Vitamin C & Antioxidants

Ascorbic acid (vitamin C) is a water-soluble vitamin that is both a critical enzyme cofactor and the principal water-phase antioxidant in plasma. This topic ties together collagen biochemistry, iron metabolism, scurvy, and the cellular free-radical defence system (SOD, catalase, glutathione peroxidase) — a favourite high-recall zone for NEET PG.

Chemistry & basic facts

Vitamin C is L-ascorbic acid, a six-carbon lactone structurally related to glucose. Most mammals synthesise it from glucose via the uronic acid pathway, but humans, primates and guinea pigs cannot because they lack L-gulonolactone oxidase (the terminal enzyme). This is why humans depend on dietary intake, and why the guinea pig is the classic animal model for scurvy.

• Active forms in redox cycling: ascorbate (reduced) ⇌ semidehydroascorbate ⇌ dehydroascorbic acid (oxidised).
• RDA (India/ICMR): 40 mg/day adults; smokers need more (+35 mg). Body pool ~1500 mg; symptoms of scurvy appear when pool falls below ~300 mg.
• Sources: amla (Indian gooseberry — richest natural source), guava, citrus fruits, capsicum, green leafy vegetables, sprouts. Easily destroyed by heat, alkali, and storage (heat-labile, water-soluble).
• Absorption: active sodium-dependent transport (SVCT1/SVCT2) in the ileum.

High-yield: Humans lack L-gulonolactone oxidase, so vitamin C is a dietary essential. Amla is the richest natural source; vitamin C is the most heat-labile vitamin.

Biochemical functions — hydroxylation & antioxidant roles

Vitamin C acts mainly as a reducing agent / electron donor, keeping metal ions of certain enzymes in their reduced (active) state.

Function	Enzyme / process	Cofactor role of vitamin C
Collagen synthesis	Prolyl & lysyl hydroxylase	Keeps Fe²⁺ reduced for hydroxylation of proline/lysine
Carnitine synthesis	Two dioxygenases	Cofactor → fatty-acid β-oxidation
Catecholamine synthesis	Dopamine β-hydroxylase	Dopamine → noradrenaline
Bile acid synthesis	7-α-hydroxylase	Cholesterol → bile acids
Tyrosine metabolism	p-hydroxyphenylpyruvate hydroxylase	Tyrosine catabolism
Iron absorption	Reduces Fe³⁺ → Fe²⁺	Enhances non-haem iron uptake
Folate metabolism	Stabilises tetrahydrofolate	Prevents folate oxidation
Antioxidant	Scavenges ROS in plasma	Regenerates vitamin E

Collagen hydroxylation (the most tested concept)

Collagen synthesis requires hydroxylation of proline → hydroxyproline and lysine → hydroxylysine, catalysed by prolyl hydroxylase and lysyl hydroxylase. These enzymes need Fe²⁺, molecular O₂, α-ketoglutarate, and ascorbate. Ascorbate keeps the iron in the reduced ferrous state.

• Hydroxyproline stabilises the triple helix via interchain hydrogen bonds.
• Hydroxylysine is the site for glycosylation and for cross-link formation (lysyl oxidase, a copper enzyme, then forms the cross-links).
• Without hydroxylation, under-hydroxylated procollagen is unstable, is not secreted, and is degraded → defective collagen → the bleeding, poor wound healing and bone defects of scurvy.

High-yield: Vitamin C is needed for prolyl and lysyl hydroxylase (post-translational hydroxylation). Lysyl oxidase (cross-linking) is copper-dependent, NOT vitamin C–dependent — a classic trap.

Iron absorption

Ascorbate reduces dietary ferric iron (Fe³⁺) to the absorbable ferrous form (Fe²⁺) and forms a soluble chelate, markedly enhancing absorption of non-haem (plant) iron. This is why vitamin C is co-prescribed with oral iron and why citrus with meals improves iron status — exploited in treating iron-deficiency anaemia.

The antioxidant defence system

A free radical is any species with an unpaired electron (e.g. superoxide O₂•⁻, hydroxyl OH•). Reactive oxygen species (ROS) also include non-radical oxidants like hydrogen peroxide (H₂O₂). They are generated by mitochondrial electron transport leak, the respiratory burst of phagocytes (NADPH oxidase), and xanthine oxidase. Uncontrolled ROS cause lipid peroxidation, protein oxidation, and DNA damage.

Antioxidants are classified as enzymatic and non-enzymatic.

