Unit Converter
Glycine (Gly)
(Simple Non-Essential Amino Acid – Key Role in Collagen, Neurotransmission & Inborn Errors of Metabolism)
Synonyms
- Glycine
- Gly
- Aminoacetic acid
- Simplest amino acid
- Plasma glycine
Units of Measurement
- µmol/L
- mg/L
- mg/dL
- mg/100 mL
- mg%
- µg/mL
Key Conversions
(Molecular Weight of Glycine ≈ 75.07 g/mol)
1 mg/L = 13.32 µmol/L
1 mg/dL = 133.2 µmol/L
1 µg/mL = 1 mg/L
mg/dL = mg% = mg/100 mL
1 µmol/L = 0.075 mg/L
Description
Glycine is the simplest amino acid, non-essential, and widely distributed in:
- Collagen (1/3 of collagen is glycine)
- Muscle
- Plasma
- CNS (as a neurotransmitter)
It serves critical functions in metabolism, neurotransmission, detoxification, and protein synthesis.
Clinically, glycine is measured mainly in metabolic disorders, amino-acid profiling, and non-ketotic hyperglycinemia (NKH) diagnosis.
Physiological Role
1. Structural Protein Component
- Key constituent of collagen, skin, bone, tendons
- Supports connective tissue stability
2. Neurotransmitter
- Inhibitory neurotransmitter in:
- Brainstem
- Spinal cord
- Brainstem
- Co-agonist for NMDA receptors (excitatory modulation)
3. Detoxification
Required for conjugation reactions:
- Hippurate formation
- Bile salt conjugation (glycocholate, etc.)
4. Immune & Antioxidant Functions
- Precursor for glutathione synthesis
- Supports antioxidant defenses
5. Metabolic Integration
Easily converts to:
- Serine
- Threonine
- Pyruvate
- Glutathione precursors
Clinical Significance
High Glycine (Hyperglycinemia)
Most clinically important.
1. Non-Ketotic Hyperglycinemia (NKH) - Primary Use
Defect in glycine cleavage enzyme system.
Severe neonatal disorder.
Features:
- Intractable seizures
- Hypotonia
- Apnea
- Lethargy
- Burst-suppression pattern on EEG
Lab Finding:
- Plasma glycine very high
- CSF glycine ↑↑
- High CSF:plasma glycine ratio (>0.08)
2. Organic Acidemias
- Propionic acidemia
- Methylmalonic acidemia
- Isovaleric acidemia
Glycine increases as detoxification mechanism (glycine conjugation).
3. Urea Cycle Disorders
Hyperammonemia may increase glycine levels.
4. Valproate Toxicity
Interferes with mitochondrial pathways → glycine elevation.
5. Renal Failure
Reduced clearance.
Low Glycine (Hypoglycinemia)
Seen in:
- Severe malnutrition
- Chronic liver disease
- Inherited glycine metabolism defects (rare)
- Low protein intake
- Critical illness or sepsis
- Neonatal amino-acid disorders
Reference Intervals
(Tietz 8E + Mayo + ARUP + ACMG)
Plasma Glycine
- 150 – 350 µmol/L (adults)
- 200 – 450 µmol/L (newborns; physiologically higher)
Critical Values
- >1000 µmol/L → strongly suggests NKH or organic acidemia
- CSF:Plasma ratio >0.08 → diagnostic for NKH
Diagnostic Uses
1. Non-Ketotic Hyperglycinemia (Primary Use)
- Plasma glycine ↑↑
- CSF glycine ↑↑
- Elevated CSF:plasma ratio
2. Organic Acidurias
Glycine used for detoxification → glycine-conjugated metabolites.
Examples:
- Methylmalonic acidemia
- Propionic acidemia
3. Hyperammonemia Disorders
Glycine rises with nitrogen overload.
4. Nutrition & Malnutrition Assessment
Part of essential amino-acid panel.
5. Critical Illness
Depletion occurs with high metabolic demand.
6. Liver Disease
Altered amino-acid ratios in hepatic failure.
Analytical Notes
- Use fasting plasma, placed immediately on ice
- Preferred tubes: EDTA/heparin
- Rapid deproteinization required (amino acids are metabolically active)
- Measured by HPLC or tandem mass spectrometry
- CSF glycine essential for NKH evaluation
Clinical Pearls
- NKH shows high glycine in both plasma & CSF, while organic acidemias show high plasma glycine but normal CSF glycine.
- Glycine is a precursor for glutathione, so low glycine may worsen oxidative stress.
- Elevated glycine in infants with seizures should prompt urgent metabolic workup.
- Glycine levels often change with nutritional support (TPN).
- Gly:Ser ratio is helpful for diagnosing some metabolic disorders.
Interesting Fact
Glycine is so small that it fits into tight turns of collagen, giving connective tissue its unique strength and flexibility.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Amino Acids.
- ACMG Guidelines - Amino Acid Disorders.
- Mayo Clinic Laboratories - Plasma Amino Acids.
- ARUP Consult - Hyperglycinemia Disorders.
- NIH Genetics - Glycine Metabolism.
- MedlinePlus / NIH - Amino Acid Testing.