Question

Which of the following amino acid is a precursor of nicotinic acid?

• A. Tyrosine
• B. Tryptophan
• C. Threonine
• D. Phenyl alanine

Hint:

Amino Acid Precursors:
Tryptophan $\rightarrow$ Niacin (Vitamin $\text{B}_3$), Serotonin, Melatonin.
Tyrosine $\rightarrow$ Epinephrine, Norepinephrine, Dopamine, Melanin, Thyroxine.
Histidine $\rightarrow$ Histamine.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
De novo synthesis of pyridine nucleotide coenzymes ($NAD^+/NADP^+$) proceeds via the kynurenine pathway from the essential amino acid tryptophan.
Key Formula or Approach:
\[ 60\text{ mg Dietary L-Tryptophan} \xrightarrow{\text{Kynurenine Pathway}} 1\text{ mg Niacin (Nicotinic Acid / Vitamin B}_3) \]

Step 2: Detailed Explanation:
In animal metabolic biochemistry:
- The essential indole amino acid L-Tryptophan is converted via the liver kynurenine-quinolinate pathway into quinolinic acid, which serves as the endogenous biosynthetic precursor for Nicotinic Acid (Niacin Vitamin $\text{B}_3$) and $\text{NAD}^+/\text{NADP}^+$.
- Quantitatively, approximately $60\text{ mg}$ of dietary tryptophan yields $1\text{ mg}$ of niacin equivalent.

Step 3: Final Answer:
Thus, Tryptophan is the precursor of nicotinic acid, corresponding to option (B).