Genetic complexity, structure, and characterization of highly active bovine intestinal alkaline phosphatases

Overview on the Development of Alkaline-Phosphatase-Linked Optical Immunoassays

The drive to achieve ultrasensitive target detection with exceptional efficiency and accuracy requires the advancement of immunoassays. Optical immunoassays have demonstrated significant potential in clinical diagnosis, food safety, environmental protection, and other fields. Through the innovative and feasible combination of enzyme catalysis and optical immunoassays, notable progress has been made in enhancing analytical performances. Among the kinds of reporter enzymes, alkaline phosphatase (ALP) stands out due to its high catalytic activity, elevated turnover number, and broad substrate specificity, rendering it an excellent candidate for the development of various immunoassays. This review provides a systematic evaluation of the advancements in optical immunoassays by employing ALP as the signal label, encompassing fluorescence, colorimetry, chemiluminescence, and surface-enhanced Raman scattering. Particular emphasis is placed on the fundamental signal amplification strategies employed in ALP-linked immunoassays. Furthermore, this work briefly discusses the proposed solutions and challenges that need to be addressed to further enhance the performances of ALP-linked immunoassays.

Post-Assay Chemical Enhancement for Highly Sensitive Lateral Flow Immunoassays: A Critical Review

Lateral flow immunoassay (LFIA) has found a broad application for testing in point-of-care (POC) settings. LFIA is performed using test strips-fully integrated multimembrane assemblies containing all reagents for assay performance. Migration of liquid sample along the test strip initiates the formation of labeled immunocomplexes, which are detected visually or instrumentally. The tradeoff of LFIA's rapidity and user-friendliness is its relatively low sensitivity (high limit of detection), which restricts its applicability for detecting low-abundant targets. An increase in LFIA's sensitivity has attracted many efforts and is often considered one of the primary directions in developing immunochemical POC assays. Post-assay enhancements based on chemical reactions facilitate high sensitivity. In this critical review, we explain the performance of post-assay chemical enhancements, discuss their advantages, limitations, compared limit of detection (LOD) improvements, and required time for the enhancement procedures. We raise concerns about the performance of enhanced LFIA and discuss the bottlenecks in the existing experiments. Finally, we suggest the experimental workflow for step-by-step development and validation of enhanced LFIA. This review summarizes the state-of-art of LFIA with chemical enhancement, offers ways to overcome existing limitations, and discusses future outlooks for highly sensitive testing in POC conditions.

Progress in Electrochemical Immunosensors with Alkaline Phosphatase as the Signal Label

Electrochemical immunosensors have shown great potential in clinical diagnosis, food safety, environmental protection, and other fields. The feasible and innovative combination of enzyme catalysis and other signal-amplified elements has yielded exciting progress in the development of electrochemical immunosensors. Alkaline phosphatase (ALP) is one of the most popularly used enzyme reporters in bioassays. It has been widely utilized to design electrochemical immunosensors owing to its significant advantages (e.g., high catalytic activity, high turnover number, and excellent substrate specificity). In this work, we summarized the achievements of electrochemical immunosensors with ALP as the signal reporter. We mainly focused on detection principles and signal amplification strategies and briefly discussed the challenges regarding how to further improve the performance of ALP-based immunoassays.

Deglycosylation Differentially Regulates Weaned Porcine Gut Alkaline Phosphatase Isoform Functionality along the Longitudinal Axis

Gut alkaline phosphatases (AP) dephosphorylate the lipid moiety of endotoxin and other pathogen-associated-molecular patterns members, thus maintaining gut eubiosis and preventing metabolic endotoxemia. Early weaned pigs experience gut dysbiosis, enteric diseases and growth retardation in association with decreased intestinal AP functionality. However, the role of glycosylation in modulation of the weaned porcine gut AP functionality is unclear. Herein three different research approaches were taken to investigate how deglycosylation affected weaned porcine gut AP activity kinetics. In the first approach, weaned porcine jejunal AP isoform (IAP) was fractionated by the fast protein-liquid chromatography and purified IAP fractions were kinetically characterized to be the higher-affinity and lower-capacity glycosylated mature IAP (p < 0.05) in comparison with the lower-affinity and higher-capacity non-glycosylated pre-mature IAP. The second approach enzyme activity kinetic analyses showed that N-deglycosylation of AP by the peptide N-glycosidase-F enzyme reduced (p < 0.05) the IAP maximal activity in the jejunum and ileum and decreased AP affinity (p < 0.05) in the large intestine. In the third approach, the porcine IAP isoform-X1 (IAPX1) gene was overexpressed in the prokaryotic ClearColiBL21 (DE3) cell and the recombinant porcine IAPX1 was associated with reduced (p < 0.05) enzyme affinity and maximal enzyme activity. Therefore, levels of glycosylation can modulate plasticity of weaned porcine gut AP functionality towards maintaining gut microbiome and the whole-body physiological status.

