Purification and partial characterization of alkaline phosphatase of matrix vesicles from fetal bovine epiphyseal cartilage. Purification by monoclonal antibody affinity chromatography

Calcium phosphate mineralization through homogenous enzymatic catalysis: Investigation of the early stages

HYPOTHESIS

The crystallization of calcium phosphate (CaP) is a ubiquitous process that occurs in several mineralized tissues and involves a variety of biochemical and chemical reactions. This issue has been hitherto continuously studied from supersaturated solutions (chemical procedure), i.e. by adding calcium and orthophosphate ions in a homogenous phase. Yet, both in vivo and in vitro investigations have clearly shown the implication of enzymes, namely alkaline phosphatase (ALP), to initiate the mineralization process by generating orthophosphate ions.

EXPERIMENTS

We report a thorough investigation on the mechanism of enzyme-induced mineralization in homogenous phase (enzymatic procedure). For this purpose, ALP is introduced in Ca²⁺/Mg²⁺-containing solution (pH = 7.4; 37 °C), and its activity modulated by the concentration of its substrate.

FINDINGS

Results show that after 24 h of mineralization both chemical and enzymatic procedures lead to the formation of well-crystalline hydroxyapatite nano-objects, however with noticeable impact on their shape and dimensions. Remarkably enough, by combining in situ monitoring and ex situ characterizations, we identify several intermediate phases, including amorphous phase, dicalcium phosphate dehydrate phase (DCPD or brushite) and Whitlockite (WH). Besides, mineralized nano-objects with a core-shell structure is observed, and hydroxyapatite platelets are shown to grow on the surface of their shell.

Effects of GPI-anchored TNAP on the dynamic structure of model membranes

Tissue-nonspecific alkaline phosphatase (TNAP) plays a crucial role during skeletal mineralization, and TNAP deficiency leads to the soft bone disease hypophosphatasia. TNAP is anchored to the external surface of the plasma membranes by means of a GPI (glycosylphosphatidylinositol) anchor. Membrane-anchored and solubilized TNAP displays different kinetic properties against physiological substrates, indicating that membrane anchoring influences the enzyme function. Here, we used Electron Spin Resonance (ESR) measurements along with spin labeled phospholipids to probe the possible dynamic changes prompted by the interaction of GPI-anchored TNAP with model membranes. The goal was to systematically analyze the ESR data in terms of line shape changes and of alterations in parameters such as rotational diffusion rates and order parameters obtained from non-linear least-squares simulations of the ESR spectra of probes incorporated into DPPC liposomes and proteoliposomes. Overall, the presence of TNAP increased the dynamics and decreased the ordering in the three distinct regions probed by the spin labeled lipids DOPTC (headgroup), and 5- and 16-PCSL (acyl chains). The largest change was observed for 16-PCSL, thus suggesting that GPI-anchored TNAP can give rise to long reaching modifications that could influence membrane processes halfway through the bilayer.

Induction of calcification in rabbit aortas by high cholesterol diets: roles of calcifiable vesicles in dystrophic calcification

Atherosclerotic calcification may weaken the aorta wall and thereby lead to rupture of the vessel. The mechanism whereby aortas undergo calcification remains unclear. Previous reports in this laboratory showed that, after 2 months of cholesterol-supplemental feeding, an increase in calcifiability of membrane vesicles isolated from rabbit aortas precedes substantial arterial calcification. Further, the mineral was deposited by isolated calcifiable vesicles as an amorphous phase similar to minerals in human aortas at an early stage of atherosclerosis. In the current study, atherosclerotic calcification was induced by exposing rabbits to a 1% cholesterol-rich diet for 3 or 6 months. After 3 months of dietary interventions, atherosclerotic lesions were fully developed. Fatty streaks were evident in areas proximal to the heart and became less frequent in the distal areas. However, calcification was not yet identifiable histologically or by using Fourier transform spectroscopy (FT-IR). After 6 months of high cholesterol treatment, aortas were partially calcified. Histochemical staining for mineral revealed that calcification appeared to occur predominantly in the intimal areas immediately adjacent to the media. Fourier Transform Imaging analysis demonstrated that the mineral deposited in atherosclerotic rabbit aortas was a hydroxyapatite-like phase. To determine whether aorta vesicles play a role in mineral formation in aortas, vesicles were isolated from calcified aortas and then their calcifiability was compared to that in normal vesicles. Interestingly, during the course of vesicle isolation, we found that calcifiable vesicles with much higher calcifiability than normal vesicles could be readily isolated from atherosclerotic aortas simply by suspending minced tissues in PBS. The characteristics of the calcification process and the enzymatic contents of isolated vesicles were similar to those obtained using collagenase digestion. Correlatively, mineral deposited by calcifiable vesicles isolated from the calcified aortas was also of hydroxyapatite-like phases. Altogether, these observations indicate that (1) aortic calcification is a later event during atherogenesis, (2) calcifiable vesicles are loosely bound to the matrices of the lesions as the result of the disease process and (3) similarities in the mineral phases between those in aortas and by vesicles during atherogenesis further support the role of calcifiable vesicles in dystrophic calcification.

