Urease Reaction Systems, Urea Conversion, and Ammonia Nitrogen Quantitative Analysis
Urease Reaction Systems, Urea Conversion, and Ammonia Nitrogen Quantitative Analysis
Urease catalyzes the hydrolysis of urea to produce ammonia and carbon dioxide. It is a key enzymatic node in soil nitrogen cycling, microbial metabolism, clinical testing, dairy quality analysis, fermentation process control, and environmental ammonia nitrogen monitoring. Detection systems established around urease activity, urea consumption, and ammonia nitrogen generation can evaluate urea metabolism from three analytical levels: enzymatic capacity, substrate conversion, and product accumulation.
Keywords: urease activity; urea metabolism; ammonia nitrogen; urea nitrogen; indophenol blue method; Berthelot reaction; diacetyl monoxime method; soil urease
1 Urease-Catalyzed Reactions and Fundamentals of Urea Metabolism
1.1 Catalytic Mechanism of Urease
(1) Substrate conversion
Urease is a nickel-containing metalloenzyme that catalyzes urea hydrolysis. During the reaction, urea is activated in the enzyme active site, followed by nucleophilic attack by water, leading to the decomposition of urea into ammonia and carbamate. Carbamate is further decomposed into ammonia and carbon dioxide. This reaction increases the ammonia nitrogen level in the system and is often accompanied by an increase in pH.
(2) Forms of ammonia nitrogen
Ammonia generated by the urease reaction exists in dynamic equilibrium with ammonium ions in aqueous solution. In analytical testing, “ammonia nitrogen” usually includes both free ammonia and ammonium nitrogen. Their relative proportions are affected by pH, temperature, ionic strength, and buffer composition. Under alkaline conditions, the proportion of free ammonia increases, whereas under acidic or neutral conditions, ammonium ions predominate.
(3) Relationship between enzyme activity and products
Urease activity reflects the ability to hydrolyze urea per unit time. Urea content reflects the remaining substrate level, while ammonia nitrogen content reflects product accumulation. These three parameters are related but cannot be directly equated. Ammonia volatilization, microbial assimilation, ammonium adsorption, nitrification, and background ammonia in the matrix may all alter endpoint results.
1.2 Research Scenarios for Urea Metabolism
(1) Soil and agricultural systems
Urea is a commonly used nitrogen fertilizer. After entering soil, it can be rapidly hydrolyzed by urease, releasing ammonium nitrogen, which further participates in ammonia volatilization, nitrification, denitrification, and plant uptake. Soil urease activity can be used to evaluate soil nitrogen transformation capacity, fertilizer release rate, and the effectiveness of urease inhibitors.
(2) Microbial and fermentation systems
Many bacteria, fungi, and yeasts can utilize urea. Microbial urease activity can alter culture system pH, nitrogen source utilization, cell growth status, and metabolite composition. Urease-positive microorganisms such as Helicobacter pylori can hydrolyze urea to produce ammonia, thereby buffering acidic environments and promoting colonization.
(3) Dairy and biological samples
Milk urea nitrogen can reflect protein intake, rumen nitrogen metabolism, and feeding management in dairy cows. Urea or urea nitrogen levels in urine, serum, and tissue samples are associated with the hepatic urea cycle, renal excretion, and overall nitrogen metabolism.
(4) Environmental water and wastewater systems
Ammonia nitrogen in water bodies is an important indicator for evaluating water quality, aquaculture systems, wastewater treatment, and nitrogen pollution. Urea hydrolysis, microbial ammonification, organic nitrogen degradation, and exogenous nitrogen-containing pollution can all increase ammonia nitrogen levels. Therefore, results should be interpreted in relation to sample source.
2 Methods for Urease Activity Detection
2.1 Ammonia Generation Method
(1) Detection principle
The ammonia generation method uses urea as the substrate. Under defined temperature, pH, and reaction time, urease in the sample catalyzes urea hydrolysis, and the generated ammonia nitrogen is then measured. The amount of ammonia nitrogen produced per unit time, per unit sample mass, per unit volume, or per unit protein amount can be used to calculate urease activity.