Electrophoretic mobility of individual molecules of alkaline phosphatase

The electrophoretic mobilities and catalytic rates of individual molecules of bovine intestinal alkaline phosphatase were determined in CHES and borate buffers of identical pH using a capillary electrophoresis based method. Both properties were found to be heterogeneous. In the presence of CHES, the mobility and rate were found to be -1.9 ± 0.2 × 10^-9 m² V^-1 s^-1 and 9.8 ± 7.4 × 10⁴ min^-1 (N = 38), respectively. In the presence of borate, the mobility and rate were found to be -6.9 ± 0.5 × 10^-9 m² V^-1 s^-1 and 2.0 ± 1.3 × 10⁴ min^-1 (N = 41), respectively. The means and variances for both properties were found to differ significantly between the two buffers. The difference in average mobility was attributed to an increase in negative charge caused by borate complexing with the carbohydrate moieties attached to the enzyme. The difference in variance was attributed to heterogeneous complexation with borate due to heterogeneity in the glycosylation. The differences in mean values for the catalytic rate were attributed to the inhibitory effect of borate and the difference in variance may suggest that the K_I of this binding may also be heterogeneous.

Cosolutes Modify Alkaline Phosphatase Catalysis through Osmotic Stress and Crowding Mechanisms

Examining the effects of different cosolutes on in vitro enzyme kinetics yielded glimpses into their potential behavior when functioning in their natural, complex, in vivo milieu. Viewing cosolute in vitro influences on a model enzyme, calf intestinal alkaline phosphatase, as a combination of competitive and uncompetitive behaviors provided quantitative insights into their effects on catalysis. Observed decreases in the apparent specificity constant, K _asp, caused by the presence of polyethylene glycols or betaine in the reaction solution, indicated interference with enzyme-substrate complex formation. This competitive inhibition appeared to be driven by osmotic stress. Dextran 6 K and sucrose strongly impeded the subsequent conversion of the bound substrate into a free product, which was marked by sharp reductions in V _max, uncompetitive inhibition. For the same step, smaller noncarbohydrate cosolutes, triethylene glycol, polyethylene glycol 400, and betaine, also behaved as uncompetitive inhibitors but to a lesser extent. However, polyethylene glycol 8000 and 20,000 were uncompetitive activators, increasing V _max. Polyethylene glycol of molecular weight 1000 displayed intermediate effects between these two groups of noncarbohydrate cosolutes. These results suggested that crowding has a strong influence on free product formation. The combination of competitive and uncompetitive effects and mixed behaviors, caused by the cosolutes on calf intestinal alkaline phosphatase kinetics, was consistent with the trends seen in similar enzyme-cosolute studies. It is proposed that the double-displacement mechanism of alkaline phosphatases, shared by many other enzymes, could be the root of this general observation.

Over-expression, characterization, and modification of highly active alkaline phosphatase from a Shewanella genus bacterium

We isolated a Shewanella sp. T3-3 bacterium that yielded highly active alkaline phosphatase (APase). We then cloned the APase gene from Shewanella sp. T3-3 (T3-3AP), and expressed and purified the enzyme from Escherichia coli. Recombinant T3-3AP showed high comparative reactivity on colorimetric (pNPP) and luminescent substrates (PPD and ASP-5). Subsequently, we improved the residual activity after maleimide activation by introducing amino acid substitutions of two Lys residues that were located near the active site. The double mutant enzyme (K161S + K184S) showed much higher residual specific activity after maleimide activation than the wild type enzyme, and had approximately twofold increased sensitivity on sandwich enzyme linked immunosorbent assays (ELISA) compared with calf intestinal APase (CIAP), which is routinely used as a labeling enzyme for ELISA.

