Direct Determination of Phosphatase Activity from Physiological Substrates in Cells

A direct and continuous approach to determine simultaneously protein and phosphate concentrations in cells and kinetics of phosphate release from physiological substrates by cells without any labeling has been developed. Among the enzymes having a phosphatase activity, tissue non-specific alkaline phosphatase (TNAP) performs indispensable, multiple functions in humans. It is expressed in numerous tissues with high levels detected in bones, liver and neurons. It is absolutely required for bone mineralization and also necessary for neurotransmitter synthesis. We provided the proof of concept that infrared spectroscopy is a reliable assay to determine a phosphatase activity in the osteoblasts. For the first time, an overall specific phosphatase activity in cells was determined in a single step by measuring simultaneously protein and substrate concentrations. We found specific activities in osteoblast like cells amounting to 116 ± 13 nmol min-1 mg-1 for PPi, to 56 ± 11 nmol min-1 mg-1 for AMP, to 79 ± 23 nmol min-1 mg-1 for beta-glycerophosphate and to 73 ± 15 nmol min-1 mg-1 for 1-alpha-D glucose phosphate. The assay was also effective to monitor phosphatase activity in primary osteoblasts and in matrix vesicles. The use of levamisole – a TNAP inhibitor- served to demonstrate that a part of the phosphatase activity originated from this enzyme. An IC50 value of 1.16 ± 0.03 mM was obtained for the inhibition of phosphatase activity of levamisole in osteoblast like cells. The infrared assay could be extended to determine any type of phosphatase activity in other cells. It may serve as a metabolomic tool to monitor an overall phosphatase activity including acid phosphatases or other related enzymes.


Introduction
Among the enzymes having a phosphatase activity and releasing P i , tissue non-specific alkaline phosphatase (TNAP) performs indispensable, multisystemic functions in humans [1]. It is expressed with high levels in bones, liver and neurons. It is absolutely required for bone mineralization and also necessary for neurotransmitter synthesis. So far there is no direct methodological approach to determine phosphate in the living cells without the need of labelling. Most standard methods for measuring inorganic phosphate or pyrophosphate are based on coupled enzyme assays, colorimetric methods, conductance, or radioactivity labeling which do not allow one-step determination in the cells. Protein determination and activity measurements must be performed separately. For example, the screening of TNAP inhibitors relied on the determination of the recombinant TNAP activity at alkaline pH using para-nitrophenylphosphate (pNPP), which is not a physiological substrate. A 1,000-fold more sensitive and 10-fold faster than the pNPP assay has been developed with pNPP dioxetane-based substrate [2][3]. However, TNAP inhibitors selected by using non physiological substrates may act differently on the living cells. Therefore, there is a strong demand for flexible and fast strategies to select inhibitors with improved prospects for clinical success [4]. Infrared (IR) spectroscopy [5] has been employed to identify specific finger-like signatures in microbial cells [6][7][8][9][10] in tumour cells [11][12][13][14][15][16] or in tissues [17][18][19] giving information of overall structural and biochemical cell composition or changes induced by anti-tumour drugs in cells [20][21][22][23][24][25][26][27][28]. Alkaline phosphatase activity in sera [29] has been determined by IR, suggesting the possibility of using IR for quantitative determination of an enzymatic activity in whole cells. Here, we report a continuous IR assay to determine phosphatase activity in osteoblasts and in matrix vesicles released by chondrocytes, characterized by high TNAP activity. We provided the proof of concept that infrared spectroscopy is a reliable assay to determine a phosphatase activity in the osteoblasts. For the first time, an overall specific phosphatase activity in a single step by measuring simultaneously protein and substrate concentrations was determined.

