Recombinant Mouse Protein Dihydroorotate dehydrogenase (quinone), mitochondrial (Dhodh)

What is the basic structure and function of mouse DHODH?

Mouse DHODH is a monomeric flavoenzyme that belongs to the dihydroorotate dehydrogenase family, Type 2 subfamily. It spans amino acids 11-395 and contains an uncleaved transit peptide required for mitochondrial targeting and proper membrane integration . The enzyme catalyzes the fourth step in pyrimidine de novo synthesis, converting dihydroorotate to orotate with quinone serving as the electron acceptor . This reaction is essential for UMP biosynthesis via the de novo pathway, making DHODH crucial for rapidly proliferating cells that require pyrimidine synthesis for DNA/RNA production.

How does mouse DHODH differ from human and rat DHODH?

Mouse DHODH shows structural homology to human and rat DHODH but exhibits distinct kinetic properties. Kinetic analysis reveals that the mouse enzyme has slightly elevated constants compared to rat and human orthologs:

These differences in substrate affinity suggest species-specific variations in enzyme-substrate interactions that may be relevant when using mouse models for studying DHODH inhibitors .

What is the subcellular localization of mouse DHODH?

Mouse DHODH is an integral protein of the inner mitochondrial membrane that faces the intermembrane space and functionally connects to the respiratory chain via ubiquinone . The N-terminal bipartite sequence consists of a mitochondrial targeting sequence and adjacent hydrophobic domain, both necessary for proper import, localization, and fixation of the enzyme in the inner mitochondrial membrane . This mitochondrial localization is consistent with its classification as a Class 2 DHODH, similar to other mammalian DHODHs.

What expression systems are suitable for recombinant mouse DHODH production?

Recombinant mouse DHODH can be successfully expressed in both cell-free systems and bacterial systems like Escherichia coli . For full-length protein (amino acids 11-395), cell-free expression systems have demonstrated effectiveness, yielding protein with ≥85% purity suitable for SDS-PAGE analysis . For N-terminal-truncated constructs lacking the mitochondrial targeting sequence and hydrophobic domain, E. coli expression systems have proven efficient, particularly when combined with C-terminal histidine tags to facilitate purification .

What purification strategies are most effective for recombinant mouse DHODH?

For N-terminal-truncated constructs expressed in E. coli, metal-chelate affinity chromatography under native conditions has proven highly effective. This approach allows purification without detergents to a specific activity exceeding 100 μmol × min⁻¹ × mg⁻¹ at pH optimum of 8.0-8.1 . The purification protocol typically involves:

• Expression of C-terminal-histidine-tagged constructs in E. coli

• Cell lysis under native conditions

• Metal-chelate affinity chromatography

• Elution with imidazole gradient

• Buffer exchange to remove imidazole

This strategy yields highly pure enzyme suitable for kinetic studies and inhibitor screening .

How can researchers verify the integrity and activity of purified recombinant mouse DHODH?

Several methods can be employed to verify the integrity and activity of purified recombinant mouse DHODH:

• SDS-PAGE analysis: To assess purity and molecular weight (approximately 41 kDa)

• Flavin analysis by UV-visible spectrometry: Native enzymes should show fairly stoichiometric ratios of 0.6-1.2 mol flavin per mol protein

• Activity assay: Using 2,6-dichloroindophenol (DPIP) reduction during oxidation of dihydroorotate. A typical assay mixture contains:

  • 2 mM L-Dihydroorotic acid

  • 0.2 mM Decylubiquinone (electron acceptor)

  • 0.12 mM DPIP

  • Assay buffer: 50 mM Tris, 150 mM KCl, 0.1% Triton X-100, pH 8.0

Activity is monitored by measuring the decrease in absorbance at 600 nm as DPIP is reduced during the reaction .

What are the optimal conditions for measuring mouse DHODH enzymatic activity?

Mouse DHODH activity is optimally measured at pH 8.0-8.1 in buffers containing Tris, KCl, and a mild detergent like Triton X-100 . The enzyme requires FMN as a cofactor and ubiquinone derivatives (such as decylubiquinone) as electron acceptors . A standard activity assay protocol includes:

• Prepare assay buffer: 50 mM Tris, 150 mM KCl, 0.1% Triton X-100, pH 8.0

• Dilute enzyme to 0.4 μg/mL in assay buffer

• Prepare substrate mixture containing 2 mM L-dihydroorotic acid, 0.2 mM decylubiquinone, and 0.12 mM DPIP in assay buffer

• Initiate reaction by mixing equal volumes of enzyme and substrate mixture

• Monitor decrease in absorbance at 600 nm for 5 minutes

• Calculate specific activity using appropriate extinction coefficients

Temperature optimization is typically performed at 25-37°C, with 30°C being commonly used for standardized assays .

How do the kinetic properties of mouse DHODH compare to other species?