Generic phosphatase activity detection using zinc mediated aggregation modulation of polypeptide-modified gold nanoparticles

A challenge in the design of plasmonic nanoparticle-based colorimetric assays is that the change in colloidal stability, which generates the colorimetric response, is often directly linked to the biomolecular recognition event. New assay strategies are hence required for every type of substrate and enzyme of interest. Here, a generic strategy for monitoring of phosphatase activity is presented where substrate recognition is completely decoupled from the nanoparticle stability modulation mechanism, which enables detection of a wide range of enzymes using different natural substrates with a single simple detection scheme. Phosphatase activity generates inorganic phosphate that forms an insoluble complex with Zn(2+). In a sample containing a preset concentration of Zn(2+), phosphatase activity will markedly reduce the concentration of dissolved Zn(2+) from the original value, which in turn affects the aggregation of gold nanoparticles functionalized with a designed Zn(2+) responsive polypeptide. The change in nanoparticle stability thus provides a rapid and sensitive readout of the phosphatase activity. The assay is not limited to a particular enzyme or enzyme substrate, which is demonstrated using three completely different phosphatases and five different substrates, and thus constitutes a highly interesting system for drug screening and diagnostics.

Catalytic signature of a heat-stable, chimeric human alkaline phosphatase with therapeutic potential

Recombinant alkaline phosphatases are becoming promising protein therapeutics to prevent skeletal mineralization defects, inflammatory bowel diseases, and treat acute kidney injury. By substituting the flexible crown domain of human intestinal alkaline phosphatase (IAP) with that of the human placental isozyme (PLAP) we generated a chimeric enzyme (ChimAP) that retains the structural folding of IAP, but displays greatly increased stability, active site Zn²⁺ binding, increased transphosphorylation, a higher turnover number and narrower substrate specificity, with comparable selectivity for bacterial lipopolysaccharide (LPS), than the parent IAP isozyme. ChimAP shows promise as a protein therapeutic for indications such as inflammatory bowel diseases, gut dysbioses and acute kidney injury.

Cloning and expression of a highly active recombinant alkaline phosphatase from psychrotrophic Cobetia marina

Alkaline phosphatase catalyzes the hydrolysis of phosphomonoesters and is widely used in molecular biology techniques and clinical diagnostics. We expressed a recombinant alkaline phosphatase of the marine bacterium, Cobetia marina, in Escherichia coli BL21 (DE3). The recombinant protein was purified with a specific activity of 12,700 U/mg protein, which is the highest activity reported of any bacterial alkaline phosphatase studied to date. The molecular mass of the recombinant protein was 55-60 kDa, as determined by SDS-PAGE, and was observed to be a dimer by gel filtration analysis. The enzyme was optimally active at 45°C and the recombinant alkaline phosphatase efficiently hydrolyzed a phosphoric acid ester in luminescent and fluorescent substrates. Therefore, this enzyme can be considered to be extremely useful as a label conjugated to an antibody.

Effects of amines and aminoalcohols on bovine intestine alkaline phosphatase activity

Bovine intestine alkaline phosphatase (BIALP) is widely used as a signaling enzyme in sensitive assays such as enzyme immunoassay (EIA). In this study, we evaluated the effects of various aminoalcohols and amines on the activity of BIALP in the hydrolysis of p-nitrophenyl phosphate (pNPP) at pH 9.8, at 20 °C. The k(cat) values at 0.05 M diethanolamine, 0.1 M triethanolamine, and 0.2 M N-methylethanolamine were 190±10, 840±30, and 500±10 s(-1), respectively. The k(cat) values increased with increasing concentrations of diethanolamine, triethanolamine, and N-methylethanolamine and reached 1240±60, 1450±30, and 2250±80 s(-1), respectively, at 1.0M. On the other hand, the k(cat) values at 0.05-1.0M ethanolamine, ethylamine, methylamine, and dimethylamine were in the range of 100-600 s(-1). These results indicate that diethanolamine, triethanolamine and N-methylethanolamine highly activate BIALP and might be suitable as a dilution buffer of BIALP in EIA. Interestingly, the K(m) values increased with increasing concentrations of diethanolamine and N-methylethanolamine, but not triethanolamine: the K(m) value at 1.0M diethanolamine (0.83±0.15 mM) was 12-fold higher than that at 0.05M (0.07±0.01 mM), and that at 1.0M N-methylethanolamine (2.53±0.20 mM) was 14-fold higher than that at 0.2M (0.18±0.02 mM), while that at 1.0M triethanolamine (0.31±0.01 mM) was similar as that at 0.2M (0.25±0.01 mM), suggesting that the mechanisms of BIALP activation are different between the aminoalcohols.