(2) Applicable samples
This method is suitable for soil, microbial cultures, plant tissues, cell lysates, crude enzyme extracts, fermentation broth, and enzyme preparations. For complex samples, substrate-free blanks, inactivated sample blanks, and sample background blanks should be included to subtract non-enzymatic sources of ammonia nitrogen.
(3) Result expression
Soil samples are commonly normalized to dry soil mass, and results may be expressed as the amount of ammonia nitrogen produced per gram of dry soil per unit time. Enzyme solutions and biological samples can be normalized to protein amount or sample volume. When comparing different systems, temperature, pH, substrate concentration, and reaction time should be standardized.
2.2 pH Indicator Method
(1) Detection principle
After urease catalyzes urea to generate ammonia, the system becomes alkaline, and a pH indicator changes color as pH increases. This method is commonly used for screening urease-positive bacteria, identification using urea media, and rapid qualitative assessment.
(2) Application scenarios
The pH indicator method is suitable for bacterial identification, preliminary screening of urease-positive microorganisms, and color observation in culture media. For example, urea agar or Christensen’s urea agar can be used to determine whether strains possess urea-hydrolyzing capacity.
(3) Method limitations
This method is strongly influenced by the buffering capacity of the culture medium, inoculum amount, incubation time, and other alkaline metabolites. It is not suitable for precise quantification of urease activity. If differences in urease activity among samples need to be compared, ammonia nitrogen colorimetric methods or enzyme-coupled methods should be used.
2.3 Inhibitor Evaluation Method
(1) Design principle
Urease inhibitor evaluation usually uses urea as the substrate and compares ammonia nitrogen generation, residual urea level, or urease activity between inhibitor-treated groups and control groups. Inhibition rate can be calculated based on reduced product formation or decreased enzyme activity.
(2) Positive controls
Flurofamide, NBPT, acetohydroxamic acid, and related compounds can be used as reference inhibitors in urease inhibition experiments. Different inhibitors vary in mechanism of action, effective concentration, and sample compatibility, and should be optimized separately for soil, microbial, or purified enzyme systems.
(3) Result interpretation
A reduction in ammonia nitrogen is not necessarily caused solely by urease inhibition. It may also result from reduced ammonia volatilization, enhanced ammonium adsorption, or altered microbial assimilation. A more complete inhibitor evaluation should simultaneously measure urease activity, residual urea, and ammonia nitrogen generation.
3 Methods for Urea Content and Urea Nitrogen Detection
3.1 Urease-Berthelot Method
(1) Detection principle
Urea is first hydrolyzed by urease to generate ammonia. The ammonia then forms a colored product through the Berthelot reaction or a modified Berthelot system. Absorbance is related to the amount of ammonia nitrogen generated from urea hydrolysis and can be further converted into urea or urea nitrogen content.
(2) Application range
This method is suitable for detecting urea or urea nitrogen in urine, serum, culture medium, fermentation broth, dairy products, and other liquid samples. The reaction system is mature and convenient for microassay or colorimetric platforms.
(3) Precautions
Pre-existing ammonia nitrogen in samples can cause background interference; therefore, sample blanks should be included. If samples contain high protein, pigments, or reducing substances, interference should be reduced by centrifugation, deproteinization, dilution, or blank subtraction.
3.2 Diacetyl Monoxime Method
(1) Detection principle
Under acidic conditions, urea reacts with diacetyl monoxime to generate a colored product, which is quantified by absorbance measurement. This method does not rely on the urease reaction and is suitable for direct detection of urea content.
(2) Application scenarios
The diacetyl monoxime method can be used to detect urea in fermentation broth, culture medium, environmental samples, biological samples, and complex matrices. Compared with urease-based methods, it can serve as an independent verification system for urea measurement.
(3) Experimental control
The acidic color development conditions are relatively strong, and heating time, color development temperature, and acid concentration must be strictly controlled. Pigments, turbidity, and protein precipitation in samples may affect absorbance. Sample blanks and necessary pretreatment should therefore be included.
3.3 Urea Nitrogen Detection
(1) Detection target
Urea nitrogen is a urea-related indicator expressed in terms of nitrogen content. It is commonly used in clinical samples, urine, dairy products, and nutritional metabolism evaluation. Urea content and urea nitrogen content can be converted based on molecular weight relationships, but reporting units and calculation basis should be clearly specified.
