B American Society for Mass Spectrometry, 2017

J. Am. Soc. Mass Spectrom. (2017) 28:1175Y1181 DOI: 10.1007/s13361-016-1591-x

FOCUS: HONORING R. G. COOKS' ELECTION TO THE NATIONAL ACADEMY OF SCIENCES: RESEARCH ARTICLE

Ambient Ionization Mass Spectrometry Measurement of Aminotransferase Activity Xin Yan,1,2

Xin Li,1 Chengsen Zhang,1 Yang Xu,1,3 R. Graham Cooks1

1

Department of Chemistry, Purdue University, West Lafayette, IN, USA Department of Chemistry, Stanford University, Stanford, CA, USA 3 College of Life Sciences, Jilin University, Changchun, China 2

Abstract. A change in enzyme activity has been used as a clinical biomarker for diagnosis and is useful in evaluating patient prognosis. Current laboratory measurements of enzyme activity involve multi-step derivatization of the reaction products followed by quantitative analysis of these derivatives. This study simplified the reaction systems by using only the target enzymatic reaction and directly detecting its product. A protocol using paper spray mass spectrometry for identifying and quantifying the reaction product has been developed. Evaluation of the activity of aspartate aminotransferase (AST) was chosen as a proof-of-principle. The volume of sample needed is greatly reduced compared with the traditional method. Paper spray has a desalting effect that avoids sprayer clogging problems seen when examining serum samples by nanoESI. This very simple method does not require sample pretreatment and additional derivatization reactions, yet it gives high quality kinetic data, excellent limits of detection (60 ppb from serum), and coefficients of variation <10% in quantitation. Keywords: Clinical enzymology, Paper spray, Multiple reaction monitoring, Kinetics, Enzyme activity, Liver Disease, Point-of-care measurements, Diagnostics Received: 25 August 2016/Revised: 3 November 2016/Accepted: 29 December 2016/Published Online: 31 January 2017

Introduction

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any diseases that cause tissue damage result in an increased release of intracellular enzymes into the plasma [1]. The activities of many of these enzymes are routinely determined for diagnostic purposes in diseases of the heart, liver, skeletal muscle, and other tissues [2]. This interest extends from primary injury to damage due to metabolism of therapeutic drugs or drugs of abuse. The level of specific enzyme activity in plasma frequently correlates with the extent of tissue damage [3–6]. Thus, determining the degree of elevation of the activity of a particular enzyme in plasma is often useful in evaluating patient prognosis (Figure 1). Examples of clinically important enzymes for diagnostics are amylase for acute pancreatitis diagnosis [7, 8]; creatine phosphokinase (CPK) for muscular dystrophy and myocardial infarction diagnosis [9–11]; Electronic supplementary material The online version of this article (doi:10. 1007/s13361-016-1591-x) contains supplementary material, which is available to authorized users. Correspondence to: Xin Yan; e-mail: [email protected], R. Cooks; e-mail: [email protected]

gamma glutamyl transferase (GGT) for hepatitis and malignant tumor diagnosis [12]; lactate dehydrogenase (LDH) for myocardial infarction and hepatitis diagnosis [13]; lipase for acute pancreatitis and bile duct obstruction diagnosis [14]; alkaline phosphatase for some bone diseases and bone tumor diagnosis [15]; prostatic acid phosphatase for prostate cancer diagnosis [16]. Enzyme activity is measured in a unit (UI) defined as the amount of enzyme that catalyzes the conversion of 1 μm of substrate to product per min [17]. Understanding enzymatic catalysis is also important in biochemistry and essential for enzyme-related biotechnologies ranging from biocatalysis to drug development [18–21]. In the clinic, standard laboratory measurements of enzyme activity involve sample preparation, enzymatic reaction with a biomarker, multi-step derivatization of the reaction products, and analysis of these derivatives typically by optical detection [22]. For example, evaluation of activity of enzyme aspartate aminotransferase (AST) has been used for detection of primary liver injury or drug-related liver damage [23]. AST detection has been studied using a number of different methods such as colorimetric analysis, spectroscopic measurement, and electrochemical detection [24]. Since available analytical techniques