Isolation of calcifiable vesicles from aortas of rabbits fed with high cholesterol diets

Advanced arterial wall calcification in atherosclerosis imposes a serious rupturing effect on the aorta. However, the mechanism of dystrophic calcification linked to hyperlipidemia, that causes atherosclerosis remains unknown. Emerging morphological and biochemical evidence reveals that calcifiable vesicles may have a role in plaque calcification. To determine whether a high cholesterol diet can induce arterial calcification and produce or activate calcifiable vesicles in aortas, a rabbit model was used. After 2 months of daily high lipid feeding (supplemented with 2% cholesterol and 6% peanut oil), typical atherosclerotic lesions developed. However, the mineral, if present in aortas, was insufficient to be detected by Fourier transform-infrared spectroscopy (FT-IR) or alizarin red staining, indicative of a non-calcifying stage of atherosclerosis. Small segments of thoracic aortas were digested in a crude collagenase solution to release calcifiable vesicles. Vesicles were also isolated from normal aortas as control to consider the possibility that membrane vesicles may be produced by crude collagenase digestion, which could cause the degradation of some cells. Calcifiable vesicles were precipitated at 300,000 x g after subcellular particles were removed by centrifugation at 30,000 x g. Calcifiability of isolated vesicles was then tested using calcifying media containing physiological levels of Ca2+ and Pi and 1 mM ATP. Electron microscopic observations showed that the isolated vesicles were heterogeneous in size and shape and capable of depositing electron dense particles. Fourier transform infrared spectroscopic analysis of the deposited particles revealed the presence of an amorphous mineral phase. The spectroscopic mineral to matrix ratios, related to the amount of mineralization, indicated that vesicles from cholesterol-fed rabbits produced more minerals than control vesicles obtained from the normal aortas. Alizarin red staining for mineral further demonstrated substantially higher calcifiability of the experimental vesicles. A 3-5 h exposure of the vesicles to calcifying media caused significant deposition of 45Ca and 32Pi in a vesicle protein-concentration dependent manner. Similar to previously reported observations with human atherosclerotic aorta vesicles, rabbit vesicles were enriched in ATP-hydrolyzing enzymes including Mg2+- or Ca2+-ATPase and NTP pyrophosphohydrolase that are implicated in normal and pathological calcification. Altogether, these observations suggest that accumulation of the released calcifiable vesicles, as a result of high cholesterol diets, may have a role in dystrophic calcification in hyperlipidemia-related atherosclerosis.

Streptozotocin-induced diabetes: significant changes in the kinetic properties of the soluble form of rat bone alkaline phosphatase

A soluble form of an alkaline phosphatase, obtained from the osseous plate of streptozotocin-induced diabetic rats, was purified 90-fold with a yield of 26%. The calculated Mr of the purified enzyme was 80,000 by denaturing polyacrylamide gel electrophoresis and 160,000 by gel filtration on Sephacryl S-300, suggesting a dimeric structure for its native form. In the absence of metal ions, the p-nitrophenylphosphatase activity of the purified enzyme was 4223.1 U/mg. Magnesium or calcium ion concentrations up to 2 mM increased the specific activity of the enzyme to 9896.5 and 10,796.2 U/mg, respectively. The enzyme was stimulated to a lesser extent by MnCl2 (5390.1 U/mg) and CoCl2 (5088.2 U/mg). The purified soluble alkaline phosphatase showed a broad substrate specificity, and among the less hydrolyzed substrates were pyrophosphate (1517.6 U/mg) and bis-p-nitrophenylphosphate (499.6 U/mg). The enzyme was relatively stable at 45 degrees for periods as long as 180 min, but was denatured rapidly above 50 degrees, following first order kinetics with T1/2 values ranging from 3.5 to 57.7 min. The results reported herein suggested that the soluble form of alkaline phosphatase from streptozotocin-induced diabetic rats had its kinetic properties altered, apparently as a consequence of changes in metal-binding properties.

Isolation of calcifiable vesicles from human atherosclerotic aortas

Advanced mineralization can cause brittleness of aortic walls with decreased elasticity thereby causing the wall to rupture. Although the precise mechanisms of dystrophic calcification remain unknown, morphological evidence reveals the presence of mineral-associated vesicles in the lesions and defective bioprosthetic valves. In an attempt to demonstrate the calcifiability of the vesicles, small segments of human atherosclerotic aortas with calcified lesions were removed at autopsy and then digested in a crude collagenase solution to release vesicles. A differential centrifugation was then used to isolate calcifiable vesicles, which was precipitated at 300,000 x g for 20 min. An exposure of the vesicles to a calcifying medium containing physiologic levels of Ca2+, Pi, and 1 mM ATP caused Ca deposition in a vesicle protein-concentration dependent manner. The calcifiability of the vesicles was further demonstrated by electron microscopy. Fourier transform spectroscopic analysis of the deposited mineral revealed the presence of a hydroxyapatite phase, closely resembling the native form of mineral in atherosclerotic plaques. In addition, calcifiable vesicles were enriched in ATP-hydrolyzing enzymes including Mg2+ or Ca2+-ATPase and NTP pyrophosphohydrolase that may be involved in normal and pathological calcification. Triton X-100 at 0.01% abolished 80% of both ATPase activity and ATP-initiated calcification. A comparison of vesicles isolated from non-atherosclerotic and atherosclerotic aortas indicated that atherosclerotic vesicles tended to have higher calcifiability. These observations suggest that the calcifiable vesicles play a part in dystrophic calcification of aortas in atherosclerosis.