(2) Dairy testing
Milk urea nitrogen can be used to evaluate dairy cow protein nutrition, rumen nitrogen utilization, and feeding management. Fat, protein, and turbidity in dairy samples may affect detection, so assay kits or pretreatment methods compatible with dairy matrices should be selected.
(3) Urine testing
Urea content in urine is relatively high and usually requires dilution into the linear detection range. The urine matrix is complex, and salts and color may affect colorimetric results. Appropriate blanks and quality control samples should be included.
4 Methods for Ammonia Nitrogen Generation Detection
4.1 Indophenol Blue Method
(1) Detection principle
Under alkaline conditions, ammonia reacts with phenolic compounds or salicylate and hypochlorite in the presence of a catalyst to form a blue or blue-green product. Absorbance is proportional to ammonia nitrogen concentration and can be used for quantitative ammonia nitrogen analysis.
(2) Applicable samples
The indophenol blue method is suitable for detecting ammonia nitrogen in water samples, soil extracts, fermentation broth, microbial culture medium, enzyme reaction solutions, and plant extracts. This method has relatively high sensitivity and is suitable for samples with low to medium ammonia nitrogen concentrations.
(3) Interference control
Color development is affected by pH, temperature, reaction time, and oxidant concentration. Dark-colored samples, high-protein samples, and samples containing strong reducing substances may generate background interference and require sample blanks or pretreatment.
4.2 Salicylate-Hypochlorite Method
(1) Method characteristics
The salicylate-hypochlorite method is a modified indophenol blue system that uses salicylate instead of the traditional phenol system, reducing reagent toxicity and improving operational safety. Ammonia nitrogen forms a blue-green product under alkaline oxidative conditions.
(2) Application range
This method is suitable for detecting ammonia nitrogen in environmental water samples, soil extracts, culture media, and fermentation samples. For batch testing and routine laboratory applications, the salicylate-based system offers good practicality.
(3) Quality control
Hypochlorite stability affects color development. Long-term storage may reduce oxidative capacity. A standard curve should be established for each batch of testing, and reference materials should be used for quality control.
4.3 Nessler’s Reagent Method
(1) Detection principle
Nessler’s reagent reacts with ammonia or ammonium ions to form a yellow to brown complex, enabling ammonia nitrogen concentration to be determined by colorimetry. This method is relatively simple and is commonly used for ammonia nitrogen determination in water samples and simple matrices.
(2) Application scenarios
Nessler’s reagent method is suitable for ammonia nitrogen detection in environmental water, wastewater, soil extracts, and simple reaction systems. In urease reaction systems, it can be used as an endpoint detection method for ammonia nitrogen generation.
(3) Limitations
Nessler’s reagent contains mercury, and experimental waste must be treated as hazardous mercury-containing waste. Calcium and magnesium ions, turbidity, color, and certain organic substances in samples may affect results. For green laboratory workflows or high-throughput detection, the salicylate-hypochlorite method or kit-based systems are preferred.
4.4 Electrode Method and Ion Chromatography
(1) Ammonia-selective electrode
An ammonia-selective electrode can detect free ammonia in solution. Adjusting pH to convert ammonium ions into free ammonia can improve electrode response. This method is suitable for water samples, fermentation broth, and online monitoring scenarios.
(2) Ion chromatography
Ion chromatography can analyze ammonium ions and other inorganic nitrogen species. It is suitable for environmental analysis requiring simultaneous detection of ammonia nitrogen, nitrate nitrogen, nitrite nitrogen, and other components.
(3) Method compatibility
Electrode and chromatographic methods require appropriate instrumentation, sample purification, and standard curves. For routine batch testing, colorimetric and kit-based methods are more convenient. For high-accuracy verification or complex nitrogen speciation analysis, instrumental methods have advantages.
5 Sample Pretreatment and Matrix Interference Control
5.1 Soil Samples
(1) Moisture content and dry soil conversion
Soil urease activity is commonly normalized to dry soil mass; therefore, soil moisture content must be measured or converted. If moisture levels vary significantly among samples, direct calculation based on wet soil mass will affect result comparability.