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Figure 1. Increased enzyme activity is a marker for disease/tissue damage and its measurements serve for clinical diagnosis

cannot directly detect the product of the aminotrasferase reaction, further derivatization reactions are used to characterize the products and create species suitable for detection. The detection of the reaction products is based on an intricate series of reactions; however, the results tend to suffer from poor signal discrimination from interferents in serum samples used in the derivatization reactions [24, 25]. The protocol is typically timeconsuming, expensive, and can delay diagnosis and treatment. The procedure also requires well-trained technicians and a special well-equipped laboratory [24]. This has led to the reasonable demand for a faster protocol that can be used for rapid diagnosis, prevention, and treatment of disease. Mass spectrometry (MS) is recognized for its capabilities for chemical analysis, including high selectivity and sensitivity [26–28]. It has been used to reveal the activities of enzymes, including the screening of enzyme inhibitors [29, 30], the study of metabolism of biomolecules [31, 32], exploration of the diversity of possible substrates, and searches for optimal reaction conditions. The solutions that are analyzed in these studies often contain high concentrations of salts derived from buffers, which must be used to maintain the activity of the enzymes. However, the salts adversely affect MS performance, likely making ion formation less reproducible, causing severe adduct formation, ion suppression, and clogging problems in electrospray ionization (ESI) [33]. Therefore, methods for molecular separation are routinely used before MS analysis such as solid phase extraction, liquid–liquid extraction, molecularly imprinted polymers, and liquid chromatography [33, 34]. Ambient ionization mass spectrometry refers to direct ionization of analytes prior to MS analysis of complex mixtures in their ambient state without sample extraction or chemical preparation [26, 35]. A series of ambient ionization methods [36–44] has been developed to replace or supplement MS methods that rely on separation of analytes prior to MS analysis. Among these methods, paper spray (PS) ionization [45] has been demonstrated to be capable of direct analysis of blood samples for therapeutic drugs and metabolites with highly quantitative performance [46–51]. In this work, a simple and general method for evaluation of enzyme activity using few reagents and without sample pretreatment or derivatization has been developed based on PS-MS. Evaluation of activity of AST is chosen here as a proof-of-principle of this MS analysis approach (Eq. 1 shows structures of reagents and products).

ð1Þ

Experimental Chemicals and Reagents All reagents and solvents were of analytical grade or higher and were used directly without further purification. L-glutamate, αketoglutaric acid, L-aspartate, L-2-aminoadipic acid, pyridoxal 5'-phosphate hydrate, Trizma base, β-nicotinamide adenine dinucleotide, reduced disodium salt hydrate, and HPLC grade methanol (MeOH) were purchased from Sigma-Aldrich (St. Louis, MO, USA). L-lactic dehydrogenase, L-aspartate aminotransferase, and L-malic dehydrogenase were purchased from NZYTech – Genes and Enzymes and Sigma-Aldrich (St. Louis, MO, USA). Fresh human serum was purchased from Innovative Research. Water was purified and deionized using a Milli-Q system (Millipore, Bedford, MA, USA). The filter paper used for paper spray ionization was from Whatman (Whatman International Ltd., Maidstone, England).

Enzyme Activity Evaluation by Paper Spray Mass Spectrometry The enzyme substrates and co-enzyme were mixed in the TrisHCl buffer solution and pre-incubated to the optimized temperature. The reaction was initiated with the addition of serum with AST or purified AST in buffer. Aliquots of sample were taken and mixed with internal standard in PS elution solvent to follow the progress of the reaction. This was done by depositing the mixture on a small paper triangle (~1 cm2), applying a high voltage (3.5 kV) to the paper, and quantifying the product using multiple reaction monitoring (MRM) in a triple quadrupole mass spectrometer. The procedure used for evaluation of enzyme activity by PS-MS is illustrated in Figure 2.