Isolation of matrix vesicles
Matrix vesicles (MVs) were prepared according to Wuthier's method [30]. Femurs from twenty 17-day-chicken embryos were taken. Slices (1-3 mm thick) of growth plates and ephyseal cartilage were cut. They were washed five times in synthetic cartilage lymph (SCL) buffer containing 1.42 mM NaH 2 PO 4Á H 2 O, 1.83 mM NaHCO 3 , 12.7 mM KCl, 0.57 mM MgCl 2 , 5.55 mM Dglucose, 63.5 mM sucrose, 16.5 mM TES, 100 mM NaCl, 0.57 mM Na 2 SO 4 , pH 7.4. The slices were incubated at 37°C for 180 min by mixing continuously in SCL buffer containing 200-500 units g -1 tissue of type I collagenase from Clostridium histolyticum (Sigma) and 1 mM CaCl 2Á H 2 O. Then, it was filtered through a nylon filter. The filtrate was centrifuged at 600 g for 15 min at 4°C. After the first centrifugation the debris were discarded and the supernatant was centrifuged at 20 000 g for 20 min at 4°C. A second centrifugation at 80 000g for 60 min at 4°C was then performed. The supernatant was discarded and the pellet was gently washed with 1 mL Tris HCL (100 mM, pH 8.0) containing 5 μM ZnCl 2 and 5 mM MgCl 2 . Washing medium was discarded and the intact pellet (around 0.05 mL) was suspended in 0-2 mL Tris HCL (100 mM, pH 8.0) containing 5 μM ZnCl 2 and 5 mM MgCl 2 (called throughout buffer A). The protein concentration in MVs was determined by Bradford [31] was in the range of 0.2 to 8 mg MV protein mL -1 . MVs were freshly prepared for the IR measurements.

Saos-2 Cells
Human osteosarcoma Saos-2 cells (ATCC HTB-85) were cultured in DMEM medium supplemented with 100 U mL -1 penicillin, 100 g mL -1 streptomycin (both from Sigma) and 10% FBS (v:v, Gibco). Stimulation of Saos-2 cells was induced by culturing the confluent cells in growth medium supplemented with 50 μg mL -1 ascorbic acid (AA) (Sigma) and 7.5 mM β- . Calvaria were dissected aseptically and cells were isolated using sequential digestion at 37°C with trypsin/EDTA 0.05% during 20 min and then with liberase 0.8 U mL -1 during 20 min. The first two digests were discarded, and cells obtained after two 45 min digestions with liberase 0.8 U mL -1 were collected, pooled and then filtered through a 100 μm cell strainer. The cells were plated at a density of 1 × 10 5 cells per well in 22.1 mm culture dish in DMEM containing 15% FBS (v:v, Gibco), 100 U mL -1 penicillin and 100 μg mL -1 streptomycin (both from Sigma) and switched 24h later to growth medium i.e., DMEM containing 10% FBS (v:v, Gibco), supplemented with 50 μg mL -1 ascorbic acid (Sigma). After six days of culture, the medium was further supplemented with 7.5 mM β-glycerophosphate (Sigma) and 50 μg mL -1 ascorbic acid during one week. Primary cultures were used without passage. Cultures were maintained in a humidified atmosphere consisting of 95% air/5% CO 2 at 37°C. After a six-day incubation, primary cells were detached with trypsin (Sigma) and 1 mM CaCl 2Á H 2 O. The cells were washed with 1 mL buffer A or with 1 mL DMEM buffer. After centrifugation, the supernatant was discarded. The washing and centrifugation procedures were repeated three times so that trypsin was completely removed. An aliquot of 0.5 to 1.5 mg of freshly prepared osteoblasts as determined by weighting was taken and kept in Eppendorf tubes for the IR measurements. The cells were freshly prepared for IR measurements.