(2) Background ammonium nitrogen subtraction
Soil itself may contain ammonium nitrogen. A substrate-free blank should be included to subtract the original ammonia nitrogen background in soil. If urease inhibitors are being studied, an inhibitor-free control and inhibitor blank should also be included.
(3) Incubation condition control
Soil urease reactions are strongly affected by temperature, pH, substrate concentration, and incubation time. When comparing different soils or treatment groups, reaction conditions should be standardized, and the reaction should remain within the linear phase.
5.2 Microbial and Fermentation Samples
(1) Distinguishing cells from supernatant
Microbial urease may be intracellular, extracellular, or cell-surface-associated. It should be clearly defined whether the tested sample is a whole-cell suspension, culture supernatant, cell lysate, or crude enzyme solution. Different sample types correspond to different metabolic interpretations.
(2) Culture medium background
Peptone, yeast extract, ammonium salts, urea, or other nitrogen-containing substances in culture media may contribute background ammonia nitrogen. Uninoculated medium blanks and substrate-free controls should be included.
(3) pH changes
Urea hydrolysis alkalizes the system, while pH changes can in turn affect urease activity, ammonia/ammonium equilibrium, and microbial metabolic status. In kinetic experiments, an appropriate buffer system should be used, and pH before and after the reaction should be recorded.
5.3 Biological and Dairy Samples
(1) Protein and turbidity
Serum, tissue homogenates, cell lysates, and milk samples contain relatively high protein levels, which may cause turbidity, precipitation, or elevated colorimetric background. Centrifugation, deproteinization, dilution, or dedicated assay kit processing can be performed before detection.
(2) Endogenous ammonia
Biological samples often contain endogenous ammonia or ammonium ions. If urea or urease activity is inferred from ammonia generation, the initial ammonia nitrogen background should be measured and subtracted from endpoint results.
(3) Sample storage
Ammonia is volatile, and urea may also degrade under microbial contamination or enzyme contamination. Samples should be stored at low temperature, repeated freeze-thaw cycles should be minimized, and testing should be performed as soon as possible after sampling.
6 Experimental Design and Result Calculation
6.1 Standard Curves and Quality Control
(1) Selection of standards
Ammonia nitrogen standard solutions or ammonia nitrogen reference materials in water can be used to establish ammonia nitrogen standard curves. Urea or urea nitrogen detection should use urea standard solutions or urea nitrogen reference materials. Urease activity detection requires establishing calculation relationships based on product ammonia nitrogen or substrate urea changes.
(2) Linear range
The standard curve must cover the concentration range of the tested samples. If sample absorbance exceeds the linear range, the sample should be diluted and retested. Direct extrapolation should not be used.
(3) Quality control samples
Environmental water samples, dairy samples, and urine samples should include quality control samples or reference materials to evaluate method accuracy and inter-batch stability. For complex matrices, spike recovery experiments are recommended.
6.2 Calculation of Urease Activity
(1) Amount generated per unit time
Urease activity can be calculated based on the amount of ammonia nitrogen generated per unit time. If results are expressed as NH₃-N, NH₄⁺, or nitrogen element, units should remain consistent, and the conversion method should be clearly specified.
(2) Sample normalization
Soil samples can be normalized to dry soil mass; tissue and cell samples can be normalized to protein amount; enzyme solutions and culture media can be normalized to volume. Results based on different normalization methods should not be directly compared.
(3) Verification of reaction linearity
Reaction time should remain within the linear phase. Incubation that is too short may reduce sensitivity, while incubation that is too long may cause nonlinearity due to substrate depletion, pH changes, or product inhibition.
6.3 Relationship Between Urea Consumption and Ammonia Nitrogen Generation
(1) Theoretical relationship
Complete hydrolysis of urea generates two ammonia molecules. Theoretically, urea decrease and ammonia nitrogen generation have a stoichiometric relationship. However, in real samples, ammonia volatilization, ammonium adsorption, microbial assimilation, and further nitrification can affect this relationship.
(2) Sources of deviation
If urea decreases markedly but ammonia nitrogen does not increase accordingly, ammonia volatilization, ammonium adsorption, microbial assimilation, or nitrification may be involved. If ammonia nitrogen increases while urea changes little, background ammonia release or decomposition of other nitrogen-containing organic compounds may be present.