Allosteric modulation of pyrophosphatase activity of rat osseous plate alkaline phosphatase by magnesium ions

Pyrophosphatase activity of rat osseous plate alkaline phosphatase was studied at different concentrations of calcium and magnesium ions, with the aim of characterizing the modulation of enzyme activity by these metals. In the absence of metal ions, the enzyme hydrolysed pyrophosphate following "Michaelian" kinetics with a specific activity of 36.7 U/mg and K0.5 = 88 microM. In the presence of low concentrations (0.1 mM) of magnesium (or calcium) ions, the enzyme also exhibited "Michaelian" kinetics for the hydrolysis of pyrophosphate, but a significant increase in specific activity (123 U/mg) was observed, K(m) values remained almost unchanged. Quite different behavior occurred in the presence of 2 mM magnesium (or calcium) ions. In addition to low-affinity sites (K0.5-40 and 90 microM, for magnesium and calcium, respectively), high-affinity sites were also observed with K0.5 values 100-fold lower. The high-affinity sites observed in the presence of calcium ions represented about 10% of those observed for magnesium ions. This was correlated with the fact that only magnesium ions triggered conformational changes yielding a fully active enzyme. These results suggested that the enzyme could hydrolyse pyrophosphate, even at physiological concentrations (4 microM), since magnesium concentrations are high enough to trigger conformational changes increasing the enzyme activity. A model, suggesting the involvement of magnesium ions in the hydrolysis of pyrophosphate by rat osseous plate alkaline phosphatase is proposed.

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.

Dependence of divalent metal ions on phosphotransferase activity of osseous plate alkaline phosphatase

Kinetic evidence for the role of divalent metal ions in the phosphotransferase activity of polidocanol-solubilized alkaline phosphatase from osseous plate is reported. Ethylenediamine tetreacetate, 1,10-phenanthrolin, and Chelex-100 were used to prepare metal-depleted alkaline phosphatase. Except for Chelex-100, either irreversible inactivation of the enzyme or incomplete removal of metal ions occurred. After Chelex-100 treatment, full hydrolase activity of alkaline phosphatase was recovered upon addition of metal ions. On the other hand, only 20% of transferase activity was restored with 0.1 microM ZnCl2, in the presence of 1.0 M diethanolamine as phosphate acceptor. In the presence of 0.1 mM MgCl2, the recovery of transferase activity increased to 63%. Independently of the phosphate acceptor used, the transferase activity of the metal-depleted alkaline phosphatase was fully restored by 8 microM ZnCl2 plus 5 mM MgCl2. In the presence of diethanolamine as phosphate acceptor, manganese, cobalt, and calcium ions did not stimulate the transferase activity. However, manganese and cobalt-enzyme catalyzed the transfer of phosphate to glycerol and glucose.

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.

Alkaline phosphatase isozymes in insects and comparison with mammalian enzyme

Studies of insect alkaline phosphatases (ALPs) are reviewed, including general insect isozyme papers from earlier periods. Results of biochemical and genetic investigations of the silkworm midgut ALPs are described. The membrane-bound (m-ALP) and soluble form (s-ALP) are controlled by distinct genes on the same chromosome. These isozymes were different in tissue localization, antigenicity, stability under alkaline conditions and sugar chains. Compared with mammalian ALPs, silkworm ALPs represented specificity in the monomeric structure, tissue localization and inhibition by amino acids. The amino acid sequence deduced from cDNA sequence of silkworm m-ALP showed 42.7-44.6% homology to three human types of ALP. Comparison of the amino acid sequences in functionally important parts of various ALP isozymes showed a significant conservation. Physiological roles of ALPs were discussed and the significance of the study in temporal and spatial regulations of both silkworm ALP genes was pointed out. In addition, the evolutionary relationship among various genes was discussed.

In vitro Ca deposition by rat matrix vesicles: is the membrane association of alkaline phosphatase essential for matrix vesicle-mediated calcium deposition?