Infrared spectra
To start the reaction, an aliquot of either MV (0.5 to 8 mg protein mL -1 ), Saos-2 cells (6 to 28 mg protein mL -1 ) or primary osteoblast (20 to 45 mg protein mL -1 ) was taken and mixed with the reaction medium containing 40-80 mM final PP i or one of other substrates (AMP, ADP, ATP, UTP, α-D-glucose 1-phosphate (G-1P), β-glycerophosphate (β-GP) and paranitrophenylphosphate (pNPP)) in buffer A. Final protein concentrations in MVs or in cells were determined directly by measuring the intensity of the amide-II band at 1550-cm -1 with the concentration absorption coefficient ε = 3.6 mg -1 mL cm -1 . ε was determined by using bovine serum albumin as a protein Five μL of the reaction mixture containing either MVs or Saos-2 cells or primary osteoblast cells were taken and deposited between two BaF 2 windows of a demountable thermostated cell (model Harrick) separated with a 12 μm (for IR spectra of substrates) or 6 μm (for IR spectra of cells with substrates) Teflon spacer. IR data were acquired with a Thermo Scientific Nicolet iS10 spectrometer equipped with a DTGS detector. The IR spectra were recorded with a thermostated IR cell kept at 37°C with 128 interferograms at 4 cm -1 resolution each and then Fourier transformed. During data acquisition, the spectrometer was continuously purged with dry filtered air (Balston regenerating desiccant dryer, model 75-45 12 VDC). At least three independent measurements have been performed to obtain the kinetics parameters. To minimize cell to cell variation, affecting TNAP activity, the same cell preparations were used to compare with the controls. For the determination of IC 50 of levamisole, relative activities were measured to take into account of the cell to cell variations.

Results and Discussion
IR spectra of phosphatase substrates IR spectra of phosphatase substrates such as AMP, ADP, ATP, UTP, α-D-glucose 1-phosphate (G-1P), β-glycerophosphate (β-GP), pNPP and PP i in aqueous Tris-HCl buffer (pH 8) containing 5 mM MgCl 2 and 5 μM ZnCl 2 (buffer A) present sufficient differences in band shapes and positions as compared with the IR spectrum of phosphate ( Fig. 1) to be used for analytical application. The IR spectrum of P i in buffer A revealed two bands at 1076 and 990 cm -1 (Fig. 1) that are assigned respectively to asymmetric and symmetric stretching vibrations of O-P-O, respectively [32]. The position of the phosphate bands is sensitive to their ionic environment as in the case of nucleotides [33] or phospholipids [34,35]. To illustrate the potential of the IR assay in assessing phosphatase activity in cells, we measured IR spectrum immediately (with 3 min delay) after mixing 50 mM substrate with osteoblast-like Saos-2 cells, (Fig. 2, dashed lines) and after 30 min of incubation (Fig. 2, full lines).
In all cases (with AMP, ADP, ATP, UTP, G-1P, β-GP), pNPP or PP i ), the intensity of one or both P i bands at 1076-1080 and 990 cm -1 increased, indicating the phosphatase activity in the whole cells (Fig. 2). It was clearly observed in the successive difference spectra (spectrum measured at the indicated time minus that measured immediately after mixing 50 mM substrate with Saos-2 cells) (Fig. 3).
Negative peaks indicated the disappearance of the substrate while positive peaks at 1076-1080 and 990 cm -1 illustrated the formation of P i (Fig. 3). Hydrolysis of ATP, UTP and ADP in the Saos-2 cells produced P i (positive 1088-1080-cm -1 and 990-cm -1 bands) and AMP (positive 977-cm -1 band) (Fig. 3). In the case of AMP hydrolysis, the P i bands (located 1088-1080 cm -1 and 990 cm -1 ) overlapped with the AMP bands (located at 1088 cm-1 and 977 cm-1) rendering difficult to observe the Pi bands. We also observed hydrolysis of G-1P, β-GP and pNPP and PP i in Saos-2 cells indicating a broad phosphatase activity. Buffer A has the disadvantage of inducing cell aggregation after several hour incubation but its absorption in the 1200-1000-cm -1 region (Fig. 4a, dashed line) is lower than that of DMEM cellular medium (Fig. 4a, full line).
The feasibility to monitor the hydrolysis of PP i by Saos-2 cells in DMEM was verified. Typical IR spectra of 50 mM PP i with Saos-2 cells in buffer A (Fig. 4b) or in DMEM (Fig. 4c) immediately after mixing PP i with Saos-2 cells (full trace) and after 30 min incubation (dashed trace) indicated both the decrease of the 1100-cm -1 band of PP i and the increase of the 1076-1080cm -1 band of P i , confirming the PP i hydrolysis. Furthermore, the 1550-cm -1 protein band      allowed us to determine directly protein concentration using the concentration absorption coefficient ε = 3.6 mg -1 mL cm -1 . Saos-2 osteoblast-like cells were selected due to their high TNAP activity, making it easier to monitor phosphatase activity. However they have different phenotypic properties in comparison with the corresponding healthy osteoblasts. Primary osteoblasts from mouse calvaria exhibited also PP i hydrolysis both in buffer A (Fig. 4d) and in DMEM but it was much lower than that of Saos-2 cells. This was confirmed qualitatively by the magnitude of the difference in IR spectra (spectrum measured after 30 min minus that measured immediately after mixing PP i with cells) recorded for Saos-2 cells in buffer A (Fig. 5a) and in DMEM medium (Fig. 5b) which were higher than that those of primary osteoblasts in buffer A (Fig. 5c) and in DMEM (Fig. 5d) respectively.