(3) Value of combined detection
Simultaneous detection of urease activity, residual urea, and ammonia nitrogen generation can distinguish different processes, such as enhanced enzyme activity, increased substrate utilization, increased product accumulation, and altered nitrogen transformation pathways.
7 Selection of Key Reagents and Detection Systems
7.1 Products for Urease, Urea Nitrogen, and Ammonia Nitrogen Detection
Application Module | Catalog No. | Product Name | Specification/Grade | Methodological Positioning | Applicable Samples and Scenarios |
Urease Standard Enzyme and Method Validation | Urease (URH) | EnzymoPure™, ActiBioPure™, Recombinant, Bioactive, High Performance, >150 U/mg protein; expressed in E.coli | Recombinant urease standard enzyme, positive enzyme source, and reference for reaction system validation | Suitable for establishing urease activity assay systems, standard enzyme reaction validation, inhibitor screening, and recombinant enzyme activity evaluation | |
Urease Standard Enzyme and Method Validation | Urease (URH) from Jack Bean | Bioactive,ActiBioPure™,Native,High Performance,EnzymoPure™,≥200 U/mg enzyme powder; ≥2000 U/mg protein | High-activity natural urease standard enzyme, positive control, and reference for method sensitivity validation | Suitable for constructing high-activity urease reaction systems, inhibitor IC₅₀ determination, urea hydrolysis kinetics, and enzyme activity method validation | |
Urease Standard Enzyme and Method Validation | Urease from Canavalia ensiformis (Jack bean) | Bioactive,ActiBioPure™,Native,High Performance,EnzymoPure™,≥100 U/mg enzyme powder | Natural jack bean urease enzyme source for routine activity assays and inhibition experiment reference | Suitable for routine urease activity determination, urease inhibitor screening, substrate conversion experiments, and establishment of urea hydrolysis models | |
Urease Standard Enzyme and Method Validation | Urease from Canavalia ensiformis (Jack bean) | EnzymoPure™, ≥45 units/mg dry weight | Basic natural urease enzyme source for reaction system setup and routine experimental reference | Suitable for basic urease activity experiments, urea hydrolysis reaction validation, culture medium reaction system construction, and low- to medium-activity enzyme source controls | |
Urease activity detection | Urease (UE) Activity Assay Kit (Micro Method) | BioReagent | Quantitative detection of urease activity in microscale systems | Suitable for evaluating urease activity in low-volume samples, enzyme solutions, microbial samples, tissues, or extracts | |
Soil urease activity detection | Soil Urease (S-UE) Activity Assay Kit (Micro Method) | BioReagent | Microassay detection of soil urease activity | Suitable for evaluating soil nitrogen cycling, urea fertilizer conversion, and urease inhibitor effects | |
Urease inhibitor evaluation | Flurofamide | ≥95% | Positive control for urease inhibition or reference compound for inhibitor screening | Suitable for urease inhibition experiments, inhibitor screening, and regulation of urea hydrolysis | |
Urea content detection | Urea Content Assay Kit (Urease-Berthelot, Micro Method) | BioReagent | Urease-Berthelot microassay for urea detection | Suitable for urea quantification, urea consumption analysis, and metabolic studies in low-volume samples | |
Urea content detection | Urea Content Assay Kit (Urease-Berthelot, Colorimetric Method) | BioReagent | Urease-Berthelot colorimetric method for urea detection | Suitable for conventional spectrophotometer platforms and batch urea sample testing | |
Urinary urea detection | Urinary Urea Content Assay Kit (Urease-Berthelot, Colorimetric Method) | BioReagent | Colorimetric detection of urea in urine samples | Suitable for urinary nitrogen metabolism analysis, urea excretion evaluation, and physiological metabolism research | |
Urea content detection | Urea Content Assay Kit (Diacetyl Monoxime, Micro Method) | BioReagent | Diacetyl monoxime microassay for urea detection | Suitable for urea detection in low-volume samples, fermentation broth, culture media, and complex matrices | |
Urea content detection | Urea Content Assay Kit (Diacetyl Monoxime, Colorimetric Method) | BioReagent | Colorimetric quantification of urea by the diacetyl monoxime method | Suitable for routine urea content detection and methodological comparison with urease-based methods | |
