Polyol pathway

Key takeaways

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

Reference excerpt

The polyol pathway is a two-step process that converts glucose to fructose. In this pathway glucose is reduced to sorbitol, which is subsequently oxidized to fructose. It is also called the sorbitol-aldose reductase pathway. The pathway is implicated in diabetic complications, especially in microvascular damage to the retina, kidney, and nerves. Sorbitol cannot cross cell membranes, and, when it accumulates, it produces osmotic stresses on cells by drawing water into the insulin-independent tissues.

Pathway

Cells use glucose for energy. This normally occurs by phosphorylation from the enzyme hexokinase. However, if large amounts of glucose are present (as in diabetes mellitus), hexokinase becomes saturated and the excess glucose enters the polyol pathway when aldose reductase reduces it to sorbitol. This reaction oxidizes NADPH to NADP+. Sorbitol dehydrogenase can then oxidize sorbitol to fructose, which produces NADH from NAD+. Hexokinase can return the molecule to the glycolysis pathway by phosphorylating fructose to form fructose-6-phosphate. However, in uncontrolled diabetics that have high blood glucose - more than the glycolysis pathway can handle - the reactions mass balance ultimately favors the production of sorbitol. Activation of the polyol pathway results in a decrease of reduced NADPH and oxidized NAD+; these are necessary co-factors in redox reactions throughout the body, and under normal conditions they are not interchangeable. The decreased concentration of these NADPH leads to decreased synthesis of reduced glutathione, nitric oxide, myo-inositol, and taurine. Myo-inositol is particularly required for the normal function of nerves. Sorbitol may also glycate nitrogens on proteins, such as collagen, and the products of these glycations are referred-to as AGEs - advanced glycation end-products. AGEs are thought to cause disease in the human body, one effect of which is mediated by RAGE (receptor for advanced glycation end-products) and the ensuing inflammatory responses induced. They are seen in the hemoglobin A1C tests performed on known diabetics to assess their levels of glucose control.

Pathology While most cells require the action of insulin for glucose to gain entry into the cell, the cells of the retina, kidney, and nervous tissues are insulin-independent, so glucose moves freely across the cell membrane, regardless of the action of insulin. The cells will use glucose for energy as normal, and any glucose not used for energy will enter the polyol pathway. When blood glucose is normal (about 100 mg/dL or 5.5 mmol/L), this interchange causes no problems, as aldose reductase has a low affinity for glucose at normal concentrations. In a hyperglycemic state, the affinity of aldose reductase for glucose rises, causing much sorbitol to accumulate, and using much more NADPH, leaving less NADPH for other processes of cellular metabolism. This change of affinity is what is meant by activation of the pathway. The amount of sorbitol that accumulates, however, may not be sufficient to cause osmotic influx of water. NADPH acts to promote nitric oxide production and glutathione reduction, and its deficiency will cause glutathione deficiency. A glutathione deficiency, congenital or acquired, can lead to hemolysis caused by oxidative stress. Nitric oxide is one of the important vasodilators in blood vessels. Therefore, NADPH prevents reactive oxygen species from accumulating and damaging cells. Excessive activation of the polyol pathway increases intracellular and extracellular sorbitol concentrations, increased concentrations of reactive oxygen species, and decreased concentrations of nitric oxide and glutathione. Each of these imbalances can damage cells; in diabetes there are several acting together. It has not been conclusively determined that activating the polyol pathway damages the microvascular systems.

Endogenous fructose production While the polyol pathway is traditionally associated with diabetic complications due to sorbitol accumulation, recent research has highlighted the metabolic role of the fructose produced. Under conditions of hyperglycemia, the activation of this pathway can lead to significant production of "endogenous fructose" in the liver and kidney. Review articles suggest that this endogenously produced fructose is metabolized by ketohexokinase, leading to ATP depletion, uric acid generation, and the stimulation of lipogenesis. This mechanism has been proposed as a contributing factor to the development of fatty liver and metabolic syndrome.

References

Further references Harper's Illustrated Biochemistry(Published by LANGE) Dinesh Puri's Medical Biochemistry(Published by ELSEVIER)

Worked examples

Example 1 — a first encounter with Polyol pathway

Start with the simplest possible case. Write down what Polyol pathway claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, 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 Polyol 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 Polyol 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 Polyol pathway

In research: Polyol pathway appears in chemistry 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 Polyol 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: Polyol pathway is common in secondary-school and first-year university syllabi. It links to neighbouring topics Clinical chemistry, Diabetes, Metabolic pathways, so understanding it makes those chapters shorter.
In everyday life: Look for Polyol 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.

How to study Polyol pathway in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Polyol 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 Polyol pathway out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Polyol pathway in simple terms?

The polyol pathway is a two-step process that converts glucose to fructose. In this pathway glucose is reduced to sorbitol, which is subsequently oxidized to fructose.

Why does Polyol pathway matter?

Because it connects several chemistry 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 Polyol 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 Polyol pathway.