Amino Acid Metabolism II: Key Concepts and Clinical Correlations

N-acetylglutamate (an allosteric activator of CPS-I) deficiency can be treated with carbamoylglutamate (an analog) that activates CPS-I.
Deficiency of other urea cycle enzymes can also occur; limit protein intake to reduce ammonia load.
Blood Urea Nitrogen (BUN) is normally $\text{BUN} \approx 10-20\ \text{mg/dL}$ ; normally ~ $10\ \text{g of urea/day}$ excreted in urine.
In hyperammonemia: ↑ blood glutamine and ↓ BUN indicate a urea cycle defect.
Defects:
- CPS-I or N-acetylglutamate synthetase deficiency → hyperammonemia, mental retardation, death; treat with carbamoylglutamate to activate CPS-I.
- Ornithine transcarbamylase (OTC) deficiency and defects in argininosuccinate synthetase/lyase/arginase → severe outcomes; OTC deficiency can cause orotic aciduria and hyperuricemia due to excess carbamoyl-P entering pyrimidine synthesis.
Therapies to remove ammonia and limit intake:
- Protein restriction; ammonia removal strategies.
- Ammonia scavengers: benzoylglycine (benzoyl glycine) or phenylacetate; leads to excretion as phenylacetylglutamine or related conjugates.
Summary of enzymes (deficiencies can be lethal early): OTC, argininosuccinate synthetase, argininosuccinate lyase, arginase.

Starvation/starved muscle: proteolysis supplies amino acids; carbon skeletons fuel glycolysis/TCA; nitrogen delivered to liver as alanine.
Alanine cycle: alanine from muscle → liver via alanine aminotransferase; in liver, alanine becomes pyruvate and then glucose (gluconeogenesis); glucose returns to muscle.
Cori cycle (lactate cycle): during low O₂, muscle converts pyruvate to lactate via LDH; lactate travels to liver, is converted back to glucose via gluconeogenesis.
Brain dependence: after ~1.5 days of starvation, glucose delivery to brain increases.
Liver handles ammonium via the urea cycle during these cycles.

Catecholamines (dopamine, norepinephrine, epinephrine) derive from tyrosine; synthesized in CNS chromaffin cells and adrenal medulla.
Rate-limiting step: tyrosine hydroxylase; requires molecular oxygen and tetrahydrobiopterin (THB, sometimes written as BH4).
Pathway: Tyrosine → DOPA → dopamine → norepinephrine → epinephrine (with DBH and PNMT steps).
Inactivation: COMT (cytosolic, SAM-dependent methylation) and MAO (mitochondrial, oxidative deamination) act sequentially.
End products: dopamine → homovanillic acid (HVA); norepinephrine/epinephrine → vanillylmandelic acid (VMA).
Functions: Epinephrine promotes fight/flight responses; increases glycogenolysis and lipolysis, among other effects.

Precursors: tryptophan → serotonin (5-HT) and then melatonin.
Rate-limiting enzyme: tryptophan hydroxylase; requires O₂ and tetrahydrobiopterin (THB).
Serotonin: synthesized in brain and enterochromaffin cells; platelets take up circulating serotonin.
Melatonin: synthesized from serotonin in the pineal gland via two steps – N-acetyltransferase (NAT) and a methyltransferase; NAT is upregulated in darkness, regulating circadian rhythm.

GABA: formed by decarboxylation of glutamate via glutamate decarboxylase; requires pyridoxal phosphate (vitamin B6); major inhibitory neurotransmitter in brain.
Histamine: formed from histidine by histidine decarboxylase; requires pyridoxal phosphate; functions in brain and periphery (vasodilation, wakefulness, appetite); H1/H2 receptors activate; H3 is inhibitory. Degradation: HNMT (in many tissues) or DAO in others.
Creatine phosphate (Cr-P): energy reservoir in brain and muscle; phosphocreatine transports high-energy phosphate to ATP formation via creatine kinase; tissue-specific CK isoforms (BB in brain, MM in skeletal muscle, MB in heart).
Glutathione (GSH): maintains protein thiols in reduced state; detoxifies xenobiotics in liver.
Nitric Oxide (NO): vasodilator and neurotransmitter; produced by NOS isoforms in endothelium (eNOS), brain (nNOS), and macrophages (iNOS).

Phenylketonuria (PKU): deficiency of phenylalanine hydroxylase → phenylpyruvate/phenylacetate accumulate; symptoms include seizures, fair skin/hair/eye color, musty odor, microcephaly, developmental delay.
Epilepsy: chronic seizure disorder due to abnormal neural activity; GABA system modulation is a therapeutic target (e.g., phenobarbital enhances GABA receptor activity).
Parkinson’s disease: degeneration of substantia nigra → dopamine loss; alpha-synuclein aggregates implicated; treatment includes L-DOPA (dopamine precursor) that crosses the blood–brain barrier; tremor, rigidity, bradykinesia.
Huntington’s disease: progressive neurodegenerative disorder due to CAG trinucleotide repeat expansion; polyglutamine tract in huntingtin causes chorea, dementia.
Albinism: tyrosinase deficiency → impaired melanin synthesis; increased UV risk and skin damage.
Depression: altered serotonin metabolism; treated with selective serotonin reuptake inhibitors (e.g., fluoxetine/Prozac).
Pheochromocytoma: tumor of chromaffin tissue → excess catecholamines; presents with hypertension, palpitations, headaches.

Urea cycle disorders: N-acetylglutamate deficiency treated with carbamoylglutamate; other enzyme defects treated with ammonia-scavengers (e.g., benzoylglycine, phenylacetate).
Glucose-Alanine vs. Cori cycles: muscle nitrogen management via alanine; Cori cycle via lactate; both connect muscle and liver for energy and nitrogen handling.
Bioamine pathways: precursors and cofactors define synthesis; catabolism via COMT/MAO/HNMT/DAO provides clinical markers (e.g., HVA, VMA) and pharmacological targets.
Key precursors: Tyrosine → catecholamines; Tryptophan → serotonin/melatonin; Histidine → histamine; Glutamate → GABA; Arginine → NO; Glycine/Arginine → Creatine-P.
Enzyme defects cause disorders (PKU, albinism, epilepsy, Parkinson’s, Huntington’s); Huntington’s due to CAG repeat expansions.