ ATPase in these compartments, we labeled LLC-PK1 cells for 24 h with [35S]methionine and used a Percoll gradient to obtain fractions enriched for endosomes or lysosomes. H+ ATPase immunoprecipitated from both fractions with monoclonal anti-H+ ATPase antibodies had labeled polypeptides of 70, 56, and 31 kDa. On two-dimensional gels, a comparison of the H+ ATPase from the endosomal and lysosomal fractions revealed that the 70-, 56-, and 31-kDa subunits were similar in both fractions. The results show that the vacuolar H+ ATPase in these cells is distributed primarily in endosomes and lysosomes and that the structure of the enzyme is similar in both compartments.

Involvement of plasma membrane glycoproteins in the contact-dependent inhibition of growth of human fibroblasts

The human embryonal lung fibroblasts used in this study showed a pronounced inhibition of growth when reaching a critical cell density. This effect has been mimicked by the addition of glutaraldehyde-fixed human fibroblasts to sparsely seeded growing cells. Inhibition of growth was not observed when glutaraldehyde-fixed cells were pretreated with galactosidase or with galactose-specific lectins, or when glutaraldehyde-fixed human or rabbit erythrocytes were added to the proliferating fibroblasts. In addition, glutaraldehyde-fixed mitotic cells were without effect on the proliferation, while cells prepared from sparse culture had lesser potency than cells prepared from confluent cultures. Plasma membranes, isolated from cells of confluent cultures, when added to growing cultures of human fibroblasts inhibited DNA synthesis in a concentration-dependent manner. On the other hand, plasma membranes isolated from sparsely seeded cells had only minor inhibitory potency. When the plasma membranes were isolated from cells treated previously with tunicamycin, an antibiotic which inhibits the synthesis of the oligosaccharide portion of asparagine-linked glycoproteins, the inhibitory effect was abolished. The same effect was observed when plasma membranes were pretreated with galactosidase. These data indicate that the growth of cells in vitro is regulated by specific cell-cell contacts. They also show that one of the molecular reactants in this process are membrane glycoproteins with asparagine-linked oligosaccharides.

Isolation of surface lectins of GH3 cells from whole cells and isolated plasma membranes

Lectins localized in the plasma membranes seem to be of special importance for the intercellular interaction mechanisms. We describe the isolation of mannose-binding proteins by Triton X-100 extraction and affinity chromatography on agarose-bound mannose. The isolation procedure was performed with whole GH3 cells as well as with isolated plasma membranes. For the isolation of plasma membranes of GH3 cells a mechanical pump was used for the disruption. After differential centrifugation an enriched plasma membrane fraction was achieved by discontinuous sucrose gradient centrifugation. The whole fractionation procedure was controlled by total balance sheets for the marker enzymes of the different cell organelles. The plasma membrane fraction was further characterized by gel electrophoresis and electron microscopy. The SDS gel electrophoresis patterns of the proteins, resulting from the Triton X-100 extraction and the affinity chromatography, are nearly identical for whole cells as well as for the enriched plasma membrane fraction. Therefore we presume these mannose-specific proteins to be plasma membrane bound, showing the molecular properties of integral proteins and having a molecular weight of Mr 67 000, 57 000, 47 000.

Integration of alkaline phosphatase in the intestinal brush border membrane

Alkaline phosphatase has been solubilized from porcine intestinal mucosa by two different methods: treatment of the mucosa by Emulphogen BC 720 and papain hydrolysis of enterocyte brush border membrane vesicles. Two differnt enzyme forms have been obtained by these methods. The two enzyme forms ('detergent form' and 'papain form') have been purified to homogeneity by similar techniques and exhibit closely related molecular characteristics. However, the detergent form displays a hydrophobic behaviour and aggregates in media free of detergent. The two forms can be differentiated by their electrophoretic mobility on polyacrylamide gel in the absence of sodium dodecyl sulphate. By electrophoresis on polyacrylamide gel in the presence of sodium dodecyl sulphate, it has been shown that the detergent and papain forms of alkaline phophatase are dimers consisting of two apparently identical subunits whose molecular weights are 64 000 and 61 000, respectively. The difference between these molecular weights has been attributed to the existence of a hydrophobic region in the detergent form which is present on each subunit.