L-Citrulline: Properties, Uses & FAQs

L-Citrulline is a non-proteinogenic alpha-amino acid that plays a central role in the urea cycle and serves as a metabolic precursor to L-arginine and, subsequently, nitric oxide in mammalian physiology.

CAS Number

372-75-8

Molecular Formula

C6H13N3O3

Molecular Weight

175.19 g/mol

Appearance

White crystalline powder

Solubility

Freely soluble in water

Storage Condition

Store at room temperature, keep dry and away from light

Aladdin SKU

C109228

What Is L-Citrulline?

L-Citrulline (CAS 372-75-8) is a naturally occurring, non-essential alpha-amino acid first isolated from watermelon (Citrullus lanatus), from which its name is derived. Unlike the 20 standard proteinogenic amino acids, L-Citrulline is not directly incorporated into proteins during ribosomal translation; instead, it functions primarily as a key metabolic intermediate.

Structurally, L-Citrulline bears a ureido group (–NH–CO–NH₂) on its side chain, distinguishing it from L-Ornithine and L-Arginine, its close metabolic relatives. Its molecular formula is C₆H₁₃N₃O₃ and its molecular weight is 175.19 g/mol.

Biochemical Role and Metabolic Pathways

L-Citrulline occupies a pivotal position in two interconnected metabolic pathways: the urea cycle and the nitric oxide (NO) synthesis pathway.

Urea Cycle: In the hepatic urea cycle, L-Citrulline is synthesized in the mitochondrial matrix from L-Ornithine and carbamoyl phosphate by the enzyme ornithine transcarbamylase (OTC). It is then exported to the cytoplasm, where it condenses with aspartate to form argininosuccinate, a reaction catalyzed by argininosuccinate synthetase. Subsequent cleavage by argininosuccinate lyase yields L-Arginine and fumarate, completing a critical branch of nitrogen metabolism and enabling ammonia detoxification.

Nitric Oxide Synthesis: In peripheral tissues, L-Citrulline is produced as a co-product alongside NO when L-Arginine is oxidized by nitric oxide synthase (NOS) enzymes. Because L-Citrulline can be recycled back to L-Arginine via the citrulline–NO cycle, it is considered an endogenous reservoir that sustains NO bioavailability. This makes it a subject of considerable interest in cardiovascular, immunological, and exercise physiology research.

Research Applications of L-Citrulline

L-Citrulline is employed across a wide spectrum of life-science research disciplines:

• Nitric Oxide Biology: Used as a tracer and substrate to study NOS activity and NO-mediated signaling in endothelial, neuronal, and immune cell models.
• Urea Cycle Disorders Research: Serves as a reference standard and supplementation agent in cell and animal models of OTC deficiency and other urea cycle disorders.
• Cardiovascular Research: Investigated for effects on vascular smooth muscle relaxation, blood pressure regulation, and endothelial function through its role as an arginine precursor.
• Exercise and Muscle Physiology: Used in studies examining skeletal muscle protein synthesis, fatigue resistance, and ammonia buffering during intense exercise.
• Immunology: Studied in the context of macrophage activation and citrullination reactions relevant to autoimmune disease models (e.g., rheumatoid arthritis research via protein citrullination, though this involves peptidylarginine deiminase rather than free L-Citrulline directly).
• Analytical Standards: Used as an HPLC and mass spectrometry reference standard for amino acid profiling and metabolomics studies.

Handling, Storage, and Safety

L-Citrulline (Aladdin SKU: C109228) is supplied as a white crystalline powder. Recommended handling and storage practices include:

• Storage: Store at room temperature (15–25 °C) in a tightly sealed container, protected from moisture and direct light. Under these conditions, the compound is stable for an extended period.
• Solubility: L-Citrulline is freely soluble in water. Aqueous stock solutions can typically be prepared at concentrations up to 50 mM without difficulty. For stability, prepare fresh solutions or store aliquots at –20 °C.
• Handling: Standard laboratory precautions apply. Avoid inhalation of dust and contact with skin or eyes. Use appropriate personal protective equipment (gloves, lab coat, safety glasses) consistent with good laboratory practice.
• Disposal: Dispose of waste material in accordance with local institutional and governmental regulations.

