U01.01.122 Organic acidemias

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Learning Objectives

  • Recognize the clinical hallmarks of organic acidemias: anion gap metabolic acidosis and hyperammonemia.
  • Explain the biochemical block in Propionic Acidemia and the resulting metabolite levels.
  • Differentiate between Propionic and Methylmalonic Acidemia using methylmalonic acid levels.
  • Identify the VOMIT amino acids and fatty acids that must be restricted in the diet.

1. General Features of Organic Acidemias

Organic acidemias are a group of metabolic disorders characterized by the accumulation of organic acids in the blood and tissues. They typically present in early infancy after the initiation of protein feeds.

  • The “Metabolic Triple Threat”:
    • High Anion Gap Metabolic Acidosis: Organic acid accumulation leads to ketoacidosis.
    • Hypoglycemia: Organic acids inhibit gluconeogenesis.
    • Hyperammonemia: Organic acids inhibit the urea cycle, leading to secondary elevations in ammonia.
  • Clinical Presentation: Poor feeding, vomiting, hypotonia (“floppy baby”), hepatomegaly, and seizures.

2. Propionic vs. Methylmalonic Acidemia

These two conditions occur in the same metabolic pathway but at different enzymatic steps.

Condition Deficient Enzyme/Cofactor Key Lab Findings
Propionic Acidemia Propionyl-CoA Carboxylase (Biotin) ↑ Propionyl-CoA; Normal Methylmalonic acid
Methylmalonic Acidemia Methylmalonyl-CoA Mutase (Vitamin B12) ↑ Propionyl-CoA; ↑ Methylmalonic acid

Activity

Memory Hook: Remember VOMIT for the substances you must restrict:

Valine, Odd-chain fatty acids, Methionine, Isoleucine, Threonine.

3. Metabolic Pathway and Treatment

The pathway involves the breakdown of specific amino acids and fatty acids into Propionyl-CoA, which eventually enters the TCA cycle as Succinyl-CoA.

  • Path: VOMIT → Propionyl-CoA $Latex\xrightarrow{Biotin}$ Methylmalonyl-CoA $Latex\xrightarrow{B12}$ Succinyl-CoA.
  • Treatment:
    • Low-protein diet (specifically limiting VOMIT).
    • Biotin or Vitamin B12 supplementation (if the defect is cofactor-responsive).
    • Alkalinization and hydration during acute crises.

Clinical Notes & Corrections:

  • Differential: If a patient has elevated methylmalonic acid and megaloblastic anemia, consider Vitamin B12 deficiency rather than a primary enzyme defect.
  • Hyperammonemia: Always check ammonia in an infant with an unexplained high anion gap acidosis; it distinguishes organic acidemias from simple starvation ketosis.

Activity