Mouse DHODH demonstrates distinct kinetic properties compared to rat and human orthologs:

These differences are particularly relevant when studying species-specific responses to inhibitors and when using mouse models to investigate human diseases involving DHODH . The higher Km values for mouse DHODH indicate lower affinity for both dihydroorotate and ubiquinone compared to rat and human enzymes.

What role does the N-terminal domain play in mouse DHODH function?

The N-terminal domain of mouse DHODH consists of a bipartite sequence with two crucial elements:

• A mitochondrial targeting sequence that directs the protein to mitochondria

• An adjacent hydrophobic domain necessary for proper integration into the inner mitochondrial membrane

Studies with N-terminal truncated variants have shown that while these constructs retain catalytic activity in vitro, the N-terminus is essential for proper subcellular localization and membrane integration in vivo . Additionally, the N-terminal domain may influence the efficacy of certain inhibitors. For instance, while the presence of the N-terminus is irrelevant for the efficacy of malononitrilamides (A77-1726, MNA715, MNA279), it significantly affects the efficacy of the dianisidine derivative redoxal .

How can inhibition of mouse DHODH be effectively monitored in research settings?

Inhibition of mouse DHODH can be monitored through several approaches:

• In vitro enzyme assays: Using purified recombinant enzyme and measuring activity inhibition with spectrophotometric methods based on DPIP reduction

• Cellular assays: Treating mouse cell lines (e.g., EL4) with inhibitors and measuring:

  • Accumulation of upstream metabolites (dihydroorotate and carbamoyl-aspartate)

  • Reduction in downstream pyrimidine synthesis

  • Effects on cell proliferation (as DHODH inhibition impacts rapidly dividing cells)

• In vivo biomarker measurement: Monitoring dihydroorotate (DHO) levels in blood and urine of treated mice. DHO accumulation serves as a robust biomarker of DHODH inhibition, with urine measurements providing the most sensitive detection .

What is the significance of dihydroorotate as a biomarker for DHODH inhibition studies?

Dihydroorotate (DHO) serves as an excellent biomarker for monitoring DHODH inhibition both in vitro and in vivo. When DHODH is inhibited, DHO accumulates due to blocked conversion to orotate. Key findings regarding DHO as a biomarker include:

• Treatment of mammalian cells with DHODH inhibitors leads to dose-dependent increases in DHO levels, with the extent of accumulation correlating with inhibitor potency

• In mouse studies, treatment with leflunomide (a prodrug of the DHODH inhibitor teriflunomide) causes substantial increases in DHO levels:

  • Up to 16-fold increase in blood

  • Up to 5,400-fold increase in urine

• The magnitude of DHO accumulation correlates with the degree of DHODH inhibition, making it possible to quantitatively assess target engagement

• Urine measurements provide more dramatic and easily detectable changes compared to blood measurements, making urine DHO analysis particularly valuable for in vivo studies

This biomarker approach represents an important advance for monitoring on-target effects in both preclinical and clinical applications of DHODH inhibitors .

How do different inhibitors compare in their efficacy against mouse DHODH?

Different inhibitors demonstrate varying efficacy against mouse DHODH:

Pharmacokinetic studies have shown that teriflunomide reaches unbound plasma levels 20-85-fold above the mouse DHODH IC50, while DSM265 only achieves levels 1.6-4.2-fold above its IC50 for mouse DHODH, barely reaching IC90 concentrations . This explains why leflunomide/teriflunomide produces robust DHO accumulation in mice while DSM265 does not, despite both compounds inhibiting mouse DHODH in vitro .

What cell lines are recommended for studying mouse DHODH inhibition?

For studying mouse DHODH inhibition, the EL4 mouse lymphoma cell line has been validated and is recommended . Key characteristics of this cell line for DHODH inhibition studies include:

• Population doubling time of approximately 15 hours, making it suitable for proliferation-based assays

• Demonstrates robust accumulation of DHO and carbamoyl-aspartate upon DHODH inhibition

• Well-characterized response to various DHODH inhibitors

• Genetically verified identity through sequencing of the DHODH gene

When using this cell line, researchers typically treat cells with inhibitors for 48 hours prior to analysis, using 0.75% DMSO as a matched control .

What are the recommended protocols for cloning and expressing mouse DHODH for structure-function studies?

For cloning and expressing mouse DHODH for structure-function studies, the following protocol is recommended:

• Cloning:

  • Obtain complete cDNA of mouse DHODH

  • Design primers for amplification (e.g., forward primer 5'-TGGCCGACTACCTGGTG-3' and reverse primer 5'-AGGTGAGGGCCGTGTA-3')

  • Verify sequence against mouse reference genome

• Expression constructs:

  • For full-length protein: Include amino acids 11-395

  • For N-terminal truncated variants: Remove the mitochondrial targeting sequence and hydrophobic domain

  • Add C-terminal histidine tag for purification

• Expression systems:

  • Cell-free systems for full-length protein

  • E. coli for N-terminal truncated variants

• Purification:

  • Use metal-chelate affinity chromatography under native conditions

  • Optimize purification to achieve ≥85% purity

This approach allows for detailed structure-function studies, including analysis of the role of specific domains and residues in enzyme activity and inhibitor sensitivity .

