Niacin and NAD/NADP

Niacin: Introduction - The B3 Boss

• Vitamin B3 (Niacin): A water-soluble vitamin, crucial for metabolism. Exists as:
  • Nicotinic acid ($C_6H_5NO_2$)
  • Nicotinamide ($C_6H_6N_2O$)
• Dietary Sources: Widely available.
  • Animal: Meat (esp. liver, poultry), fish (tuna, salmon).
  • Plant: Fortified/enriched grains, legumes, nuts, seeds.
• Endogenous Synthesis: Unique among B vitamins.
  • Precursor: Tryptophan (essential amino acid).
  • Cofactors: Vitamin B2 (Riboflavin) & B6 (Pyridoxine) are vital.
  • 📌 Ratio: 60 mg Tryptophan $\approx$ 1 mg Niacin.
• Recommended Dietary Allowance (RDA):
  • Adults: 14-16 mg NE/day (NE = Niacin Equivalents).

⭐ Niacin can be synthesized endogenously from the amino acid Tryptophan; approximately 60 mg of Tryptophan yields 1 mg of Niacin.

NAD/NADP: Synthesis & Roles - Electron Shuttles

• Synthesis from Niacin (B3) & Tryptophan:
  • Preiss-Handler Pathway: Nicotinic Acid → NAD+.
  • Salvage Pathway: Nicotinamide → NAD+.
  • Tryptophan (de novo) also yields NAD+.
  • NAD+ $\xrightarrow{\text{NAD+ kinase, ATP}}$ NADP+.

• Structure: Dinucleotide core.
  • NAD+: Nicotinamide, Adenine, 2 Ribose, 2 $P_i$.
  • NADP+: NAD+ with an additional $P_i$ on adenosine's 2'-ribose.
• Roles: Electron Shuttles (Coenzymes):
  • Carry $2e^-$ and $H^+$.
  • NAD+ (Catabolic): Oxidative reactions (glycolysis, TCA cycle).
    • $NAD^+ + 2e^- + 2H^+ \rightleftharpoons NADH + H^+$
  • NADP+ (Anabolic): Reductive biosynthesis (fatty acid, steroid synthesis), antioxidant.
    • $NADP^+ + 2e^- + 2H^+ \rightleftharpoons NADPH + H^+$
    • 📌 NADPH: Pentose Pathway, Protective, Producing.

⭐ NADP+ differs from NAD+ by an additional phosphate group on the 2'-hydroxyl of the adenosine moiety, and NADPH is a major electron donor in reductive biosynthesis and detoxification pathways.

NAD/NADP: Metabolic Functions - Niacin's Busy Day

Niacin (B3) derivatives $NAD^+$ & $NADPH$: key electron carriers in metabolism.

⭐ The ratio of $NAD^+/NADH$ is generally high, favoring oxidative reactions, while the ratio of $NADP^+/NADPH$ is generally low, favoring reductive biosynthetic reactions.

Niacin: Clinical Aspects - Too Little, Too Much

• Pellagra (Deficiency): 📌 3 D's: Dermatitis (photosensitive, Casal's necklace), Diarrhea, Dementia. Can progress to Death (4th D).
  • Causes: Dietary lack (maize/sorghum), malabsorption, Hartnup disease, Carcinoid syndrome, Isoniazid.
  • Diagnosis: Clinical; urinary N-methylnicotinamide.
  • Treatment: Niacin/Nicotinamide (e.g., 300-500 mg/day).

⭐ Hartnup disease, an autosomal recessive disorder affecting neutral amino acid transport (including Tryptophan), can lead to Pellagra-like symptoms due to impaired Niacin synthesis.

• Toxicity (Excess):
  • Niacin flush: Prostaglandin-mediated (PGD2); ↓ with aspirin.
  • Peptic Ulcer Disease (PUD), Gout, Hepatotoxicity.
• Therapeutic Uses (Pharmacological Doses):
  • Hyperlipidemia: ↓ LDL, ↓ TGs, ↑ HDL.
    • Mechanism: Inhibits VLDL synthesis & adipose tissue lipolysis.
    • Dose: 1-3 g/day.

High‑Yield Points - ⚡ Biggest Takeaways

• Niacin (B3) is precursor for coenzymes NAD+ & NADP+, vital in redox reactions.
• Deficiency causes Pellagra: the 3 Ds (Dermatitis, Diarrhea, Dementia).
• Tryptophan is a precursor (60:1 ratio); deficiency seen in Hartnup disease & carcinoid syndrome.
• NAD+ is key in catabolism (e.g., glycolysis); NADPH (from NADP+) in anabolism (e.g., FA synthesis) & antioxidant defense.
• Therapeutic doses treat hyperlipidemia; common side effect is flushing.