Determination of specific activity of phosphatase in Saos-2 cells
The kinetic parameters are easily obtained from IR spectra. For example, the PP i hydrolysis by Saos-2 cells is followed by an increase of the 990-cm -1 band of P i (Fig. 6a) and the concomitant decrease of the 1120-1100-cm -1 band of PP i (Fig. 6b).
The absorbance changes of 1120-1100-cm -1 band of PP i (Fig. 6b) and the 990-cm -1 band of P i (Fig. 6a) served to determine the PP i hydrolysis ( Table 1).
As expected, increasing the amount of Saos-2 cells, from 7.5 mg mL -1 , 12 mg mL -1 to 18 mg mL -1 , as determined by the protein band, increased the PP i hydrolysis but the specific activity remained at the level of 102 ± 4 nmol min -1 mg -1 as expected ( Fig. 6c and Table 1). There is not a statistically significant difference between the specific activities (P value is at least greater than 0.513). This compares well with the specific PP i hydrolysis of 130 nmol min -1 mg -1 in mammalian HeLa cells at pH 8.5 (36) as assayed by the modified method of Fiske and Subbarow. Quantitative determinations were also obtained for AMP, G-1P, β-GP, pNPP using both P i bands at 990 and 1076-1080 cm -1 ( Table 1). The substrate bands were also used to determine enzyme specific activity, as in the case of AMP (977 cm -1 ), pNPP (980 cm -1 ) and PP i (1107 cm -1 ) ( Table 1). Specific activities determined either from P i or from substrate-bands were identical within experimental errors except for G-1P and β-GP due to strong band overlapping. To check Table 1. Specific activity of phosphatase activity of several substrates by osteoblast-like Saos-2 cells in buffer A at 37°C as determined by the infrared band of substrate or Pi (990 and 1070 cm -1 ).

Substrates
Band position cm -1 (molecule) Mol. Abs. coeff. L.mol -1 .cm -1 Specific activity nmol.mg -1 min -1 Average nmol.mg -1 .min -1 if the pyrophosphatase activity originates from TNAP-enriched cells, levamisole (a TNAP inhibitor) was added to the medium. At the concentration of 5 mM, levamisole almost completely inhibited the PP i hydrolysis by Saos-2 cells (Fig. 6d), suggesting that TNAP contributed mostly to the PP i hydrolysis of Saos-2 cells. Relative activity instead of specific activity was used to take into account of cell variability. 100% corresponded to the specific activity without the addition of inhibitor. An IC 50 value of 1.16 ± 0.03 mM was obtained for the inhibition of phosphatase activity of levamisole which is comparable with the reported concentration of 1 mM levamisole that blocked calcification in cultures of aortas from uremic rats [37].