How can researchers study species differences in DHODH to optimize drug development?

To study species differences in DHODH for drug development optimization, researchers can implement the following approaches:

• Comparative enzyme kinetics: Express and purify recombinant DHODH from multiple species (mouse, rat, human) and compare kinetic parameters (Km, Vmax) for substrates and inhibitors

• Cellular models: Test DHODH inhibitors across cell lines from different species:

  • Human: Jurkat (doubling time ~24 hours)

  • Mouse: EL4 (doubling time ~15 hours)

  • Other species: SIRC rabbit cell line (doubling time ~30 hours)

• Structural analysis: Compare crystal structures or homology models of DHODH from different species to identify structural differences that can be exploited for selective targeting

• Biomarker studies: Compare DHO accumulation patterns across species in response to the same inhibitors:

  • In vitro cell culture models

  • In vivo animal models

  • Clinical samples (when available)

• Pharmacokinetic/pharmacodynamic correlation: Compare unbound plasma levels of inhibitors to species-specific IC50 values to understand differences in in vivo efficacy

This multi-faceted approach helps identify species-specific differences that may impact drug development, enabling better translation from preclinical models to human applications.

How can recombinant mouse DHODH be used to study the role of pyrimidine metabolism in immune disorders?

Recombinant mouse DHODH provides a valuable tool for studying pyrimidine metabolism in immune disorders through several approaches:

• Inhibitor screening: Testing potential therapeutic compounds against recombinant mouse DHODH helps identify candidates for treating autoimmune disorders before advancing to in vivo studies

• Mechanistic studies: Comparing the effects of DHODH inhibitors on mouse immune cells with human cells helps elucidate species-specific responses, which is crucial since many DHODH inhibitors (e.g., leflunomide/teriflunomide) are used clinically for autoimmune disorders

• Transgenic models: Knowledge of mouse DHODH structure and function enables the creation of genetically modified mice with altered DHODH activity to study the impact on immune function

• Biomarker validation: Using recombinant mouse DHODH inhibition studies to validate DHO as a biomarker helps develop tools for monitoring treatment efficacy in both preclinical models and potentially in clinical settings

• Structure-activity relationship studies: Comparing inhibitor binding to recombinant mouse and human DHODH helps design species-selective compounds for research purposes or broad-spectrum inhibitors for therapeutic applications

These applications are particularly relevant given that DHODH inhibitors like leflunomide are used clinically for treating autoimmune disorders such as rheumatoid arthritis .

What are the challenges in translating findings from mouse DHODH studies to human applications?

Several challenges exist when translating findings from mouse DHODH studies to human applications:

• Species-specific kinetic differences: Mouse DHODH has different kinetic parameters (higher Km values for both dihydroorotate and ubiquinone) compared to human DHODH, potentially affecting inhibitor efficacy

• Differential inhibitor sensitivity: Some inhibitors show markedly different potencies against mouse versus human DHODH. For example, antimalarial compounds like DSM265 have different efficacy profiles across species

• Biomarker interpretation: While DHO accumulation serves as a biomarker in both species, the magnitude of changes and compartmental distribution (blood vs. urine) may vary, requiring careful interpretation of translational data

• Pharmacokinetic differences: Inhibitors may have different absorption, distribution, metabolism, and excretion profiles in mice versus humans, affecting target engagement even when in vitro enzyme inhibition is comparable

• Physiological context: The role of DHODH in rapidly proliferating cells may have species-specific nuances, particularly in immune cell proliferation, which is often the target of therapeutic intervention

Researchers can address these challenges by conducting parallel studies with both mouse and human DHODH, carefully correlating in vitro findings with in vivo observations, and using biomarkers like DHO to confirm target engagement across species .

How does the flavin cofactor influence mouse DHODH activity and inhibitor binding?

The flavin cofactor (FMN) plays a crucial role in mouse DHODH activity and inhibitor binding:

• Stoichiometry: Mouse DHODH contains approximately 0.6-1.2 mol of flavin per mol of protein, which is essential for its catalytic function

• Electron transfer mechanism: During catalysis, FMN accepts electrons from dihydroorotate and transfers them to ubiquinone, serving as an essential electron carrier in the conversion of dihydroorotate to orotate

• Structural stability: The flavin cofactor contributes to the structural integrity of the enzyme, with proper flavin incorporation being essential for obtaining functionally active recombinant protein

• Inhibitor binding site: Many DHODH inhibitors interact with regions adjacent to the flavin binding site, and the orientation of the flavin cofactor can influence inhibitor binding affinity and specificity

• Species differences: While the flavin binding pocket is generally conserved across mammalian DHODHs, subtle species-specific differences in the surrounding residues can affect inhibitor interactions and explain differential sensitivity to certain compounds

Understanding these aspects of flavin-DHODH interaction is crucial for designing effective inhibitors and interpreting structure-activity relationships across different mammalian species.