SH groups

SH (thiol) groups: glutathione and protein thiols, thiol/disulfide balance and oxidative stress.

SH groups (thiol or sulfhydryl groups) are chemical groups of one sulphur and one hydrogen atom found in many biologically active molecules: the amino acid cysteine, the tripeptide glutathione, coenzyme A, lipoic acid and many proteins, especially albumin. Thiols are easily oxidised and are among the body's most important antioxidants: they neutralise reactive oxygen species and peroxides, take part in detoxification of foreign compounds and protect enzymes and membranes. When oxidised, two SH groups join into a disulfide bond (S–S); this reversible thiol/disulfide balance is an indicator of oxidative stress.

Oxidative stressAntioxidantsGlutathione

Main plasma thiolalbumin (cysteine-34)

Main cellular thiolglutathione

Measured asnative and total thiols, disulfides

Reference valuesmethod-dependent

Figure 1. Thiol groups are oxidised to disulfides and reduced back. The balance between them reflects the body's antioxidant capacity.

Measurement

Thiols are measured spectrophotometrically, classically with Ellman's reagent. An automated method measures native thiols and total thiols; the difference gives the disulfides, and their ratios describe thiol/disulfide homeostasis. Reference values depend on the method, so results must be compared with the laboratory's or the application's own range.

Clinical significance

Reduced thiol levels and a shift towards disulfides have been described in many conditions linked to oxidative stress: diabetes, cardiovascular disease, chronic kidney and liver disease, inflammatory and autoimmune diseases, sepsis, cancer and smoking, and after exposure to toxins and radiation. Thiol-containing compounds (for example N-acetylcysteine, which replenishes glutathione) are used as antidotes in paracetamol poisoning. A thiol test is a marker of oxidative balance, not a diagnosis of a specific disease.

Why it matters

A balanced diet rich in vegetables and fruit, adequate protein, not smoking, limiting alcohol and treating chronic diseases help maintain the body's antioxidant defences.

References

Erel O, Neselioglu S. A novel and automated assay for thiol/disulphide homeostasis. Clin Biochem. 2014;47:326–332. doi:10.1016/j.clinbiochem.2014.09.026
Jocelyn PC. Biochemistry of the SH Group. London: Academic Press; 1972.

This article is for general information and does not replace a consultation with a doctor. Laboratory results must be interpreted by a physician together with symptoms and other tests.

Published on 22 April 2024