1. Phosphatidylinositol phospholipase C (PI-PLC) treatment of rachitic rat matrix vesicles (MVs) released about 80% of membrane-bound alkaline phosphatase (ALP), AMPase, PPiase into the media. 2. About 20% hydrolytic activity was not released from MV membranes by PI-PLC treatment. 3. SDS-polyacrylamide gel electrophoresis and Western blot analysis showed only one immunoreactive protein corresponding to the molecular weight of ALP present in the soluble fraction after PI-PLC treatment. 4. The specific activity of the released ALP was at least 5-fold higher than the residual activity. 5. After PI-PLC treatment, MVs also demonstrated an 80% reduction of AMP- or beta GP-dependent calcium deposition. 6. The soluble fraction containing 80% of ALP activity was unable to support calcium deposition. The mixing of the soluble and insoluble fractions after PI-PLC treatment failed to fully restore calcium-depositing activity.

Allosteric modulation by ATP, calcium and magnesium ions of rat osseous plate alkaline phosphatase

Alkaline phosphatase from rat osseous plate is allosterically modulated by ATP, calcium and magnesium at pH 7.5. At pH 9.4, the hydrolysis of ATP and PNPP follows Michaelis-Menten kinetics with K0.5 values of 154 microM and 42 microM, respectively. However, at pH 7.5 both substrates exhibit more complex saturation curves, while only ATP exhibited site-site interactions. Ca(2+)-ATP and Mg(2+)-ATP were effective substrates for the enzyme, while the specific activity of the enzyme for the hydrolysis of ATP at pH 7.5 was 800-900 U/mg and was independent of the ion species. ATP, but not PNPP, was hydrolyzed slowly in the absence of metal ions with a specific activity of 140 U/mg. These data demonstrate that in vitro and at pH 7.5 rat osseous plate alkaline phosphatase is an active calcium or magnesium-activated ATPase.

Electrophoretic separation of alkaline phosphatase isoenzymes in synovial fluid and serum from patients with rheumatoid arthritis

The alkaline phosphatase enzyme in both serum and synovial fluid from 28 cases of rheumatoid arthritis and from the serum of 30 controls was measured. The enzyme was further studied by separating its isoenzymes to clarify their origin in both synovial fluid and serum of 10 patients with elevated level of the enzyme in their sera. The level of the enzyme in serum was elevated in 37% of patients confirming previous reports on that point. The most abundant isoenzyme in the synovial fluid (66.9%) was found to be bone in origin while in serum the most abundant isoenzyme was found to be hepatic (60.5%). This may be responsible for increased bone turn-over in rheumatoid joints whether in formation or resorption.

ATP-induced chondrocalcinosis

OBJECTIVE

To determine whether adult articular cartilage mineralizes in the presence of ATP.

METHODS

Intact adult porcine articular cartilage and monolayers of chondrocytes were cultured in physiologic media containing ATP, and mineralization was measured as retention of 45Ca. Cartilage was analyzed by electron microscopy.

RESULTS

Articular cartilage sequestered 45Ca when incubated with 100 microM ATP: Use of the ATP analog alpha, beta-methylene ATP did not promote mineralization and addition of pyrophosphatase inhibited mineralization, indicating that hydrolysis of ATP to AMP and inorganic pyrophosphate is necessary for the process to occur. Mineral was concentrated in articular cartilage vesicles in the perichondral area.

CONCLUSION

Adult articular cartilage mineralizes in the presence of ATP, in a manner similar to that found with isolated matrix or articular cartilage vesicles. This supports the notion that these structures have a role in chondrocalcinosis.

Phosphotransferase activity associated with rat osseous plate alkaline phosphatase: a possible role in biomineralization

1. Alkaline phosphatase from rat osseous plate catalyzed the transfer of phosphate from p-nitrophenylphosphate to glycerol, ethanolamines, Tris, glucose and 1-amino-1-methyl-2-propanol, in a wide range of pH. Serine did not stimulate phosphotransferase activity of the enzyme. 2. The best phosphotransferase acceptors were diethanolamine and glycerol while glucose was the poorest phosphotransferase acceptor used. 3. Diethanolamine and glycerol affected both VM and KM of p-nitrophenylphosphate hydrolysis with activation constants (KA) of 0.25 and 0.85 M, respectively. 4. A kinetic model was proposed for the phosphotransferase reaction observed with alkaline phosphatase from rat osseous plates.

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.

Different forms of alkaline phosphatase in adult rat femur. Effect of a vitamin D3-deficient diet and of a sorbitol-enriched diet

In the femoral extremities of the adult rat containing the metaphysis, the epiphyseal cartilage, and the epiphysis, four alkaline phosphatase (AP) forms were distinguished on polyacrylamide gel electrophoresis. Two soluble forms were present in the 160,000 g supernatant: one of Mr 165 kDa and another of Mr 110-115 kDa, which exhibited a strong catalytical activity. Moreover, from the pellet, three membrane-bound forms of Mr 130, 110-115, and 100 kDa could be extacted with sodium deoxycholate. When denaturated AP was visualized by postelectrophoretic autoradiography of the phosphorylated intermediates, subunits always appeared as three monomers of Mr 75-80, 60-70, and 50-60 kDa. As four native forms but only three types of subunits were found to be present in the femur, it seems that, apart from homodimers, some heterodimers could also occur. Three types of diets were administered to three groups of rats for 5 weeks. Two are known to disturb bone mineralization: (1) a vitamin D3-deficient diet, and (2) the same as (1) but enriched with 12% sorbitol. The third was a normal diet containing vitamin D3. Concerning the effects on AP of dietary sorbitol and the vitamin D3-deficient diet, it was found that rats receiving the diet supplemented with sorbitol showed a substantial rise in the activity of the Mr 165 kDa form with the concomitant appearance of a new monomer of Mr 100 kDa. In contrast, rats fed the vitamin D3-deficient diet always displayed an increase in enzyme activity, principally of the Mr 100 and 110 kDa forms.(ABSTRACT TRUNCATED AT 250 WORDS)

