Vitamin B6 and Transamination

Vitamin B6 Forms & Sources - Pyridoxine Powerhouse

• Vitamers (3 forms):
  • Pyridoxine (PN): Plant origin.
  • Pyridoxal (PL).
  • Pyridoxamine (PM): Animal origin.
  • All convert to phosphate esters (PNP, PLP, PMP).
• Active Coenzyme: Pyridoxal Phosphate (PLP).

  ⭐ Pyridoxal Phosphate (PLP) is the biologically active coenzyme form of Vitamin B6.

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• Sources & RDA:

PLP Coenzyme Function - Amino Acid Ace

• Pyridoxal Phosphate (PLP): B6's active coenzyme, vital for amino acid (AA) metabolism.
• Mechanism: Schiff Base Cycle
  • Internal Aldimine: PLP's aldehyde group + enzyme's Lysine residue: $\text{E-CHO} + \text{H}_2\text{N-Lys} \rightleftharpoons \text{E-CH=N-Lys} + \text{H}_2\text{O}$.
  • External Aldimine: Incoming AA substrate displaces enzyme's Lysine: $\text{E-CH=N-Lys} + \text{R-CH(NH}_2\text{)-COOH} \rightleftharpoons \text{E-Lys-NH}_2 + \text{R-CH(COOH)-N=CH-PLP}$.
  • Labilizes AA $\alpha$-carbon bonds, enabling diverse reactions.
• PLP-Dependent Reactions:
  • Transamination
  • Decarboxylation
  • Racemization
  • Deamination
  • Side-chain modifications

⭐ The aldehyde group of PLP is the reactive part, forming a Schiff base with amino groups.

Transamination Mechanism & Significance - Amino Group Shuffle

• General Reaction: Transfer of an amino group ($ ext{-NH}_2$) from an amino acid to an $\alpha$-keto acid, yielding a new amino acid and a new $\alpha$-keto acid. $\text{Amino Acid}_1 + \alpha\text{-Keto Acid}_2 \rightleftharpoons \alpha\text{-Keto Acid}_1 + \text{Amino Acid}_2$

• Mechanism: 'Ping-pong bi-bi' reaction. PLP (Pyridoxal Phosphate) is the coenzyme.

  • Enzyme-PLP + Amino Acid$_1$ $\rightarrow$ Enzyme-PMP + $\alpha$-Keto Acid$_1$. (PLP accepts $\text{-NH}_2$, forms PMP)
  • Enzyme-PMP + $\alpha$-Keto Acid$_2$ $\rightarrow$ Enzyme-PLP + Amino Acid$_2$. (PMP donates $\text{-NH}_2$, regenerates PLP)

• Key Transaminases (Aminotransferases):

  Feature	ALT (Alanine Aminotransferase/SGPT)	AST (Aspartate Aminotransferase/SGOT)
  Reaction	Alanine + $\alpha$-KG $\rightleftharpoons$ Pyruvate + Glutamate	Aspartate + $\alpha$-KG $\rightleftharpoons$ OAA + Glutamate
  Tissue (Major)	Liver (specific)	Liver, Heart, Sk. Muscle, Kidney (Assorted)
  Clinical Sig.	$\uparrow$ in liver damage (e.g., viral hepatitis)	$\uparrow$ in liver/muscle damage, MI
  📌 Mnemonic: ALT for Liver; AST for Several Tissues (Heart, Liver, Muscle).

• Metabolic Significance:

  • Funneling amino groups to glutamate (for urea cycle or AA synthesis).
  • Synthesis of non-essential amino acids.
  • Role in gluconeogenesis (e.g., alanine $\rightarrow$ pyruvate).

• Flowchart: Ping-Pong Bi-Bi Mechanism

⭐ All transamination reactions require PLP, and glutamate is a common product, acting as a collector of amino groups for disposal or reuse.

Vitamin B6 Clinical Aspects - Deficiency & Drama

• Deficiency Causes:
  • Dietary inadequacy, chronic alcoholism.
  • Drug interactions: 📌 "I Pee On Coins" (Isoniazid, Penicillamine, Oral Contraceptives, Cycloserine).
• Deficiency Manifestations:
  • Skin: Seborrheic dermatitis-like rash, cheilosis, glossitis.
  • Neurological: Peripheral neuropathy, irritability; convulsions in infants (due to impaired GABA synthesis).
  • Hematological: Sideroblastic anemia (microcytic, hypochromic). *
  • Diagnosis: Tryptophan load test (↑ urinary xanthurenic acid).

⭐ Isoniazid, an anti-tubercular drug, can induce Vitamin B6 deficiency by forming an inactive hydrazone complex with pyridoxal phosphate (PLP), leading to its functional inactivation and increased excretion.

• Toxicity:
  • Sensory neuropathy with chronic high doses (> 200 mg/day).
• Therapeutic Uses:
  • Prevention of INH-induced neuropathy (prophylactic dose 10-25 mg/day).
  • Nausea and vomiting of pregnancy.
  • Homocystinuria (pharmacological doses).

High‑Yield Points - ⚡ Biggest Takeaways

• Vitamin B6 active form is Pyridoxal Phosphate (PLP).
• PLP is the key coenzyme for transamination reactions, crucial for amino acid metabolism.
• Aminotransferases (e.g., ALT, AST) are PLP-dependent; their levels indicate liver damage.
• Transamination involves transfer of an amino group from an amino acid to a keto acid.
• B6 deficiency leads to peripheral neuropathy, sideroblastic anemia, dermatitis, and seizures.
• Isoniazid can cause B6 deficiency by forming inactive complexes with PLP.