Exact and user-friendly kinetic analysis of the two-step rapid equilibrium Michaelis-Menten mechanism

Most enzyme kinetic experiments are carried out under pseudo-first-order conditions, that is, when one of the reactant species (the enzyme or the substrate) is in a large excess of the other species. More accurate kinetic information about the system can be gained without the restrictions of the pseudo-first-order conditions. We present a practical and general method of analysis of the common two-step rapid equilibrium Michaelis-Menten mechanism. The formalism is exact in that it does not involve any other approximations such as the steady-state, limitations on the reactant concentrations or on reaction times. We apply this method to the global analysis of kinetic progress curves for bovine alkaline phosphatase assays carried out under both pseudo-first-order and pseudo-second-order conditions.

[Inhibition of alkaline phosphatase by thioureido derivatives of methylenebisphosphonic acid]

A series of thioureido derivatives of methylenebisphosphonic acid were synthesized by the reaction of aminomethylenebisphosphonic acid with the corresponding isothiocyanates, and their effect on the activity of alkaline phosphatases from bovine small intestine mucosa (BSIM) and human placenta was studied. It was found that (3-phenylthioureido)methylenebisphosphonate is approximately one order of magnitude more effective in inhibiting the activity of alkaline phosphatase from BSIM than the alkyl derivatives of thioureidomethylenebisphosphonic acid with methyl, ethyl, tert-butyl, or cyclohexyl substituents. The introduction of substituents into the benzene ring of (3-phenylthioureido)methylenebisphosphonate decreased the effect of the inhibitor on the activity of the enzyme. The affinity of (3-phenylureido)methylenebisphosphonate to the alkaline phosphatase of BSIM was also weaker as compared with the corresponding thioureidomethylenebisphosphonate. The insertion of thioureidobisphosphonates into the active site of alkaline phosphatase of human placenta by the method of molecular docking indicated that the methylenebisphosphonate residue and the substituted amino groups of the inhibitor are involved in the mechanisms of complex formation with the enzyme. It is supposed that the improvement of the inhibitory activity of (3-phenylthioureido)methylenebisphosphonate toward alkaline phosphatase of BSIM is due to the additional fixation of the phenyl substituent in the active site of the enzyme.

A novel phosphatase upregulated in Akp3 knockout mice

Reexamination of the Akp3(-/-) mouse intestine showed that, despite the lack of intestinal alkaline phosphatase (IAP), the Akp3(-/-) gut still had considerable alkaline phosphatase (AP) activity in the duodenum and ileum. This activity is due to the expression of a novel murine Akp6 gene that encodes an IAP isozyme expressed in the gut in a global manner (gIAP) as opposed to duodenum-specific IAP (dIAP) isozyme encoded by the Akp3 gene. Phylogenetically, gIAP is similar to the rat IAP I isozyme. Kinetically, gIAP displays a 5.7-fold reduction in catalytic rate constant (k(cat)) and a 30% drop in K(m), leading to a 4-fold reduction k(cat)/K(m) compared with dIAP, and these changes in enzymatic properties can all be attributed to a crucial R317Q substitution. Western and Northern blot analyses document the expression of Akp6 in the gut, from the duodenum to the ileum, and it is upregulated in the jejunum and ileum of Akp3(-/-) mice. Developmentally, Akp3 expression is turned on during postnatal days 13-15 and exclusively in the duodenum, whereas Akp6 and Akp5 are expressed from birth throughout the gut with enhanced expression at weaning. Posttranslational modifications of gIAP have a pronounced effect on its catalytic properties. Given the low catalytic efficiency of gIAP, its upregulation during fat feeding, its sequence similarity with rat IAP I, and the fact that rat IAP I has been implicated in the upregulation of surfactant-like particles during fat intake, it appears likely that gIAP may have a role in mediating the accelerated fatty acid intake observed in Akp3(-/-) mice fed a high-fat diet.