The extracellular matrix of cartilage in the growth plate before and during calcification: changes in composition and degradation of type II collagen

Calcification occurs in the extracellular matrix of the hypertrophic zone of the growth plate when the extracellular matrix volume is reduced to a minimum and alkaline phosphatase content is maximal. The present study shows that significant quantitative and qualitative changes occur in the composition and structure of macromolecules in the extracellular matrix before and during calcification in the proximal tibial growth plate of the bovine fetus. These were detected in part by using microchemical and microimmuno-chemical analyses of sequential transverse frozen sections at defined sites throughout the growth plate. Concentrations of matrix molecules in the extracellular matrix have not previously been determined biochemically. They were measured per unit matrix volume by using combined immunochemical/chemical-histomorphometric analyses. The concentrations within the extracellular matrix of the C-propeptide of type II collagen, aggregating proteoglycan (aggrecan), and hyaluronic acid all progressively increased in the maturing and hypertrophic zones, being maximal (or near maximal) at the time of initiation of mineralization. These results for proteoglycan are contrary to some earlier reports of a loss of proteoglycan are contrary to some earlier reports of a loss of proteoglycan prior to mineralization which measured the tissue content of proteoglycan rather than that present in the extracellular matrix, the volume of which is progressively reduced as the growth plate matures. The C-propeptide data provides a quantitative confirmation of previous immunohistochemical studies. Total collagen concentration (measured as hydroxyproline) in the extracellular matrix initially increased through the proliferating and maturing zones but then rapidly decreased in the hypertrophic zone.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Increased matrix vesicle protein in rachitic rat epiphyseal growth plates

Extracellular, membrane-bound vesicles are widely regarded to be the initial site of calcification in a variety of tissues under normal and pathological conditions. Alkaline phosphatase is believed to play a vital role in this process by hydrolysing ester phosphates or mineral inhibitors, e.g. inorganic phosphates. In the present study, matrix vesicles from normal and rachitic rat growth plates were compared with regard to specific activity of alkaline phosphatase, total vesicle protein and ultrastructural distribution of alkaline phosphatase activity. Matrix vesicles were released from normal or rachitic growth plates by collagenase digestion and isolated by differential centrifugation. Enzyme cytochemical localization involving a cerium capture method was performed on vesicles collected by vacuum filtration on Millipore filters. SDS gels and Western blots on fractions of both normal and rachitic matrix vesicles showed major proteins to be almost identical and confirmed the presence of alkaline phosphatase in both. Total matrix vesicle protein ((mg total matrix vesicle protein/rat) x 10(2)) per rat was significantly greater for the rachitic animals (9.0 +/- 2.0 vs. 4.0 +/- 1.0), P less than 0.0001. Alkaline phosphatase specific activity (units alkaline phosphatase/mg vesicle protein) in the rachitic and normal matrix vesicles was 25.29 +/- 9.36 and 18.78 +/- 3.37, respectively (0.05 less than P less than 0.1). Electron dense cerium phosphate deposits were localized to the outer membrane surface of matrix vesicles derived from both types of rats. This data, the first to quantify the relationship between rickets, matrix vesicle protein and alkaline phosphatase specific activity, suggests that matrix vesicles from rachitic and normal rats have biochemical and morphological similarity.(ABSTRACT TRUNCATED AT 250 WORDS)

Alkaline phosphatase from rat osseous plates: purification and biochemical characterization of a soluble form

A soluble form of an alkaline phosphatase obtained from rat osseous plates was purified 204-fold with a yield of 24.3%. The purified enzyme showed a single protein band of Mr 80,000 on SDS-PAGE and an apparent molecular weight of 163,000 by gel filtration on Sephacryl S-300 suggesting a dimeric structure for the soluble enzyme. The specific activity of the enzyme at pH 9.4 in the presence of 2 mM MgCl2 was 19,027 U/mg and the hydrolysis of p-nitrophenyl phosphate (K0.5 = 92 microM) showed positive cooperativity (n = 1.5). The purified enzyme showed a broad substrate specificity, however, ATP, bis(p-nitrophenyl) phosphate and pyrophosphate were among the less hydrolyzed substrates assayed. Surprisingly the enzyme was not stimulated by cobalt and manganese ions, in contrast with a 20-25% stimulation observed for magnesium and calcium ions. Zinc ions exerted a strong inhibition on p-nitrophenylphosphatase activity of the enzyme. This paper provides a simple experimental procedure for the isolation of a soluble form of alkaline phosphatase which is induced by demineralized bone matrix during endochondral ossification.

