Gamma-Glutamyl Transferase (GGT)

Gamma-glutamyl transferase (GGT) is a membrane-bound enzyme that transfers gamma-glutamyl groups and plays a central role in glutathione metabolism. In the liver panel its main value is as a sensitive but non-specific marker of hepatobiliary disease: it rises in cholestasis and is the standard test used to confirm that a raised alkaline phosphatase is of hepatic rather than bony origin.

Definition

Gamma-glutamyl transferase is a cell-surface enzyme that catalyses the transfer of gamma-glutamyl moieties and participates in glutathione metabolism; its serum activity is a sensitive but non-specific marker of hepatobiliary disease, used chiefly to confirm a hepatic source of raised alkaline phosphatase.

Scope

The entry covers the biochemistry of GGT, its role in the gamma-glutamyl cycle of glutathione handling, why it rises in cholestasis, its use to localise a raised alkaline phosphatase to the liver, and the reasons for its limited specificity. It treats GGT as a clinical-biochemistry topic and is not guidance for interpreting an individual's results.

Core questions

• What reaction does GGT catalyse and what is its role in glutathione metabolism?
• Why does serum GGT rise in cholestasis?
• How is GGT used to confirm a hepatic source of a raised alkaline phosphatase?
• Why is GGT sensitive but not specific for liver disease?

Key concepts

• Transfer of gamma-glutamyl groups
• Gamma-glutamyl cycle and glutathione metabolism
• Membrane localisation in biliary epithelium and hepatocyte canaliculus
• Cholestatic marker
• Enzyme induction (alcohol, certain drugs)
• Use to localise raised alkaline phosphatase to the liver
• High sensitivity, low specificity

Mechanisms

GGT is anchored to the external surface of cell membranes, with high expression in the biliary epithelium and the canalicular pole of hepatocytes as well as in kidney, pancreas, and intestine. It catalyses the transfer of the gamma-glutamyl group of glutathione and related peptides to acceptors, the first step in the gamma-glutamyl cycle by which extracellular glutathione is broken down and its constituent amino acids salvaged. In cholestasis, retained bile acids solubilise the enzyme from membranes and increase its synthesis, raising serum activity, so GGT rises in parallel with alkaline phosphatase in hepatobiliary disease. Because bone does not contain GGT, a raised GGT alongside a raised alkaline phosphatase indicates a hepatic source, which is its main practical use. GGT is also induced by alcohol and some drugs, and is elevated in many non-hepatobiliary conditions, which is why it is sensitive but not specific.

Clinical relevance

Serum GGT is part of the extended liver panel and the standard marker for confirming the hepatobiliary origin of a raised alkaline phosphatase. This entry explains the enzyme and why its activity changes; it describes how the marker is generated and interpreted at the level of biochemistry and patterns, and is not a basis for diagnosing or treating any individual.

Epidemiology

GGT is among the most sensitive enzymes for detecting hepatobiliary disease, but its frequent elevation in alcohol use, metabolic disorders, and with enzyme-inducing drugs limits its specificity; population studies have also examined associations of serum GGT with cardiometabolic outcomes, reflecting its links to oxidative stress and glutathione metabolism.

Evidence & guidelines

A comprehensive review of gamma-glutamyl transferase and clinical guidance on abnormal liver chemistries describe the enzyme's biochemistry, its use to localise a raised alkaline phosphatase, and the limits of its specificity.

History

Gamma-glutamyl transferase was characterised through work on the gamma-glutamyl cycle of glutathione metabolism and was subsequently adopted in clinical chemistry as a sensitive marker of hepatobiliary disease and, in particular, as a complement to alkaline phosphatase for confirming a hepatic source of enzyme elevation.

Debates

What is the clinical value of GGT given its low specificity?

GGT is highly sensitive to hepatobiliary disease but rises in many other settings, so its usefulness is debated; its clearest established role is confirming the hepatic origin of a raised alkaline phosphatase rather than serving as a stand-alone screening test.

Seminal works

• whitfield-2001

Frequently asked questions

Why is GGT measured together with alkaline phosphatase?

Because bone tissue does not contain GGT, a raised GGT accompanying a raised alkaline phosphatase indicates that the ALP elevation is of hepatobiliary rather than bony origin.

Why is a raised GGT not specific for liver disease?

GGT is present in several tissues and is readily induced by alcohol and some medications, and it rises in many non-hepatobiliary conditions, so an isolated elevation must be interpreted cautiously.