Determination of specific activity of phosphatase in matrix vesicles
Matrix vesicles (MVs) extracted from growth plates and epiphyseal cartilage of 17-day-chicken embryos were also assayed for PP i hydrolysis since they have 10-40 fold higher TNAP specific activity than cells. MV size is around 100-400 nm diameter as determined by electron microscope [38]. Typical example of a series of IR spectra during PP i hydrolysis of MVs in Buffer A supplemented with 50 mM PP i monitored at 37°C are shown in Fig. 7a. The intensity decrease of the 1120-1100 cm -1 band related to the disappearance of PP i is correlated with the increase in the 1076-1080 and 990 cm -1 bands (Fig. 7a), indicating a hydrolysis of PP i by MVs and an appearance of P i . It is better seen in the difference infrared spectra, e.g. from the IR spectrum of MVs recorded at the indicated time minus that recorded immediately (Fig. 7b), which were similar with difference spectra of Saos-2 cells with PP i (Fig. 3) and also with those of alkaline phosphatase with PP i (data not shown). The calculated IR difference, i.e. spectrum of P i minus spectrum of PP i (Fig. 7c, dashed line) is almost identical to the IR difference spectrum of MVs with PP i (spectrum recorded after 60 min minus the spectrum Direct Determination of Phosphatase Activity in Cells recorded immediately) (Fig. 7c, full line). The IR changes produced by PP i hydrolysis by MVs corresponded to the calculated decrease of one PP i and to the formation of two P i . We estimated the sensitivity of the IR assay during 30-min incubation to be about 0.2 mg mL -1 MV protein. At 0.2 mg mL -1 MV protein, only faint bands not much more intensive than the noise signal are detected (Fig. 7d, dashed line), while at 2.5 mg mL -1 MV protein, the IR bands indicating disappearance of PP i and appearance of P i are well resolved (Fig. 7d, full line). Activity measurements were determined by measuring absorbance changes of the absorbance changes of PP i band located at 1107 cm -1 using ε = 2158 ± 211 M -1 cm -1 (Fig. 8a) as well as with two P i bands located at 990 cm -1 (Fig. 8b) and 1076 cm -1 (Fig. 8c), using ε = 443 ± 50 M -1 cm -1 and ε = 1215 ± 131 M -1 cm -1 , respectively. As a result we determined an apparent hydrolytic activity of MVs towards PP i of 1132 ± 370 nmol min -1 mg -1 .
For comparison, the TNAP activity of MVs determined at pH 10.4 using the pNPP as a substrate, amounted to 30 000 ± 10 000 nmol min -1 mg -1 . This is consistent with the reported values for alkaline phosphatase extracted from HeLa cells, having a PP i hydrolytic activity at pH 8.5 about 26 to 30 times smaller than the pNPP hydrolytic activity at pH 10.5 [36]. To test if the hydrolysis of PP i is specifically associated to TNAP, we incubated the reactive medium containing MVs and 50 mM PP i with 5 mM levamisole, a TNAP specific inhibitor. Addition of levamisole induced a smaller slope in the absorbance changes at 1107 cm -1 (Fig. 8a), at 990 cm -1 (Fig. 8b) and at 1076 cm -1 (Fig. 8c) as compared with the control sample without levamisole (Fig. 8). We observed a significant inhibition of PP i hydrolysis in the presence of 5 mM levamisole from 1132 ± 370 nmol min -1 mg -1 (without levamisole) to 343 ± 112 nmol min -1 mg -1 (with 5 mM Levamisole) confirming that TNAP is to a large extent responsible for PP i hydrolysis. Since the levamisole could not block completely the phosphatase activity, there is the possibility that other phosphatases such as NPP1 can catalyze the phosphatase activity. Indeed, NPP1, which usually hydrolyses phosphodiester bond can also hydrolyze phosphomonoester bond at a lesser extend [39].

Conclusion
It has been recognized earlier that IR spectroscopy has considerable potential as a tool in diagnostics since it may be used to assess biochemical changes of cells exposed to various inhibitors [20][21][22][23][24][25][26][27][28]. Here we demonstrated the ability of IR spectroscopy to directly determine in situ a phosphatase activity in osteoblasts using natural substrates without any labeling. We showed the ability of IR spectroscopy to directly determine simultaneously protein concentration and the phosphatase activity in cultured osteoblasts as well as in matrix vesicles using physiological substrates such as AMP, ADP, ATP, UTP, and PP i . The method has been validated for osteoblasts (Saos-2 cells and primary osteoblasts) and chondrocyte-derived matrix vesicles which are all TNAP enriched. This approach could be extended to determine alkaline phosphatase activity as well as any type of phosphatase activity in other cells. It may serve as a metabolomic tool to determine an overall phosphatase activity in a cell which may involve several enzymes.