Biological Assay It is known that the observed activity of a given enzyme sample varies with changes in reaction conditions, which means measurements of enzyme activity are highly method-dependent. With this in mind, the International Federation of Clinical Chemistry (IFCC) 2002 [52] specified conditions for target enzyme reactions using spectroscopic analysis. The suggested conditions are used here except for simplification of the MS method by removal of the lengthy product derivatization steps. L-glutamate, pyridoxal 5'-phosphate hydrate (PALPO), and AST were mixed in Tris-HCl (pH 7.65, 37 °C). The reaction was initiated by adding serum with AST and incubated at 37 °C. All measured values for glutamate in the aspartate

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Figure 2. Protocol for enzyme activity evaluation using PS-MS (a) initiate enzymatic reaction at 37 oC, (b) mix with internal standard and solvent (MeOH:H2O:formic acid, 9:1:0.2%); (c) analyze product using PS-MS

aminotransferase reaction were corrected for initial values in serum and in the reagents following an IFCC method [53] of blank correction for AST measurements.

Mass Spectrometry, Data Collection, and Data Processing Quantitative reaction monitoring was performed using a TSQ Quantum Access MAX (Thermo Scientific, San Jose, CA, USA) in the multiple reaction monitoring (MRM) mode [54, 55]. Each monitoring experiment interrogated precursor/ product ion pairs using m/z 0.010 windows for a period of 60 ms each, repeated 20 times for a total of 1.5 s to measure each transition. Inlet capillary temperature and voltage were 300 °C and 35 V, respectively. The most abundant fragment was chosen for MRM quantification. Data were processed using the Xcalibur Quant Browser. Peaks were integrated, and quantification was performed using the ratio of the areas under the curves for the analyte and internal standard. Trend lines were constructed using linear least-squares fits.

Calibration Curve Quantitation of the AST catalyzed product L-glutamate was achieved using a triple quadrupole mass spectrometer. L-2aminoadipic acid was used as internal standard for quantitation of L-glutamate. MRM transitions (m/z 148→84 for Lglutamate and m/z 162→98 for L-2-aminoadipic acid) were monitored using high frequency (10 Hz) switching between the two channels (See Supporting Information for the fragmentation patterns of both compounds). The intensity ratios of the two transitions were averaged for a 90 s period

Results and Discussion Paper Spray Incorporating Desalting Process with Ionization PS was used to integrate sample separation and ionization [45, 48]. This allowed the time for sample preparation and kinetic measurements to be greatly reduced. The enzymatic reaction mixture was applied onto the paper substrate, which serves as a medium to absorb the chemicals. After adding solvent methanol/water to initiate paper spray, a continuous and steady spray was obtained for quantitation of L-glutamate (Figure 3a). The optimized solvent (methanol and water, v:v, 9:1) also contributes to reducing the salt effect from buffer solutions in PS-MS. Experiments performed nanoESI (nESI) of the same reaction mixture without sample preparation were compared; clogging of the tip happened immediately following initiation of the spray (Figure 3b).

Figure 3. Comparison of ionization performance analyzing enzymatic reaction by (a) nESI and (b) paper spray in total ion chronogram (TIC), tandem mass spectra of L-glutamate (MS2 Glu), and tandem mass spectra of internal standard L-2aminoadipic acid (MS2 IS)

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glutamate in either methanol/water or in serum are linear across a wide range of concentrations with a limit of detection (LOD) of 20 ppb for purified AST sample and 60 ppb for the serum AST sample.

Catalytic Activity of Purified AST and Serum AST in Tris-HCl Buffer

Figure 4. Calibration curve of L-glutamate prepared (a) in TrisHCl buffer; (b) in serum with Tris-HCl buffer (v:v 1:9) for comparison with AST activity measurement

during which sample was being sprayed. Matrix effect was evaluated by adding known amounts of L-glutamate to fresh human serum. As seen in Figure 4, the relative responses of L-

AST is now considered to be one of the classic serum chemical tests for liver injury or dysfunction. The normal range of AST for most people is 1 ~ 40 U l–1 [52]. The grading of severity was published since 1982 by the NCI as the Common Toxicity Criteria for Adverse Events, and is described based on the factor by which AST exceeds the upper limit of the normal range (ULN) [23]. Therefore, experiments were performed using AST with 800, 200, 100, 40, and 20 U l–1, which represents limit of life-threatening, severe, moderate, mild, and healthy state separately. Known amounts of AST were added to buffer for the experiment without serum inference. For the matrix study, measured amounts of AST were added to fresh serum to generate final activities of 20, 40, 100, 200, and 800 U l–1. From the variety of principles for the determination of the catalytic activity of enzymes, kinetics methods are recommended [56]. The catalytic activity of AST is obtained by continuously monitoring product formation from the enzymatic reactions. After each aliquot was taken from the reaction mixture, formic acid and solvent were added to the mixture to prevent further reaction. Data were plotted against the known enzyme activity of the standard to build curves showing the reaction progress (Figure 5). The reaction catalyzed by AST is a twosubstrate, Ping-Pong Bi-Bi mechanism [57]. The enzyme requires pyridoxal phosphate as coenzyme. To ensure that the maximum potential catalytic activity of the sample is