Distinct structure and activity recoveries reveal differences in metal binding between mammalian and Escherichia coli alkaline phosphatases

The amino acids involved in the coordination of two Zn2+ ions and one Mg2+ ion in the active site are well conserved from EAP (Escherichia coli alkaline phosphatase) to BIAP (bovine intestinal alkaline phosphatase), whereas most of their surrounding residues are different. To verify the consequences of this heterology on their specific activities, we compared the activity and structure recoveries of the metal-free forms (apo) of EAP and of BIAP. In the present study, we found that although the sensitivities of EAP and BIAP to ions remained similar, significant differences in dimeric structure stability of apo-enzymes were observed between EAP and BIAP, as well as in the kinetics of their activity and secondary structure recoveries. After mild chelation inactive apo-EAP was monomeric under mild denaturing conditions, whereas inactive apo-BIAP remained dimeric, indicating that the monomer-monomer contact was stronger in the mammalian enzyme. Dimeric apo-EAP (0.45 microM, corresponding to 4 units/ml) recovered approx. 80% of its initial activity after 3 min incubation in an optimal recovery medium containing 5 microM Zn2+ and 5 mM Mg2+, whereas dimeric apo-BIAP (0.016 microM, corresponding to 4 units/ml) recovered 80% of its native activity after 6 h incubation in an optimal recovery medium containing 0.5 microM Zn2+ and 5 mM Mg2+. Small and different secondary structure changes were also observed during activity recoveries of apo-BIAP and apo-EAP, which were not in parallel with the activity recoveries, suggesting that distinct and subtle structural changes are required for their optimal activity recoveries.

Calix[4]arene α-Aminophosphonic Acids: Asymmetric Synthesis and Enantioselective Inhibition of an Alkaline Phosphatase

[structure: see text] Chiral calix[4]arene alpha-aminophosphonic acids were obtained through diastereoselective Pudovik-type addition of sodium ethyl phosphites to the chiral calixarene imines, removal of chiral auxiliary groups, and mild dealkylation of phosphonate fragments. The diacids obtained show inhibitory activity toward porcine kidney alkaline phosphatase that depends considerably on the absolute configuration of the alpha-carbon atoms.

Characterization of structural and catalytic differences in rat intestinal alkaline phosphatase isozymes

To understand the differences between the rat intestinal alkaline phosphatase isozymes rIAP-I and rIAP-II, we constructed structural models based on the previously determined crystal structure for human placental alkaline phosphatase (hPLAP). Our models of rIAP-I and rIAP-II displayed a typical alpha/beta topology, but the crown domain of rIAP-I contained an additional beta-sheet, while the embracing arm region of rIAP-II lacked the alpha-helix, when each model was compared to hPLAP. The representations of surface potential in the rIAPs were predominantly positive at the base of the active site. The coordinated metal at the active site was predicted to be a zinc triad in rIAP-I, whereas the typical combination of two zinc atoms and one magnesium atom was proposed for rIAP-II. Using metal-depleted extracts from rat duodenum or jejunum and hPLAP, we performed enzyme assays under restricted metal conditions. With the duodenal and jejunal extract, but not with hPLAP, enzyme activity was restored by the addition of zinc, whereas in nonchelated extracts, the addition of zinc inhibited duodenal IAP and hPLAP, but not jejunal IAP. Western blotting revealed that nearly all of the rIAP in the jejunum extracts was rIAP-I, whereas in duodenum the percentage of rIAP-I (55%) correlated with the degree of AP activation (60% relative to that seen with jejunal extracts). These data are consistent with the presence of a triad of zinc atoms at the active site of rIAP-I, but not rIAP-II or hPLAP. Although no differences in amino acid alignment in the vicinity of metal-binding site 3 were predicted between the rIAPs and hPLAP, the His153 residue of both rIAPs was closer to the metal position than that in hPLAP. Between the rIAPs, a difference was observed at amino acid position 317 that is indirectly related to the coordination of the metal at metal-binding site 3 and water molecules. These findings suggest that the side-chain position of His153, and the alignment of Q317, might be the major determinants for activation of the zinc triad in rIAP-I.

Phosphate binding in the active site of alkaline phosphatase and the interactions of 2-nitrosoacetophenone with alkaline phosphatase-induced small structural changes

To monitor structural changes during the binding of Pi to the active site of mammalian alkaline phosphatase in water medium, reaction-induced infrared spectroscopy was used. The interaction of Pi with alkaline phosphatase was triggered by a photorelease of ATP from the inactive P(3)-[1-(2-nitrophenyl)]ethyl ester of ATP. After photorelease, ATP was sequentially hydrolyzed by alkaline phosphatase giving rise to adenosine and three Pi. Although a phosphodiesterase activity was detected prior the photorelease of ATP, it was possible to monitor the structural effects induced by Pi binding to alkaline phosphatase. Interactions of Pi with alkaline phosphatase were evidenced by weak infrared changes around 1631 and at 1639 cm(-1), suggesting a small distortion of peptide carbonyl backbone. This result indicates that the motion required for the formation of the enzyme-phosphate complex is minimal on the part of alkaline phosphatase, consistent with alkaline phosphatase being an almost perfect enzyme. Photoproduct 2-nitrosoacetophenone may bind to alkaline phosphatase in a site other than the active site of bovine intestinal alkaline phosphatase and than the uncompetitive binding site of L-Phe in bovine intestinal alkaline phosphatase, affecting one-two amino acid residues.

