Deficiency of which cofactor shunts 3-OH-Kynurenine to xanthurenic acid pathway?

INI-CET EXAM -2025

Applied Nutrition & Dietetics

Medium

Appeared in: INI-CET EXAM -2025

Explanation

Pyridoxine, or Vitamin B6, is the correct cofactor.
Its active form, Pyridoxal Phosphate (PLP), is an essential coenzyme for kynureninase, an enzyme in the tryptophan degradation pathway.
A deficiency in Pyridoxine impairs the function of kynureninase, causing its substrate, 3-hydroxykynurenine, to build up.
This excess substrate is then diverted (shunted) to an alternative pathway, leading to the formation of xanthurenic acid, which is excreted in the urine.

Option A: Thiamine (Vitamin B1) is primarily involved in carbohydrate metabolism as thiamine pyrophosphate (TPP), a cofactor for enzymes like pyruvate dehydrogenase and transketolase. It does not act as a cofactor for kynureninase.
Option B: Biotin (Vitamin B7) serves as a coenzyme for carboxylase enzymes, which add a carboxyl group to substrates (e.g., pyruvate carboxylase in gluconeogenesis). It is not involved in the kynureninase reaction.
Option C: Pantothenic acid (Vitamin B5) is a component of Coenzyme A (CoA) and Acyl Carrier Protein (ACP). CoA is crucial for transferring acyl groups, as seen in the Krebs cycle (acetyl-CoA) and fatty acid metabolism. It is not a direct cofactor for kynureninase.

Visual 1: Flowchart - A diagram of the Tryptophan-Kynurenine pathway. The flowchart should clearly show Tryptophan being converted to Kynurenine, then to 3-OH-Kynurenine. At this point, an arrow should point to the enzyme 'Kynureninase' with 'PLP (from B6)' as a cofactor, leading to 3-Hydroxyanthranilic acid. A second, dotted-line arrow should branch off from 3-OH-Kynurenine, labeled 'B6 Deficiency Shunt', leading to 'Xanthurenic Acid'.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain The question tests the knowledge of the role of water-soluble vitamins as cofactors in metabolic pathways, specifically the degradation of the amino acid tryptophan as background academic context rather than a clinical decision trigger.
The excretion of xanthurenic acid in urine after a tryptophan load is a classic biochemical test used to diagnose Vitamin B6 deficiency.
Nurses should be aware that certain medications, most notably Isoniazid (INH) used to treat tuberculosis, can act as a Vitamin B6 antagonist and induce a deficiency state. Patients on long-term INH therapy are often prescribed pyridoxine supplements to prevent complications like peripheral neuropathy and the metabolic block described in the question.
What if? - What if the patient was pregnant? Pregnancy increases the demand for many nutrients, including Vitamin B6. A relative deficiency can occur, leading to increased xanthurenic acid excretion. This is one reason why pyridoxine is sometimes used to manage nausea and vomiting in pregnancy.

How to Approach the Question

First, identify the key molecules in the question: the substrate (3-OH-Kynurenine) and the alternative product (xanthurenic acid). This points to a specific metabolic pathway.
Recognize that these molecules are part of the tryptophan degradation (kynurenine) pathway.
Recall the major enzymes and their required cofactors in amino acid metabolism. Transamination and decarboxylation reactions very commonly require Pyridoxal Phosphate (PLP).
Connect the cofactor PLP to its parent vitamin, Pyridoxine (Vitamin B6).
Deduce that a deficiency of the cofactor's parent vitamin would impair the enzyme's function, causing the substrate to accumulate and be shunted to an alternative pathway.
Evaluate the other options to confirm their roles are in different types of reactions (e.g., Thiamine in decarboxylation of alpha-keto acids, Biotin in carboxylation).

Concept Tested & Keywords

Concept Tested: The question tests the knowledge of the role of water-soluble vitamins as cofactors in metabolic pathways, specifically the degradation of the amino acid tryptophan.
Stem keywords: cofactor deficiency, 3-OH-Kynurenine, xanthurenic acid pathway, tryptophan metabolism
Lead-in keywords: Deficiency of which
Negative lead-in flag: false

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