Figure 5. Kinetics data for 10 catalytic reactions using (a) purified AST with 20, 40, 100, 200, and 800 U l–1 in Tris-HCl buffer; (b) serum enzyme with 20, 40, 100, 200, and 800 U l–1 in Tris-HCl buffer. The ratio of fragment intensity of L-glutamate and L-2aminoadipic acid (MRM transitions: m/z 148→84 for L-glutamate and m/z 162→98 for L-2-aminoadipic acid) was monitored about 30 min for each reaction

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the curve). The amounts of products formed after 500 s were monitored and calculated based on L-glutamate calibration curve both in serum and in the buffer. Enzyme activity was plotted as a function of reaction rate constant (Figure 6), and it shows excellent linearity. Precision was quantified with the coefficient of variation (CV), which is defined as the standard deviation divided by the mean. CV values were <10% for AST tests in two conditions. Our method directly measures AST activity in its catalytic reaction. The sample does not need to be diluted even with an AST activity of 800 U l–1. These results demonstrate the high quality of the enzymatic activity measurements made possible by this new ambient ionization MS method.

Conclusions and Outlook Figure 6. Activity evaluation curve for (a) purified AST in TrisHCl buffer; (b) serum enzyme in Tris-HCl buffer. The rate of formation of L-glutamate in each reaction was plotted against enzyme activity at 20, 40, 100, 200, and 800 U l–1. ΔIR/s represents the ratio change of fragment intensity of L-glutamate and L-2-aminoadipic acid over time

measured, the complete saturation of the AST with pyridoxal phosphate is performed prior to measurement. Linear portion of the curve appears after allowing for mixing effect or lagphases, which cannot be eliminated by changes in the reaction conditions. Kinetics of AST catalyzed reactions can be clearly differentiated at various activities in buffer with/without serum matrix. Calculations of the catalytic activity are based upon that portion of the progress curve, which is zero-order with respect to concentration of substrates (i.e., the initial, linear portion of

An ambient mass spectrometry method based on paper spray mass spectrometry was used to develop an analytical protocol for evaluation of enzyme activity by quantifying the product of an enzymatic reaction using multiple reaction monitoring MS/ MS scans. Evaluation of the activity of the enzyme AST was chosen as an example. Paper spray has a desalting effect that avoids sprayer clogging problems, which are seen in nESI. This very simple method requires no sample pretreatment and does not use additional derivatization reactions, either prior to or accompanying PS-MS. One can imagine that the approach used here might provide the basis for a general and convenient way to evaluate enzymatic activity. It has the potential to be deployed with a miniature MS [58, 59] for use for on-site/point-of-care diagnosis directly by nurses and physicians or for personal health care by patients themselves (Figure 7a). Kinetic measurements of enzyme activity showed that the reaction proceeds with an

Figure 7. (a) Key elements of a proposed analysis system for diagnosis of liver damage by in situ measurement of aminotransferase activity, showing schematically a miniature mass spectrometer, with serum handling and paper spray ionization system. (b) Activity evaluation of AST showing limiting values of enzyme activity derived from PS-MS measurements and readout for clinical practice

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initial burst of L-glutamate release, followed by the usual zeroorder release when it reaches the steady-state limit. One method of handling the data obtained by the PS-MS method is to use enzyme zero-order kinetics and convert the measured MS intensity ratios into limiting values of enzyme activity corresponding to various stages of disease to allow direct readout and evaluation of AST in clinical practice (Figure 7b).

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Acknowledgements

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This work was supported by NSF grant CHE 13-07264. The authors acknowledge the contributions of Kassandra Moore.

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