science

Kynurenine pathway

Kynurenine pathway is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Kynurenine pathway rather than just read about it. In short: The kynurenine pathway is a metabolic pathway leading to the production of nicotinamide adenine dinucleotide (NAD+). Metabolites involved in the kynurenine pathway include tryptophan, kynurenine, kynurenic acid, xanthurenic acid, quinolinic acid, and 3-hydroxykynurenine.

Key takeaways

Kynurenine pathway belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Kynurenine pathway to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Kynurenine pathway from memory before moving on to harder problems.

Reference excerpt

The kynurenine pathway is a metabolic pathway leading to the production of nicotinamide adenine dinucleotide (NAD+). Metabolites involved in the kynurenine pathway include tryptophan, kynurenine, kynurenic acid, xanthurenic acid, quinolinic acid, and 3-hydroxykynurenine. The kynurenine pathway is responsible for about 95% of total tryptophan catabolism. Disruption in the pathway is associated with certain genetic and psychiatric disorders.

Kynurenine pathway dysfunction Disorders affecting the kynurenine pathway may be primary (of genetic origin) or secondary (due to inflammatory conditions). Peripheral inflammation can lead to a build up of kynurenine in the brain, and this is associated with major depressive disorder, bipolar disorder, and schizophrenia. Dysfunction of the pathway not only causes increase in amounts of metabolites such as quinolinic acid and kynurenic acid but also affects synthesis of serotonin and melatonin. Kynurenine clearance in exercised muscle cells can suppress the build up in the brain.

Hydroxykynureninuria Also known as kynureninase deficiency, this extremely rare inherited disorder is caused by the defective enzyme kynureninase which leads to a block in the pathway from tryptophan to nicotinic acid mononucleotide (NaMN). As a result, tryptophan is no longer a source of niacin, hence not producing NAD, leading to pellagra (niacin deficiency). Both B6-responsive and B6-unresponsive forms are known. Patients with this disorder excrete excessive amounts of xanthurenic acid, kynurenic acid, 3-hydroxykynurenine, and kynurenine after tryptophan loading and are said to suffer from tachycardia, irregular breathing, arterial hypotension, cerebellar ataxia, developmental retardation, coma, renal tubular dysfunction, renal or metabolic acidosis, and even death. The only biochemical abnormality noted in affected patients was a massive hyperkynureninuria, seen only during periods of coma or after intravenous protein loading. This disturbance was temporarily corrected by large doses of vitamin B6. The activity of kynureninase in the liver was markedly reduced. The activity was appreciably restored by the addition of pyridoxal phosphate.

Acquired and inherited enzyme deficiencies Downregulation of kynurenine 3-monooxygenase (KMO) can be caused by genetic polymorphisms, cytokines, or both. KMO deficiency leads to an accumulation of kynurenine and to a shift within the tryptophan metabolic pathway towards kynurenic acid and anthranilic acid. Deficiencies of one or more enzymes on the kynurenine pathway leads to an accumulation of intermediate metabolic products which can cause effects depending on their concentration, function and their inter-relation with other metabolic products. For example, kynurenine 3-monooxygenase deficiency is associated with disorders of the brain (such as schizophrenia and tic disorders) and of the liver. The mechanism behind this observation is typically a blockade or bottleneck situation at one or more enzymes on the kynurenine pathway due to the effects of indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO) and/or due to genetic polymorphisms afflicting the particular genes. Dysfunctional states of distinct steps of the kynurenine pathway (such as kynurenine, kynurenic acid, quinolinic acid, anthranilic acid, 3-hydroxykynurenine) have been described for a number of disorders, for example:

HIV dementia Tourette syndrome Tic disorders Attention deficit hyperactivity disorder (ADHD) Psychiatric disorders (such as schizophrenia, major depressive disorder, bipolar disorder, anxiety disorders) Multiple sclerosis Huntington's disease Encephalopathies Lipid metabolism Liver fat metabolism Systemic lupus erythematosus Glutaric aciduria Vitamin B6 deficiency Eosinophilia-myalgia syndrome Long COVID

Research Research into roles of the kynurenine pathway in human physiology is ongoing.

Neurodegenerative diseases and mental disorders Scientists are investigating the role of dysregulation of this pathway in aging, neurodegenerative diseases, mental disorders, somatic symptom disorders, and chronic fatigue syndrome (CFS).

Kynurenine/tryptophan ratio Changes in the ratio of kynurenine versus tryptophan are reported for many diseases like arthritis, HIV/AIDS, neuropsychiatric disorders, cancer and inflammations. The kynurenin/tryptophan is also an indicator for the activity of indoleamine 2,3-dioxygenase (IDO).

Methods Kynurenine metabolites can be quantified using liquid chromatography coupled to mass spectrometry.

Related substrates In some species, the kynurenine pathway also processes 6-bromotryptophan, leading to the analogous series of brominated metabolites. These and subsequent derivatives are believed to be responsible for the biofluorescence observed in the skin of the swell shark and the chain catshark.

References

Illustrations

Kynurenine pathway: The kynurenine pathway — The kynurenine pathway

Worked examples

Example 1 — a first encounter with Kynurenine pathway

Start with the simplest possible case. Write down what Kynurenine pathway claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Kynurenine pathway before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Kynurenine pathway ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Kynurenine pathway

In research: Kynurenine pathway appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Kynurenine pathway in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Kynurenine pathway is common in secondary-school and first-year university syllabi. It links to neighbouring topics Ageing, Metabolic pathways, so understanding it makes those chapters shorter.
In everyday life: Look for Kynurenine pathway outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Kynurenine pathway in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Kynurenine pathway means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Kynurenine pathway out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Kynurenine pathway in simple terms?

Why does Kynurenine pathway matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Kynurenine pathway?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Kynurenine pathway.