The alkaline phosphatase from bone: transphosphorylating activity and kinetic mechanism

For the purified alkaline phosphatase from bone, the ability to catalyze a phosphate transfer reaction from p-nitrophenyl phosphate to two different hydroxy acceptor compounds, ethanolamine and glycerol, was established by identification of the formed phosphorylated products, phosphoethanolamine and glycerol 3-phosphate, respectively. In addition, a steady-state kinetic analysis of the hydrolysis of p-nitrophenyl phosphate in the presence of an added nucleophile, diethanolamine, gave rise to the proposal of a simple model for the kinetic mechanism of the enzyme. This mechanism includes a covalent phosphoryl enzyme intermediate, the dephosphorylation of which by water (k3) or a nucleophile (k4) is rate-determining. According to this model, in the presence of diethanolamine, k3 and k4 were determined to be 4.44 s-1 M-1 and 1000 s-1 M-1, respectively. Therefore, in vitro a suitable nucleophile, such as diethanolamine, seems to be a better phosphate acceptor than water. These results may suggest that alkaline phosphatase from bone could be well suited for catalyzing phosphate transfer reactions in vivo as well.

Solubilization of membrane-bound matrix-induced alkaline phosphatase with polyoxyethylene 9-lauryl ether (polidocanol): purification and metalloenzyme properties

1. Matrix-induced alkaline phosphatase prepared from rat osseous plate was solubilized with polidocanol and purified on a Sephacryl S-300 column. 2. Purified solubilized alkaline phosphatase has a molecular weight of ca 115,000 and bind one magnesium and two zinc ions. At least 110 detergent molecules are bound to each enzyme molecule. 3. Solubilization and purification procedures did not destroy the ability of the enzyme to hydrolyze adenosine-5'-triphosphate, p-nitrophenylphosphate, pyrophosphate and bis p-nitrophenylphosphate. 4. Magnesium, manganese and cobalt ions are stimulators of PNPPase activity of solubilized enzyme whereas calcium and zinc ions are inhibitors.

Isolation of two glycosylated forms of membrane-bound alkaline phosphatase from avian growth plate cartilage matrix vesicle-enriched microsomes

Isolation of two membrane-bound alkaline phosphatase (AP) species from avian growth plate cartilage matrix vesicle (MV) fractions is described. AP was first released from the membranes by phosphatidylinositol-specific phospholipase C (PIase C), followed by chromatography on DEAE-Bio-Gel A and Reactive-Red agarose. Two AP species having apparent Mr of 81.5 and 77 kDa by SDS-PAGE were purified in high yield and specific activity by this simple method. Treatment with neuraminidase to remove sialic acid residues reduced their size slightly, but did not diminish the difference in Mr between the two species. Digestion with N-glycanase, however, decreased both AP species to a common size of 59 kDa. This reveals that both enzymes are highly glycosylated and suggests that the two forms may result from differences in degree of glycation. The amino acid compositions of the two avian enzyme forms are very similar, but are markedly enriched in serine, glycine and glutamate when compared to those reported for mammalian liver-kidney-bone AP. Possible differences in amino acid sequence between the two avian forms have not been excluded. The cross-reactivity of polyclonal antibodies to these enzymes with bovine kidney, but not intestinal AP, indicate that the avian cartilage APs are of the liver-kidney-bone isozyme type.

Cartilage macromolecules and the calcification of cartilage matrix

The calcification of cartilage matrix in endochondral bone formation occurs in an extracellular matrix composed of fibrils of type II collagen with which type X collagen is closely associated. Also present within this matrix are the large proteoglycans containing chondroitin sulfate which aggregate with hyaluronic acid. In addition, the matrix contains matrix vesicles containing alkaline phosphatase. There is probably a concentration of calcium as a result of its binding to the many chondroitin sulfate chains. At the time of calcification, these proteoglycans become focally concentrated in sites where mineral is deposited. This would result in an even greater focal concentration of calcium. Release of inorganic phosphate, as a result of the activity of alkaline phosphatase, can lead to the displacement of proteoglycan bound calcium and its precipitation. The C-propeptide of type II collagen becomes concentrated in the mineralizing sites, prior to which it is mainly associated with type II collagen fibrils and is present in dilated cisternae of the enlarged hypertrophic chondrocytes. The synthesis of type II collagen and the C-propeptide, together with alkaline phosphatase, are regulated by the vitamin D metabolites 24,25(OH)2 cholecalciferol and 1,25 (OH)2 cholecalciferol. At the time of calcification, type X collagen remains associated with type II collagen fibrils. It may play a role in preventing the initial calcification of these fibrils focusing mineral formation in focal interfibrillar sites. This process of calcification is clearly very complex, and involves different interacting matrix molecules and is carefully regulated at the cellular level.