Consult the Safety Data Sheet (SDS) for complete hazard, first-aid, and disposal information before use.

Purity, Grades, and Quality Assurance

Aladdin Scientific supplies L-Citrulline (C109228) for research use. When selecting L-Citrulline for your experiments, consider the following quality parameters:

• Purity: Research-grade L-Citrulline is typically characterized by HPLC purity ≥98%. Customers should verify the Certificate of Analysis (CoA) for the specific lot being used, as exact purity values are lot-dependent.
• Identity Confirmation: Identity is confirmed by techniques such as NMR, IR spectroscopy, and/or mass spectrometry. Optical rotation measurements confirm the L-enantiomeric form.
• Relevant Specifications to Check: Residue on ignition, heavy metals content, and water content (by Karl Fischer titration) are standard quality parameters. Review the CoA for your specific lot.
• Application Suitability: This product is intended for research and development use only and is not suitable for clinical, therapeutic, or food-additive purposes without appropriate regulatory qualification.

Frequently asked questions

What is L-Citrulline used for in research?

L-Citrulline is widely used as a biochemical tool to study the urea cycle, nitric oxide synthase (NOS) activity, and NO-mediated signaling. It also serves as an analytical reference standard in amino acid profiling, metabolomics, and as a substrate or supplementation agent in cardiovascular, exercise physiology, and immunology research models.

How should L-Citrulline be stored to maintain stability?

L-Citrulline powder should be stored at room temperature (15–25 °C) in a tightly closed container, protected from moisture and light. Aqueous solutions should be prepared fresh where possible; if storage is required, aliquot and freeze at –20 °C to minimize degradation.

What is the difference between L-Citrulline and L-Arginine for research purposes?

L-Arginine is the direct substrate for nitric oxide synthase and arginase enzymes, while L-Citrulline is a metabolic precursor that is converted to L-Arginine intracellularly via the citrulline–NO cycle. In some experimental models, L-Citrulline demonstrates superior bioavailability compared to exogenous L-Arginine because it bypasses first-pass intestinal and hepatic extraction; however, researchers should select the compound appropriate to the specific pathway or enzyme activity being studied.

Is Aladdin's L-Citrulline (C109228) suitable for cell culture experiments?

Aladdin's L-Citrulline (C109228) is a research-grade compound. For cell culture applications, researchers should verify the purity and endotoxin levels stated on the Certificate of Analysis for their specific lot, as endotoxin content can affect cell-based assay results. The product is not sterile as supplied; prepare sterile-filtered aqueous solutions using a 0.2 µm filter prior to adding to cell culture media.

How is L-Citrulline different from citrullinated proteins?

Free L-Citrulline is an amino acid that participates in the urea cycle and NO synthesis as described above. Citrullinated proteins, by contrast, arise from post-translational modification in which peptidylarginine deiminase (PAD) enzymes convert arginine residues within proteins into citrulline residues. This modification is not related to free L-Citrulline metabolism and is studied primarily in the context of autoimmune diseases such as rheumatoid arthritis, where anti-citrullinated protein antibodies (ACPAs) are diagnostic biomarkers.

References

1. Curis E, et al. Almost all about citrulline in mammals. Amino Acids. 2005;29(3):177-205. doi:10.1007/s00726-005-0235-4
2. Breuillard C, Cynober L, Moinard C. Citrulline and nitrogen homeostasis: an overview. Amino Acids. 2015;47(4):685-691. doi:10.1007/s00726-015-1932-2
3. Romero MJ, et al. Therapeutic use of citrulline in cardiovascular disease. Cardiovasc Drug Rev. 2006;24(3-4):275-290.
4. Meijer AJ, et al. Urea cycle disorders and associated metabolic pathways. Biochemical Society Transactions. 1990.

Products are supplied for research and development use only. Not for use in humans, animals, diagnosis, or therapy.