Questions: Vitamin C: Synthesis, Antioxidant Roles, and Enzyme Cofactor Function

Question 1

A patient with severe vitamin C deficiency presents with bleeding gums, old wounds reopening, and fragile capillaries. Which mechanism best explains these findings?

• A. Reduced antioxidant capacity allows ROS to damage blood vessel walls directly
• B. Vitamin C deficiency impairs prolyl and lysyl hydroxylase, preventing stable collagen formation
• C. Without vitamin C, vitamin E cannot be regenerated, causing membrane lipid peroxidation in vessel walls
• D. Vitamin C deficiency reduces NADPH production needed for glutathione recycling

Answer: (B)

Scurvy is fundamentally a structural failure, not primarily an antioxidant deficiency. Prolyl hydroxylase and lysyl hydroxylase require vitamin C as a cofactor to add hydroxyl groups to proline and lysine in procollagen chains. Without hydroxylation, collagen cannot form the stable triple-helix and cross-linked fibril structure that gives connective tissue its tensile strength. Blood vessels become fragile, wounds don't heal, and existing scar tissue can reopen. The antioxidant role of vitamin C is real but secondary to this structural failure in explaining classic scurvy symptoms.

Question 2

Vitamin C is water-soluble while vitamin E is fat-soluble and embedded in cell membranes. How does vitamin C nonetheless help regenerate oxidized vitamin E?

• A. Vitamin C is converted to a lipid-soluble form by liver enzymes before entering membranes
• B. Vitamin C acts at the membrane surface, donating electrons to oxidized vitamin E at the aqueous-lipid interface
• C. Vitamin C first regenerates glutathione, which then diffuses into membranes to reduce vitamin E
• D. Vitamin E moves from the membrane into the cytosol, where it is regenerated by vitamin C, then returns

Answer: (B)

The vitamin E radical (tocopheroxyl) formed after quenching a lipid ROS is located at the membrane surface, near the aqueous interface — accessible to water-soluble molecules. Vitamin C acts as a 'roving electron shuttle,' donating an electron to the oxidized vitamin E radical at this interface without itself needing to enter the lipid bilayer. This is why vitamins C and E are a coordinated antioxidant system: vitamin E quenches lipid-phase ROS, and vitamin C recharges it from the aqueous phase.

Question 3

Humans cannot synthesize vitamin C because, during evolution, we lost the gene encoding L-gulonolactone oxidase, the final enzyme in the biosynthetic pathway.

• T. True
• F. False

Answer: True

Most mammals produce vitamin C endogenously via L-gulonolactone oxidase in the liver. Humans, other primates, guinea pigs, and some other species have a nonfunctional pseudogene for this enzyme — a loss that occurred early in primate evolution, likely because abundant dietary fruit made endogenous synthesis energetically redundant. The consequence is total dependence on dietary intake, with no internal buffer when dietary supply is cut off for extended periods.

Question 4

Taking vitamin C supplements well above the recommended dietary intake reliably increases tissue antioxidant protection because vitamin C is stored in body fat for later use.

• T. True
• F. False

Answer: False

Vitamin C is water-soluble, not fat-soluble — it cannot be stored in adipose tissue. Tissue saturation is reached at modest doses (around 200 mg/day), and the kidneys excrete excess vitamin C via active reabsorption that becomes saturated at high plasma concentrations. Megadoses therefore provide little additional antioxidant benefit and can cause osmotic diarrhea and, in susceptible individuals, kidney stones (from oxalate, a metabolite of ascorbic acid). The body treats excess vitamin C as waste, not as a reserve.

Question 5

Explain why scurvy is described as a structural failure rather than simply an antioxidant deficiency, and what it reveals about vitamin C's role in ongoing tissue maintenance.

Model answer: Scurvy results from the inability to maintain collagen because vitamin C is an essential cofactor for prolyl and lysyl hydroxylase — enzymes that hydroxylate procollagen chains so they can form stable triple-helix and cross-linked fibril structures. Without this, existing connective tissue degrades and cannot be repaired: capillaries become fragile, wounds fail to heal, and old scars reopen. This also reveals that vitamin C is not just needed for initial collagen synthesis during growth — collagen requires continuous turnover and replacement throughout life, meaning that even adults with depleted vitamin C stores suffer structural collapse.

Distinguishing the structural role from the antioxidant role matters clinically and conceptually. The antioxidant role is important for preventing oxidative damage in plasma and tissues, but the immediate life-threatening manifestations of scurvy — hemorrhage, wound dehiscence, bone pain — stem directly from collagen failure. This is why scurvy was historically lethal on long voyages: sailors' connective tissue literally fell apart, not primarily from oxidative stress but from the structural demands of a vitamin that acts as an enzyme cofactor.