Serum biliary enzymes usually reflect the obstruction of biliary tract and damages with the biliary capillaries.
All these four serum enzymes rise mainly with different proportions in biliary tract obstruction.
Clinically, the rise in serum levels is also observed in cases with hepatocellular damages.
Of these four biliary enzymes, alkaline-phosphatase and gamma-glutamyl transpeptidase are usually used in clinical practice.
Glutathione (gamma-glutamylcysteinyl-glycine) is one of the peptides that can be hydrolized by this enzyme, thus GGTP is also named as glutathionase at first.
GGTP not only catalyzes the hydrolysis of glutathione but also catalyzes the hydrolysis of other gamma-glutmyl peptides bonds and the transfer of gamma-glutamyl group from donor peptides to suitable acceptors, therefore it was named as gamma-glutamyl transpeptidase later.
Serum GGTP was first reported by Szewzuk and Orlowski in 1960 as a clinical diagnostic procedure in the diagnosis of hepatic diseases using the synthetic substrate alpha-(N-gamma-DL-glutamyl)-aminonitrile by colorimetric method, and later similar results were reported by Goldbarg with another synthetic substrate N-(DL-gamma-glutamyl)-aniline in 1963 and by Oikawa with the method of Szewczuk in 1964. Afterwards, it has been used mostly as the marker of biliary obstruction.
Author began the study of animal experiment and clinical evaluation of the serum GGTP determination in 1968, and reported the clinical usefullness of serum GGTP determination in the diagnosis of liver diseases in 1972. After 25 years of clinical experience, I am still confident on the value of clinical application of serum GGTP determination.
In Taiwan, the NTUH laboratory first applied the test as one of the routine work-up items in clinical practice in 1977, and up to date it has been one of the routine blood biochemical tests in almost all of the hospitals on the island. However, there are a few doctors who are still not familiar with the usefulness of this test.
By the report of Orlowski, the human body contains GGTP by 2,200 units in kidney, 180 u in pancreas, 85 u in liver, 35 u in spleen, 20 u in small intestine, 10 u in brain and 1 u in heart, therefore the ratio of GGTP content is 100 : 8 : 4 for kidney, pancreas and liver.
The biological significance of GGTP still remains to be clarified, thus we still do not know the reason why no rise in serum GGTP is observed in renal disease.