Antioxidant	Type / location	Action	Cofactor
Superoxide dismutase (SOD)	Enzymatic	2 O₂•⁻ + 2H⁺ → H₂O₂ + O₂	Cu/Zn (cytosol), Mn (mitochondria)
Catalase	Enzymatic (peroxisome)	2 H₂O₂ → 2 H₂O + O₂	Haem (Fe)
Glutathione peroxidase (GPx)	Enzymatic (cytosol)	H₂O₂ + 2GSH → 2H₂O + GSSG	Selenium
Glutathione reductase	Enzymatic	GSSG + NADPH → 2GSH	FAD; needs NADPH from HMP shunt
Vitamin C	Non-enzymatic, aqueous	Direct ROS scavenger; regenerates vitamin E	—
Vitamin E (tocopherol)	Non-enzymatic, lipid/membrane	Breaks lipid peroxidation chain	—
Beta-carotene, uric acid, bilirubin	Non-enzymatic	ROS scavengers	—

Stepwise detoxification of superoxide:

O₂•⁻ → (SOD) H₂O₂ → (catalase / glutathione peroxidase) H₂O + O₂

High-yield: SOD is the FIRST-line enzymatic defence (acts on superoxide). Glutathione peroxidase is selenium-dependent; catalase is a haem enzyme. The HMP shunt supplies NADPH to regenerate reduced glutathione (GSH) — link to G6PD deficiency.

The vitamin C–vitamin E synergy

Vitamin E is the chain-breaking antioxidant inside membranes; when it neutralises a lipid peroxyl radical it becomes a tocopheryl radical. Vitamin C (in the aqueous phase) regenerates active vitamin E, sparing it. This "antioxidant network" — vitamin C ↔ vitamin E ↔ glutathione — is a recurring concept.

Link to G6PD deficiency

The HMP (pentose phosphate) shunt, via G6PD, generates NADPH. NADPH keeps glutathione reduced (GSH), which feeds glutathione peroxidase. In G6PD deficiency, RBCs cannot regenerate GSH → oxidant stress (fava beans, primaquine, infection, sulfa drugs) → haemolysis with Heinz bodies and bite cells. This is the most clinically important downstream consequence of a failed antioxidant pathway.

Scurvy — the vitamin C deficiency disease

Scurvy results from defective collagen synthesis. Connective tissue, blood vessel walls, bone matrix (osteoid), and dentine are all affected. Classic among sailors historically; today seen in elderly, alcoholics, the very poor, food faddists, infants on unsupplemented boiled cow's milk, and psychiatric patients.

Clinical features (adults)

• Bleeding manifestations: the hallmark. Perifollicular haemorrhages and corkscrew (coiled) hairs, gum bleeding/swelling (only if teeth present), subperiosteal haemorrhage, ecchymoses, splinter haemorrhages.
• Poor wound healing, reopening of old scars.
• Anaemia — multifactorial (impaired iron absorption, bleeding, associated folate deficiency).
• Hyperkeratotic, follicular papules on the skin.
• "Woody leg" oedema, arthralgia, lethargy, "scorbutic rosary".

High-yield: Perifollicular haemorrhages + corkscrew hairs + swollen bleeding gums is the classic scurvy triad. Vitamin C deficiency = defective collagen with normal lysyl oxidase.

Infantile scurvy — Barlow's disease

Seen typically at 6–24 months in infants on prolonged boiled/heated milk without supplementation (heat destroys vitamin C).

• Pseudoparalysis — the infant keeps legs flexed and abducted ("frog-leg / pithed-frog position") and resists movement because of painful subperiosteal haemorrhage.
• Scorbutic rosary — sharp-angled costochondral junctions (contrast with the rounded rachitic rosary of rickets).
• Irritability, bleeding gums (when teeth erupt).

Radiographic signs of infantile scurvy (high-yield eponyms):

Sign	Description
Trümmerfeld zone (scurvy line)	Lucent band beneath the dense provisional zone of calcification
White line of Frankel	Dense, sclerotic line at the metaphysis (zone of provisional calcification)
Wimberger ring sign	Dense sclerotic ring around the epiphysis (ground-glass centre)
Pelkan spur	Lateral metaphyseal spur/beak from healing subperiosteal bleed
Corner / angle sign	Marginal metaphyseal defect

High-yield: Wimberger ring (epiphysis), Frankel's white line (metaphysis) and Pelkan spurs are the scurvy radiographic triad. Pseudoparalysis with frog-leg posture is the classic infantile presentation.

Diagnosis & investigation

Scurvy is largely a clinical and dietary diagnosis supported by response to treatment.

• Plasma ascorbic acid — low; reflects recent intake (deficient < 0.2 mg/dL).
• Leucocyte (buffy coat) ascorbate — best index of tissue stores / body pool (investigation of choice for true deficiency).
• Capillary fragility (Hess/tourniquet) test — positive but non-specific.
• X-ray of long bones (knee) in infants → the eponymous signs above.
• Dramatic clinical response to vitamin C within days supports the diagnosis.

High-yield: Leucocyte ascorbic acid level is the most reliable indicator of body vitamin C stores; plasma level reflects only recent intake.

Management / drug of choice

Treatment → ascorbic acid supplementation.

1. Adults: vitamin C 100–300 mg/day (some regimens 500 mg–1 g/day) orally until recovery, then dietary maintenance.
2. Infantile scurvy: 100–200 mg/day of ascorbic acid; bone changes and pseudoparalysis resolve rapidly.
3. Address the underlying diet; encourage amla, citrus, guava, fresh vegetables.
4. Symptomatic bleeding resolves within 24–48 h; gum and skin changes over weeks.

