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Aspartic Acid (Asp)
(L-Aspartic Acid – Acidic Amino Acid – Urea Cycle & Neurotransmitter Role)
Synonyms
- Aspartic acid
- L-Aspartic acid
- Asp
- 2-Aminosuccinic acid
- Dicarboxylic amino acid
Units of Measurement
µmol/L, mg/L, mg/dL, mg/100mL, mg%, µg/mL
Description
Aspartic acid (Asp) is a non-essential amino acid classified as an acidic amino acid. It plays key roles in:
- Urea cycle
- Transamination reactions
- Malate–aspartate shuttle (ATP production)
- Synthesis of other amino acids (asparagine)
- Neurotransmission (excitatory amino acid)
Asp is measured as part of a quantitative plasma amino acid profile to evaluate:
- Urea cycle disorders
- Organic acidemias
- Liver disease
- Mitochondrial dysfunction
- Malnutrition or catabolic states
Physiological Role
1. Transamination & Energy Metabolism
- Converts to oxaloacetate, a key TCA cycle intermediate
- Supports gluconeogenesis
2. Urea Cycle
Asp donates nitrogen for argininosuccinate formation, vital for ammonia detoxification.
3. Malate-Aspartate Shuttle
Transfers reducing equivalents into mitochondria → essential for oxidative phosphorylation.
4. Neurotransmitter
Aspartate acts as:
- An excitatory neurotransmitter, similar to glutamate
- Precursor to N-acetylaspartate (NAA) in CNS metabolism
Clinical Significance
Elevated Aspartic Acid
1. Urea Cycle Disorders (UCD)
Asp levels may increase mildly; patterns depend on defect.
2. Hepatic Disease
Impaired nitrogen metabolism alters amino acid profiles.
3. Organic Acidemias
Especially disorders impacting TCA cycle or transamination pathways.
4. Renal Failure
Reduced clearance leads to accumulation.
5. Severe Catabolic States
Burns, trauma, sepsis → altered amino acid turnover.
Low Aspartic Acid
Less common; seen in:
1. Malnutrition / Poor Protein Intake
2. Severe Liver Dysfunction
Reduced synthesis and impaired amino acid handling.
3. Mitochondrial Disorders
Impaired malate–aspartate shuttle function.
4. Post–L-asparaginase Therapy
Indirect reduction via precursor depletion.
Reference Intervals
(Tietz 8E + Mayo & ARUP LC-MS/MS Amino Acid Panels)
Plasma Aspartic Acid
- 2 – 20 µmol/L
- ≈ 0.3 – 3.0 mg/L
Infants/Children
- Similar or slightly higher due to active nitrogen metabolism.
Unit Meanings
| Unit | Meaning |
| µmol/L | micromole per liter |
| mg/L | milligram per liter |
| mg/dL | milligram per deciliter |
| mg/100 mL | mg% |
| mg% | milligram per 100 mL |
| µg/mL | microgram per milliliter |
Diagnostic Uses
1. Evaluation of Urea Cycle Disorders
Part of amino acid panel helping differentiate:
- Citrullinemia
- Argininosuccinate lyase deficiency
- Arginase deficiency
- CPS1/OTC deficiency (Asp low–normal)
2. Organic Acidemias
Abnormal Asp occurs in:
- Propionic acidemia
- Methylmalonic acidemia
- TCA cycle defects
3. Liver Disease Assessment
Supports interpretation of:
- Hyperammonemia
- Hepatic failure metabolic patterns
4. Nutritional & Critical Care Assessment
Asp falls with:
- Protein–energy malnutrition
- Severe sepsis
- Major burns or trauma
5. CNS/Metabolic Disorders
Asp contributes to NAA and excitatory neurotransmission.
Analytical Notes
- Preferred sample: fasting plasma (EDTA)
- Avoid hemolysis (RBCs contain amino acids)
- Process immediately or freeze plasma at –20°C
- LC–MS/MS is the gold standard
- Levels influenced by TPN, dietary intake, and metabolic state
Clinical Pearls
- Asp is usually low in plasma compared to other amino acids.
- Always interpret Asp in patterns with asparagine, glutamate, glutamine, and citrulline.
- Very high ammonia + abnormal Asp pattern → suspect UCD.
- Exercise and muscle breakdown may increase Asp slightly.
Interesting Fact
Aspartic acid was one of the earliest amino acids discovered in 1827 and is responsible for the "aspartyl" component of aspartame, the artificial sweetener.
References
- Tietz Clinical Chemistry and Molecular Diagnostics, 8th Edition - Amino Acids.
- Mayo Clinic Laboratories - Plasma Amino Acid Profiling.
- ARUP Consult - Metabolic & Amino Acid Disorders.
- IFCC - Standards for Amino Acid Analysis.
- NIH / MedlinePlus - Amino Acid Overview.
- Metabolic Genetics Literature - Urea Cycle & Organic Acidemias.