Calcification of in vitro developed hypertrophic cartilage

We have recently reported that dedifferentiated cells derived from stage 28-30 chick embryo tibiae, when transferred in suspension culture in the presence of ascorbic acid, develop in a tissue closely resembling hypertrophic cartilage. Ultrastructural examination of this in vitro formed cartilage showed numerous matrix vesicles associated with the extracellular matrix (C. Tacchetti, R. Quarto, L. Nitsch, D. J. Hartmann, and R. Cancedda, 1987, J. Cell Biol. 105, 999-1006). In the present article we report that the in vitro developed hypertrophic cartilage undergoes calcification. We indicate a correlation between the levels of alkaline phosphatase activity and calcium deposition at different times of development. Following the transfer of cells into suspension culture and an initial lag phase, the level of alkaline phosphatase activity rapidly increased. In most experiments the maximum of activity was reached after 5 days of culture. When alkaline phosphatase activity and 45Ca deposition were measured in the same experiment, we observed that the increase in alkaline phosphatase preceded the deposition of nonwashable calcium deposits in the cartilage.

Correlation between alkaline and acid phosphatase activities and age-related osteopenia in murine vertebrae

Lumbar vertebrae (L4) from CW-1 female mice were examined for age-related changes in alkaline and acid phosphatase activities from young to old age. Histochemically, both enzymes were encountered along the bony surfaces of both trabecular and cortical bones with no significant age-related changes in their distribution. Biochemical determinations of bone alkaline phosphatase (Alk'ase) activity revealed that for a given unit level of bone or the bone as a whole no significant changes took place, whereas acid phosphatase (Acid'ase) activity was found to have increased significantly with age. A high positive correlative relationship was noted between the calcium content and the trabecular bone volume of the same vertebrae. It may, therefore, be proposed that age-related bone loss in mice could be attributed to an enhanced resorption rather than to a substantial reduction in the formative potential of bone cells.

Purification and partial amino acid sequencing of alkaline phosphatase from rachitic rat epiphyseal cartilage

1. Alkaline phosphatase of rachitic epiphyseal cartilage was purified to apparent homogeneity by sequential application of monoclonal affinity, DEAE-cellulose, and Sepharose CL-6B chromatography. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of the enzyme showed the presence of a dominant band corresponding to a molecular weight of 80,000. 2. The N-terminal amino acid sequence was determined as follows: Phe-Val-Pro-Glu-Lys5-Glu-Lys-Asp-Pro-Ser10-Tyr-Trp-Arg-Gln-+ ++Gln15-Ala-Gln-Glu- Thr-Leu20-Lys-Asn-Ala-Leu-Lys25-Leu-Gln-Lys-?-Asn-Val-Asn-?- Ala-Lys35-?-Ile-?- Met-Phe40-Leu-(Gly?)-Asp-(Ala/Gly?)-Met45-?-Val-?- (Val/Gly?).

Purification and partial amino acid sequencing of rat bone tumor (UMR106) alkaline phosphatase

Cultured rat osteosarcoma (UMR106) alkaline phosphatase was purified to apparent homogeneity by sequential application of polyclonal antibody affinity, DEAE-cellulose, and Sepharose CL-6B chromatography. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the enzyme preparation treated with sodium dodecyl sulfate and mercaptoethanol showed the presence of a dominant band (using silver staining) corresponding to a molecular weight of 80,000. The amino acid composition was similar to those of various alkaline phosphatases. The N-terminal amino acid sequence was determined as follows: Phe-Val-Pro-Glu-Lys-Glu-Lys- Asp-Pro-Ser-Tyr-Trp-Arg-Gln-Gln-Ala-Gln-Glu-Thr-Leu- Lys-Asn-Ala-Leu-Lys-?-Gln-Lys-?-Asn-Val-Asn-Ala-Lys.

Rat alkaline phosphatase. II. Structural similarities between the osteosarcoma, bone, kidney, and placenta isoenzymes

A mouse monoclonal antibody raised against rat osteosarcoma alkaline phosphatase (AP) was covalently coupled to protein A-Sepharose and used to purify this enzyme from preparations of rat osteosarcoma, calvaria, kidney, and placenta in a single-step procedure. The tissue-specific isoenzymes purified in this manner showed identity in the immunodiffusion reaction with a polyclonal anti-AP antibody, but differed in apparent molecular weight and degree of polydispersity on sodium dodecyl sulfate-polyacrylamide gels. Treatment with N-glycanase abolished these differences, yielding proteins with an apparent molecular weight of 52,000 Da and identical V8 protease digestion patterns. Alkaline phosphatase from these tissues showed no significant difference in amino acid composition and identity in the first 20 N-terminal amino acids. These findings provide structural evidence which supports the hypothesis that the tissue-specific alkaline phosphatase isoenzymes share a common protein sequence subject to different glycosylation pattern.