Placental and intestinal alkaline phosphatases are receptors for Aeromonas sobria hemolysin

The Aeromonas sobria hemolysin causes diarrhea following infection by this enteropathogenic bacterium. We previously identified the putative receptor for A. sobria hemolysin as a p66 protein on Intestine 407 cells (Microb. Pathog. 27 (1999) 215-221). Here, we have partially purified and obtained a peptide mass fingerprint of p66 which revealed its identity with placental alkaline phosphatase (PLAP). Recombinant PLAP expressed in 293T cells was also found to bind to hemolysin and the binding was found not to be dependent on the N-linked glycosylation of PLAP. By immunohistochemical analysis, PLAP expression was detected in human intestinal mucosa, the target tissue in disease. In addition to PLAP, hemolysin also binds to intestinal alkaline phosphatase (IAP), an enzyme that is also abundantly expressed in intestine. Thus, both PLAP and IAP are very likely involved in the pathogenesis of diarrhea caused by this bacterial toxin.

Amino acid sequence of the cold-active alkaline phosphatase from Atlantic cod (Gadus morhua)

Atlantic cod is a marine fish that lives at low temperatures of 0-10 degrees C and contains a cold-adapted alkaline phosphatase (AP). Preparations of AP from either the lower part of the intestines or the pyloric caeca area were subjected to proteolytic digestion, mass spectrometry and amino acid sequencing by Edman degradation. The primary structure exhibits greatest similarity to human tissue non-specific AP (80%), and approximately 30% similarity to AP from Escherichia coli. The key residues required for catalysis are conserved in the cod AP, except for the third metal binding site, where cod AP has the same variable residues as mammalian APs (His153 and His328 by E. coli AP numbering). General comparison of the amino acid composition with mammalian APs showed that cod AP contains fewer Cys, Leu, Met and Ser, but proportionally more Asn, Asp, Ile, Lys, Trp and Tyr residues. Three N-linked glycosylation sites were found. The glycan structure was determined as complex biantennary in type with fucose and sialic acid attached, although a trace of complex tri-antennary structure was also observed. A three-dimensional model was obtained by homology modelling using the human placental AP scaffold. Cod AP has fewer charged and hydrophobic residues, but more polar residues at the intersubunit surface. The N-terminal helix arm that embraces the second subunit in dimeric APs may be more flexible due to a replaced Pro at its base. One disulfide bridge was found instead of the two present in most other APs. This may invoke greater movement in the structure that together with weaker subunit contacts leads to improved catalytic efficiency.

Structural evidence of functional divergence in human alkaline phosphatases

The evolution of the alkaline phosphatase (AP) gene family has lead to the existence in humans of one tissue-nonspecific (TNAP) and three tissue-specific isozymes, i.e. intestinal (IAP), germ cell (GCAP), and placental AP (PLAP). To define the structural differences between these isozymes, we have built models of the TNAP, IAP, and GCAP molecules based on the 1.8-structure of PLAP(1) and have performed a comparative structural analysis. We have examined the monomer-monomer interface as this area is crucial for protein stability and enzymatic activity. We found that the interface allows the formation of heterodimers among IAP, GCAP, and PLAP but not between TNAP with any of the three tissue-specific isozymes. Secondly, the active site cleft was mapped into three regions, i.e. the active site itself, the roof of the cleft, and the floor of the cleft. This analysis led to a structural fingerprint of the active site of each AP isozyme that suggests a diversification in substrate specificity for this isozyme family.