Prevention: balanced diet; avoid prolonged boiling of milk for infants; supplement at-risk groups (elderly, alcoholics, dialysis, malabsorption).

Toxicity & other clinical angles

• High-dose vitamin C (megadoses, >2 g/day) → GI upset, diarrhoea, and oxalate kidney stones (ascorbate is metabolised partly to oxalate). Caution in renal stone formers and oxalosis.
• Can cause false-negative stool occult blood and false urine glucose results.
• In G6PD deficiency, large IV doses can precipitate haemolysis.
• May cause rebound scurvy in a neonate whose mother took megadoses in pregnancy (foetal adaptation to high levels).
• Withdrawal of long-term high doses can cause conditioned deficiency.

High-yield: Vitamin C megadoses → oxalate renal stones; avoid in G6PD deficiency and stone formers.

Key differentials

Condition	Distinguishing point
Rickets (vitamin D deficiency)	Rounded rachitic rosary, widened wrists, bowing, low calcium/phosphate, raised ALP; X-ray shows cupping/fraying — NOT the dense scurvy lines
Henoch–Schönlein purpura	Palpable purpura over buttocks/legs, arthritis, abdominal pain, normal vitamin C; IgA vasculitis
Battered child / NAI	Multiple fractures of varying age; raised by subperiosteal bleeds of scurvy mimicking trauma
Leukaemia / bleeding diatheses (ITP, haemophilia)	Abnormal counts/coagulation; gum bleeding without perifollicular pattern
Vitamin K deficiency	Prolonged PT/INR, bleeding, but normal collagen and no corkscrew hairs
Copper deficiency / Menkes	Defective lysyl oxidase cross-linking, kinky hair — collagen cross-linking fails, hydroxylation is normal

Recently asked / exam angle

• Enzyme requiring vitamin C → prolyl hydroxylase / lysyl hydroxylase (NOT lysyl oxidase). Frequently set as a single-best-answer trap.
• First enzyme acting on superoxide radical → superoxide dismutase (SOD).
• Selenium-containing antioxidant enzyme → glutathione peroxidase.
• Enzyme deficient in humans for vitamin C synthesis → L-gulonolactone oxidase.
• Frog-leg posture / pseudoparalysis in infant → infantile scurvy (Barlow's disease).
• Wimberger ring sign / Frankel's line / Pelkan spur → scurvy (differentiate from congenital syphilis where Wimberger sign refers to medial tibial metaphyseal erosion — note the overlap).
• Vitamin C enhances absorption of → non-haem iron (Fe³⁺ → Fe²⁺).
• Megadose vitamin C complication → oxalate stones.
• Antioxidant that regenerates vitamin E → vitamin C.
• Image-based questions on corkscrew hairs and perifollicular haemorrhage.

Mnemonic for scurvy — "4 H's": Haemorrhage (perifollicular/gum), Hyperkeratosis (follicular), Hair (corkscrew/coiled), Healing impaired (wounds).

Mnemonic for enzymatic antioxidants — "Some Cats Get Grumpy": SOD, Catalase, Glutathione peroxidase (and Glutathione reductase).

Vitamin C functions — "It HEALS": Iron absorption, Hydroxylation (collagen), E regeneration (vitamin E), Antioxidant, Lysine→carnitine, Steroid/catecholamine (dopamine β-hydroxylase, bile acids).

Rapid revision

1. Vitamin C = L-ascorbic acid; humans lack L-gulonolactone oxidase → dietary essential; richest source = amla.
2. Cofactor for prolyl & lysyl hydroxylase (collagen); keeps Fe²⁺ reduced — lysyl oxidase is copper-dependent, not vitamin C.
3. Needed for dopamine β-hydroxylase, carnitine synthesis, bile-acid 7-α-hydroxylase, and folate stabilisation.
4. Enhances non-haem iron absorption by reducing Fe³⁺ → Fe²⁺.
5. Most clinically useful deficiency test = leucocyte ascorbic acid; plasma reflects only recent intake.
6. Scurvy triad: perifollicular haemorrhages + corkscrew hairs + bleeding gums with poor wound healing.
7. Infantile scurvy = Barlow's disease: pseudoparalysis, frog-leg posture, scorbutic rosary (sharp-angled).
8. Scurvy X-ray eponyms: Frankel's white line, Trümmerfeld zone, Wimberger ring, Pelkan spur.
9. Treatment = ascorbic acid 100–300 mg/day (adults), 100–200 mg/day (infants); rapid response.
10. Enzymatic antioxidants: SOD (first, Cu/Zn–Mn) → catalase (haem) / glutathione peroxidase (selenium).
11. NADPH from HMP shunt regenerates GSH; failure in G6PD deficiency → oxidant haemolysis (Heinz bodies, bite cells).
12. Vitamin C regenerates vitamin E; megadoses cause oxalate stones and risk haemolysis in G6PD deficiency.