Rat alkaline phosphatase. I. Purification and characterization of the enzyme from osteosarcoma: generation of monoclonal and polyclonal antibodies

Alkaline phosphatase (AP) was purified to over 90% homogeneity from rat osteosarcoma by acetone precipitation followed by chromatography on DEAE-cellulose, Sephacryl S-200, and hydroxyapatite. The purified enzyme had a specific activity of 759 units/mg protein at its optimal pH (10.5), and a Km of 0.8 mM for p-nitrophenylphosphate. The enzyme's apparent subunit molecular mass on sodium dodecyl sulfate-polyacrylamide gel electrophoresis was 82,000 Da. The heat-inactivation profile and homoarginine inhibition were characteristic of the bone-liver-kidney AP isoenzyme. Monoclonal and polyclonal anti-AP antibodies were prepared and characterized. Polyclonal rabbit antiserum quantitatively precipitated the activity from purified AP preparations and tissue extracts but did not inhibit AP catalytic activity. This antiserum was almost 10-fold less active against heat-inactivated enzyme when tested in a competition assay using 125I-AP. Two distinct monoclonal antibodies were each partly effective in immunoprecipitating AP when tested individually; however, together they precipitated over 90% of the AP activity.

Purification and partial amino acid sequencing of bovine kidney alkaline phosphatase

Bovine kidney alkaline phosphatase (ALPase) was purified by the sequential application of monoclonal anti-bovine cartilage ALPase affinity, DEAE-cellulose, and Sepharose CL-6B chromatography. Sodium dodecyl sulfate-polyacrylamide-gel electrophoresis showed the presence of a single band corresponding to a molecular weight of 80,000. The N-terminal amino acid sequence of bovine kidney alkaline phosphatase was determined as follows: Leu-Val-Pro-Glu-Lys-Asp-Pro-?-Tyr-Trp-Arg-Asp-Gln-Ala-Gln.

Immunohistochemical study of alkaline phosphatase in growth plate cartilage, bone, and fetal calf isolated chondrocytes using monoclonal antibodies

Monoclonal antibodies prepared against alkaline phosphatase of matrix vesicles from fetal bovine growth plate cartilage were used to study the distribution of alkaline phosphatase in bovine bone and cartilage. Primary chondrocyte cultures from bovine growth plate cartilage were prepared and expression of this antigen was followed during 3 d long cultured period. Same monoclonal anti-alkaline phosphatase antibodies recognized both osteoblasts in metaphyseal bone as well as proliferating and hypertrophic chondrocytes in growth plate cartilage. Extracellular reaction was observed in lower areas of growth plate cartilage after hyaluronidase digestion of tissue sections. In isolated growth plate chondrocytes strong membrane associated reaction as well as weaker intracytoplasmic reaction were noticed and especially in large hypertrophic cells membrane reaction was clustered showing "capping-like" phenomenon. These results indicates the high similarity of alkaline phosphatase in bone and cartilage and its uneven distribution in chondrocyte plasma membrane.

Triton X-100 solubilized bone matrix-induced alkaline phosphatase

1. Solubilized and membrane-bound alkaline phosphatase showed Michaelis-Menten behavior in a wide range of different substrate concentrations. 2. Membrane-bound alkaline phosphatase has a molecular weight of 130,000 and its minimum active configuration comprises two identical subunits of about 65,000. 3. The two forms of the enzyme behave similarly with respect to NaCl, urea and guanidine HCl. 4. Catalytic groups have pK values of about 8.5 and 9.7 for both membrane-bound and solubilized enzyme.

Development and cross-reactive properties of monoclonal antibodies to bovine matrix vesicle alkaline phosphatase

Alkaline phosphatase (ALPase), concentrated in the membranes of matrix vesicles, is believed to play a role in initial calcification. To further purify, characterize, and identify this enzyme in tissue, a monoclonal antibody was developed against the ALPase of isolated fetal calf matrix vesicles. Splenic lymphocytes derived from mice immunized with Sepharose 6B-purified fetal calf matrix vesicle ALPase were fused with mouse plasmacytoma cells (line X63-Ag-8.653) using standard hybridoma technology. Hyperimmune sera and hybridoma culture supernatants were screened for the presence of specific antibody using a newly developed double-immunosorbent assay in which putative antibody is added to microtiter plate wells precoated with affinity-purified rabbit antimouse immunoglobulin. After incubation and washing, partially purified fetal calf matrix vesicle ALPase is added to each well. The enzyme adheres only to wells that contain specific anti-ALPase antibody. These wells are identified by adding the enzyme substrate p-nitrophenyl phosphate and reading the wells in a plate-reading spectrophotometer at 405 nm. A hybridoma-producing specific antibody was subsequently cloned and grown as ascities-producing tumors in pristane-primed mice. Ouchterlony analysis indicated that the cell line secretes an immunoglobulin of IgG1 class. This antibody reacts specifically with ALPase derived from calf matrix vesicles and cross-reacts with ALPase of bovine kidney, liver, and placental origin and human bone but does not cross-react with bovine intestinal ALPase or ALPase derived from matrix vesicles isolated from rachitic rat growth plate cartilage.