Inhibitor profiles of alkaline phosphatases in bovine preattachment embryos and adult tissues

The alkaline phosphatases are a small family of isozymes. Bovine preattachment embryos transcribe mRNA for two tissue-specific alkaline phosphatases (TSAP2 and TSAP3) beginning at the 4- and 8-cell stages. Whereas no mRNA has been detected in oocytes, there is maternally inherited alkaline phosphatase activity. It is not known which isozyme(s) is responsible for the maternal activity or when TSAP2 and TSAP3 form functional protein. No antibodies are available that recognize the relevant bovine alkaline phosphatases. Therefore, sensitivity to heat and chemical inhibition was used to separate the different isozymes. By screening tissues, it was determined that the bovine tissue-nonspecific alkaline phosphatase (TNAP) is inactivated by low temperatures (65C) and low concentrations of levamisole (<1 mM), whereas bovine tissue-specific isozymes require higher temperatures (90C) and levamisole concentrations (>5 mM). Inhibition by L-homoarginine and L-phenylalanine was less informative. Cumulus cells transcribe two isozymes and the pattern of inhibition suggested heterodimer formation. Inhibition of alkaline phosphatase in bovine embryos before the 8-cell stage indicated the presence of only TNAP. At the 16-cell stage the pattern was consistent with TNAP plus TSAP2 or -3 activity, and in morulae and blastocysts the pattern indicated that the maternal TNAP is fully supplanted by TSAP2 or TSAP3.

Improving Escherichia coli alkaline phosphatase efficacy by additional mutations inside and outside the catalytic pocket

We describe a strategy that allowed us to confer on a bacterial (E. coli) alkaline phosphatase (AP) the high catalytic activity of the mammalian enzyme while maintaining its high thermostability. First, we identified mutations, at positions other than those occupied by essential catalytic residues, which inactivate the bacterial enzyme without destroying its overall conformation. We transferred concomitantly into the bacterial enzyme four residues of the mammalian enzyme, two being in the catalytic pocket and two being outside. Second, the gene encoding the inactive mutant was submitted to random mutagenesis. Enzyme activity was restored upon the single mutation D330N, at a position that is 12 A away from the center of the catalytic pocket. Third, this mutation was combined with other mutations previously reported to increase AP activity slightly in the presence of magnesium. As a result, at pH 10.0 the phosphatase activity of both mutants D330N/D153H and D330N/D153G was 17-fold higher than that of the wild-type AP. Strikingly, although the two individual mutations D153H and D153G destabilize the enzyme, the double mutant D330N/D153G remained highly stable (T(m)=87 degrees C). Moreover, when combining the phosphatase and transferase activities, the catalytic activity of the mutant D330N/D153G increased 40-fold (k(cat)=3200 s-1) relative to that of the wild-type enzyme (k(cat)=80 s-1). Due to the simultaneous increase in K(m), the resulting k(cat)/K(m) value was only increased by a factor of two. Therefore, a single mutation occurring outside a catalytic pocket can dramatically control not only the activity of an enzyme, but also its thermostability. Preliminary crystallographic data of a covalent D330N/D153G enzyme-phosphate complex show that the phosphate group has significantly moved away from the catalytic pocket, relative to its position in the structure of another mutant previously reported.

Structural study on the carbohydrate moiety of calf intestinal alkaline phosphatase

Surprisingly alkaline phosphatase (AP) (EC 3.1.3.1) of calf intestine is found in large amounts, e.g. 80%, within chyme. Most of the enzyme is present as a mixture of four differently hydrophobic anchor-bearing forms and only the minor part is present as an anchorless enzyme. To investigate whether changes in the N-glycosylation pattern are signals responsible for large-scale liberation from mucosa into chyme, the glycans of the two potential glycosylation sites predicted from cDNA were investigated by matrix-assisted laser desorption/ionization and electrospray ionization mass spectrometry in combination with exoglycosidase treatment after tryptic digestion and reversed-phase chromatography. The glycans linked to Asn249 are at least eight different, mainly non-fucosylated, biantennary or triantennary structures with a bisecting N-acetylglucosamine. For the most abundant glycopeptide (40%) the following glycan structure is proposed: [carbostructure: see text]. The glycans linked to Asn410 are a mixture of at least nine, mainly tetraantennary, fucosylated structures with a bisecting N-acetylglucosamine. For the most abundant glycopeptide (35%) the following glycan structure is proposed: [carbostructure: see text]. For the structures the linkage data were deduced from the reported specificities of the exoglycosidases used and the specificities of the transglycosidases active in biosynthesis. The majority of glycans are capped by alpha-galactose residues at their non-reducing termini. In contrast to the glycans linked to other AP isoenzymes, no sialylation was observed. Glycopeptide 'mass fingerprints' of both glycosylation sites and glycan contents do not differ between AP from mucosa and chyme. These results suggest that the observed large-scale liberation of vesicle-bound glycosylphosphatidylinositol (GPI)-anchored AP from mucosa into chyme is unlikely to be mediated by alteration of glycan structures of the AP investigated. Rather, the exocytotic vesicle formation seems to be mediated by the controlled organization of the raft structures embedding GPI-AP. (c) 2001 John Wiley & Sons, Ltd.