Urea content detection | Urea Content Assay Kit (Diacetyl Monoxime, Micro Method) | BioReagent | Urea detection in small-volume systems | Suitable for low-sample-volume experiments, small-volume reaction systems, and trace sample analysis | |
Urea content detection | Urea Content Assay Kit (Diacetyl Monoxime, Colorimetric Method) | BioReagent | Conventional colorimetric detection of urea content | Suitable for urea quantification in fermentation broth, culture media, food samples, or environmental samples | |
Milk urea nitrogen detection | Milk Urea Nitrogen (MUN) Content Assay Kit (Urease, Micro Method) | BioReagent | Microassay detection of milk urea nitrogen | Suitable for dairy quality analysis, dairy cow nutritional metabolism, and MUN level evaluation | |
Milk urea nitrogen detection | Milk Urea Nitrogen (MUN) Content Assay Kit (Urease, Colorimetric Method) | BioReagent | Colorimetric detection of milk urea nitrogen | Suitable for routine dairy testing, batch MUN sample analysis, and feeding management evaluation | |
Urea nitrogen reference material | Urea Standard Solution (calculated as N) | 1000.0 mg/L in water | Standard calibration for urea nitrogen detection | Suitable for urea nitrogen standard curves, method calibration, and quality control | |
Ammonia nitrogen standard solution | Ammonia as N IC Standard | 100mg/L(as N ) in H2O | Establishment of ammonia nitrogen standard curves | Suitable for calibration of the indophenol blue method, Nessler’s reagent method, ion chromatography, and ammonia nitrogen detection methods | |
Ammonia nitrogen standard solution | Ammonia as N IC Standard | 500mg/L in H2O | Medium-concentration ammonia nitrogen standard calibration | Suitable for calibration of ammonia nitrogen detection in environmental water samples, soil extracts, and fermentation broth | |
Ammonia nitrogen standard solution | Ammonia as N IC Standard | 1000μg/ml in Water (20℃) | High-concentration ammonia nitrogen stock standard | Suitable for high-concentration standard stock solutions, dilution preparation, and method validation | |
Ammonia nitrogen reference material in water | Amino Nitrogen in Water | 100μg/ml ±2% (20℃) | Quality control for low-concentration ammonia nitrogen in water samples | Suitable for low-concentration water sample ammonia nitrogen detection, quality control, and recovery verification | |
Ammonia nitrogen reference material in water | Amino Nitrogen in Water | 500μg/ml ±1% (20℃) | Quality control for medium-concentration ammonia nitrogen in water samples | Suitable for medium-concentration ammonia nitrogen detection and environmental sample method confirmation | |
Ammonia nitrogen reference material in water | Amino Nitrogen in Water | 1000μg/ml ±1% (20℃) | High-concentration ammonia nitrogen stock standard in water | Suitable for high-concentration standard preparation, linear range verification, and inter-batch quality control | |
Water quality ammonia nitrogen standard sample | Ammonia nitrogen standard | 12.7mg/L | Quality control sample for water quality ammonia nitrogen testing | Suitable for environmental monitoring, water sample analysis, and laboratory quality control | |
Microbial urease screening | Christensen′s Urea Agar | Suitable for microbiology, CellNourish™ Basic | Culture-based screening of urease-positive microorganisms | Suitable for bacterial identification, urease-positive screening, and qualitative urea hydrolysis experiments | |
Microbial urease screening | Urease Agar Base |
| Basic culture medium system for urea hydrolysis | Suitable for preliminary screening of microbial urease activity, assessment of urea hydrolysis ability, and culture medium construction |
Urease activity, urea content, and ammonia nitrogen generation assays correspond to three analytical levels: enzymatic capacity, substrate consumption, and product accumulation. In practical applications, urease activity detection, urea content detection, urea nitrogen detection, or ammonia nitrogen standard curve systems should be selected according to sample type. Result reliability should be controlled through blank subtraction, reference material calibration, spike recovery, and reaction linearity verification.