Vitamin C (ascorbic acid)

Function & Sources - The Collagen Catalyst

• Antioxidant: Directly scavenges reactive oxygen species. Regenerates other antioxidants like Vitamin E.
• Collagen Synthesis: Essential cofactor for prolyl and lysyl hydroxylases to hydroxylate proline and lysine.
  • Reaction: $Proline + O_2 + \alpha-KG \xrightarrow{Vit\ C} 4-Hydroxyproline + Succinate$
  • This step is critical for the formation of stable collagen triple-helices.
• Cofactor in Biosynthesis:
  • Dopamine β-hydroxylase: Converts dopamine to norepinephrine.
  • Carnitine synthesis: For fatty acid transport into mitochondria.
• Iron Absorption: Enhances the absorption of non-heme iron by reducing it from ferric ($Fe^{3+}$) to the more soluble ferrous ($Fe^{2+}$) state in the gut.

⭐ Vitamin C's role in hydroxylating proline and lysine is paramount; without it, unstable pro-collagen is produced, leading to the fragile connective tissue seen in scurvy.

• Sources: Citrus fruits, bell peppers, strawberries, broccoli, potatoes.

Biochemical Role - The Electron Donor

• Primary function: Potent water-soluble antioxidant and reducing agent (electron donor).
• Mechanism: Maintains iron and copper atoms in enzymes in their reduced (active) states. Vitamin C reduces $Fe^{3+}$ → $Fe^{2+}$ and $Cu^{2+}$ → $Cu^{+}$.

• Key Cofactor Roles:
  • Collagen Synthesis: Required by prolyl and lysyl hydroxylases for post-translational hydroxylation of pro-collagen, enabling stable triple helix formation.
  • Catecholamine Synthesis: Cofactor for dopamine β-hydroxylase, which converts dopamine to norepinephrine.
  • Iron Absorption: Enhances absorption of non-heme iron in the intestine by reducing it from the ferric ($Fe^{3+}$) to the ferrous ($Fe^{2+}$) state.

⭐ Vitamin C is essential for the conversion of dopamine to norepinephrine, linking nutritional status directly to neurotransmitter synthesis.

📌 Ascorbate helps Absorb iron.

Deficiency (Scurvy) - The Sailor's Scourge

• Pathophysiology: Impaired collagen synthesis due to ↓ hydroxylation of proline & lysine residues in procollagen.
• Clinical Presentation (The 4 H's):
  • Hemorrhage:
    • Gingival bleeding, swollen gums
    • Perifollicular hemorrhage & petechiae
    • Easy bruising (ecchymoses)
    • Impaired wound healing
  • Hyperkeratosis:
    • Follicular hyperkeratosis (rough skin)
    • "Corkscrew" hairs
  • Hypochondriasis: Malaise, fatigue, depression.
  • Hematologic: Anemia (↓ iron absorption, bleeding), leukopenia.

⭐ Exam Favorite: "Corkscrew" hairs and perifollicular hemorrhages are highly specific findings for Scurvy. They result directly from weakened collagen in hair follicles and blood vessel walls.

Toxicity & Uses - Too Much of a Good Thing?

• Toxicity (megadoses >2 g/day):

  • Nausea, vomiting, diarrhea, abdominal bloating.
  • ↑ Risk of calcium oxalate kidney stones.
  • ⚠️ Can induce iron overload in susceptible patients (e.g., hemochromatosis) by enhancing iron absorption.

• Therapeutic Uses:

  • Antioxidant.
  • Adjunctive for methemoglobinemia (reduces $Fe^{3+}$ to $Fe^{2+}$).

⭐ High doses can cause a false-negative stool guaiac test, potentially masking GI bleeding.

High‑Yield Points - ⚡ Biggest Takeaways

• Vitamin C is a crucial antioxidant and cofactor in collagen synthesis (prolyl and lysyl hydroxylase).
• Facilitates iron absorption by reducing ferric (Fe³⁺) to ferrous (Fe²⁺) iron.
• Cofactor for dopamine β-hydroxylase, converting dopamine to norepinephrine.
• Deficiency leads to Scurvy: characterized by bleeding gums, impaired wound healing, and perifollicular hemorrhage.
• Excess intake can cause calcium oxalate kidney stones and iron overload.