Dissociation and unfolding of cold-active alkaline phosphatase from atlantic cod in the presence of guanidinium chloride

Cold-adaptation of enzymes involves improvements in catalytic efficiency. This paper describes studies on the conformational stability of a cold-active alkaline phosphatase (AP) from Atlantic cod, with the aim of understanding more clearly its structural stability in terms of subunit dissociation and unfolding of monomers. AP is a homodimeric enzyme that is only active in the dimeric state. Tryptophan fluorescence, size-exclusion chromatography and enzyme activity were used to monitor alterations in conformational state induced by guanidinium chloride or urea. In cod AP, a clear distinction could be made between dissociation of dimers into monomers and subsequent unfolding of monomers (fits a three-state model). In contrast, dimer dissociation of calf AP coincided with the monophasic unfolding curve observed by tryptophan fluorescence (fits a two-state model). The DeltaG for dimer dissociation of cod AP was 8.3 kcal.mol-1, and the monomer stabilization free energy was 2.2 kcal.mol-1, giving a total of 12.7 kcal.mol-1, whereas the total free energy of calf intestinal AP was 17.3 kcal.mol-1. Thus, dimer formation provided a major contribution to the overall stability of the cod enzyme. Phosphate, the reaction product, had the effect of promoting dimer dissociation and stabilizing the monomers. Cod AP has reduced affinity for inorganic phosphate, the release of which is the rate-limiting step of the reaction mechanism. More flexible links at the interface between the dimer subunits may ease structural rearrangements that facilitate more rapid release of phosphate, and thus catalytic turnover.

The two isozymes of rat intestinal alkaline phosphatase are products of two distinct genes

Rat intestinal alkaline phosphatases (IAP-I and -II) differ in primary structure, substrate specificity, tissue localization, and response to fat feeding. This study identifies two distinct genes ( approximately 5-6 kb) corresponding to each isozyme and containing 11 exons of nearly identical size. The exon-intron junctions are identical with those found in IAP genes from other species. The 1.7 and 1.2 bp of 5' flanking regions isolated from each gene, respectively, contain Sp1 and gut-enriched Kruppel-like factor (GKLF) binding sites, but otherwise show little identity. There is a potential CAAT-box 14 bp 5' to the transcriptional start site, 36 bp upstream from IAP-I, and a TATA-box 31 bp 5' to the transcriptional start site, 55 bp upstream from IAP-II. Transfection of these promoter regions (linked to luciferase as a reporter gene) into a kidney cell line, COS-7, produced the differential response to oleic acid expected from in vivo studies, i.e., threefold increase using the 5' flanking region of IAP-II, but not IAP-I. This response was not reproduced by 5,8,11,14-eicosatetraynoic acid (ETYA) or clofibrate, suggesting that peroxisome proliferator response elements are not involved. Isolation of the IAP-II gene will allow determination of the sequences responsible for dietary fat response in the enterocyte.

Alkaline phosphatase from the Antarctic strain TAB5. Properties and psychrophilic adaptations

The gene encoding alkaline phosphatase (AP) from the psychrophilic strain TAB5 was cloned, and its nucleotide sequence was determined. A single open reading frame consisting of 1125 base pairs which encodes a polypeptide consisting of signal peptide of 22 amino acids and a mature protein of 353 amino acids was identified. The deduced protein sequence of AP exhibits a 38% identity to the AP III and AP IV sequences of Bacillus subtilis and conserves the typical sequence motifs of the core structure and active sites of APs from various sources. Based on the crystal structure of the mutated Escerichia coli AP D153H, a homology-based 3D model of the TAB5 AP was constructed on the basis of which various features of the enzyme amino-acid sequence can be interpreted in terms of potential psychrophilic adaptations. The AP gene was expressed in E. coli BL21(DE3) cells, the recombinant protein was isolated to homogeneity from the membrane fraction of the cells and its properties were examined. The purified TAB5 AP shows typical features of a cold enzyme: high catalytic activity at low temperature and a remarkable thermosensitivity. The use of this heat-labile enzyme, for dephosphorylation of nucleic acids, simplifies dephosphorylation protocols.