γ-GLUTAMYL TRANSFERASE (GGT)

Tapeshwar Yadav(Lecturer)BMLT, DNHE, M.Sc. Medical Biochemistry

Other names :γ-Glutamyl transpeptidase(GGTP),γ-GT.

• GGT is one of a large group of enzymes “Peptidases”.

• A membrane bound enzyme whose active site faces the external side of cell.

• Hepatobiliary tract enzyme.

INTRODUCTION:- GGT - - GT Catalyzes the transfer of - glutamyl

group from peptides or peptide like compounds to an acceptor peptide molecule.

Eg: - glutamyl P- Nitroanilide + Glycylglycine P-nitroanilide +

( Substrate donor) ( Acceptor) - glutamylglycyl glycine (Transfer product)

GGT

GGT acts only on peptides or peptide like compounds containing a terminal glutamate residue joined to the remained compound through the terminal -carboxy.

Glycylgycine is the five times more effective as an acceptor than is either(Glycine or the tripeptide).

The rate of peptidase transfer reaction is considerably faster than that of simple hydrolysis reaction.

GGT plays a major role in glutathione metabolism.

Leukotriene Synthesis Drug Xenobiotic detoxification

• SITE :Some enzymes is present in cytosol, but the

larger fraction is located in the cell membrane. GGT found in many tissues like. Highest tissue activity of this enzyme is found in

kidney. Relatively high activity in canalicular portion of

hepatocyte, pancreas acinar, prostate, bile duct.

• FUNCTIONS : It is involved in the transfer of Amino acids across

the cell Membrane.

Resorption of Amino acids from glomerular filtrate and from the Intestinal Lumen.

In glutathione metabolism by transferring the glutamyl moiety to a variety of acceptor molecule.

BIOCHEMICAL ROLE:• Glutathione is a tripeptide• Glutamyl-cysteinyl-glycine• Synthesis occur in cytosol

GSH

GSH SYNTHETASE

ADP +PI

-GLU-CYS

ADP +PI

GLU-CYS SYNTHASE

GLUTAMATE

5-OXOPROLINASE

ATP ADP + PI

5 - OXOPROLINE

GLU Cyclo transferase

AMINOACID

GLU AMINOACID

GGT

AMINOACID

CYS - GLY

DIPEPTIDASE

CYS

GLY

GAMMA GLUTAMYL CYCLE (Meister Cycle):

LEUKOTRIENE SYNTHESIS :• Leukotriene- Conjugated trienes formed

from Eicosanoic acids in leucocytes by the lipoxygenase pathway. In response to both immunologic and non-inflammatory stimuli.

• Synthesis-LTS are synthesized from Arachidonate + Hydroxy peroxy groups to arachidonic acid and produce Hydroxy peroxy Eicosan tetraenoates (HPETE)

5HPETE Hydroxylase H2O

LTA4

Hydroxylase H2O

LTB4

Glutathione

LTC4

GGT Glutamate

LTD4

Cysteinyl-glycine dipeptidase GlycineLTE4

• Xenobiotic Detoxification : Several xenobiotics undergo detoxification by conjugation to produce less toxic and or more easily excretable compound.

• CONJUGATION : Foreign compound combines with a substance produced in the body. Eg: GlutathioneA wide range of organic compound get conjugated with cysteine of glutathione. The formation of mercapturic acid.

R-X + GSHHX GSH transferase R-SGGlutamate GGTCysteinyl glycineGlycine Cysteinyl glycine Cysteine AcetylcoAN- Acetyl TransferaseCOASH Mercapturic acid

RELEASE OF ENZYME IN TO CIRCULATION : Release of GGT into serum reflects the toxic effect of

alcohol & other drugs on microsomal structure in liver cell.

Cell damage membrane permeability causing cytosolic iso-enzymes to spill into the sinusoid and from these into peripheral blood.

Fragment of hepatocyte membrane rich in GGT activity this membrane fragmented by bile acids.

Bile acids acts as detergent, could solubilize and release GGT from plasma membrane.

SIGNS AND SYMPTOMS OF LIVER DISEASES : Yellowish skin & Eyes. Light coloured stool. Dark coloured urine. Increase fatigue. Loss of appetite. Nausea and vomiting. Itching sensation. Pain and swelling in abdomen.

ESTIMATED METHODS: GGT in diagnostic Enzymology dates from the

introduction in 1969 of a convenient method of assay by “SZASZ”

Method and modifications of it use L - glutamyl P-nitroanilide (Substrate) Glycylglycine (acceptor) serving as the -glutamyl residue

acceptor. Other Substrates:- That have been investigated include the -

glutamyl derivatives of aminopropionitrile. - Napthylamine aniline

GGPNA has been found to be most convenient because it is very sensitive and the P-nitoraniline formed can be directly measured.

Derivatives of GGPNA are also available and have been used in other methods. In these derivatives, various groupd have been introduced into the ring to increase solubility in water.

The most useful substrate is L- - glutamyl 3-carboxy- 4-nitroanilide. Which is readily soluble in water and is split by GGT at a rate comparable to that observed with L-GGPNA.

DETERMINATION OF GGT:• Carboxy substrate method (Kinetic method) Sample - Serum free from hemolysisPrinciple:L--Glutamyl 3-carboxy 4-nitroanilide L- -Glutamyl glycylglycine

+ +Glycylglycine 5-Amino-2-nitrobenzoate

• The rate of formation of 5-amino 2-Nitrobenzoate is measured as an increase in absorbance which is proportional to the GGT activity in the sample.

GGT

Addition Sequence Test (ml)Working reagent 1.0

Incubate at the assay minute

temperature for 1and add

Sample 0.1

PROCEDURE :- Pipette in to a clean dry test tube

Mix well and read the initial absorbance A0 after 1 minute. wavelength at 405 nm on ERBA Chem-5.

Calculations :GGT activity in U/L = A/min ×1158

Normal Values :Males - 10-50 U/L Females - 7-35 U/L

CLINICAL SIGNIFICANE :-• The enzyme present in serum appears to originate

primarily from the Hepatobiliary system.• GGT activity is elevated in all forms of liver disease

like• Obstructive jaundice

– Cholangitis– Cholecystitis– Biliary atresia– Infectious hepatitis

• GGT IN MI: It occurs at about the fourth day and reaches a maximum value another 4th day and probably implies liver damage secondary to cardiac insufficiently.

• GGT IN ALCOHOLICS: Elevated levels are observed alcoholic cirrhosis.

• GGT IN RECEIVING DRUGS: Such as phenytoin (Dilatin) & Phenobarbital raised levels of the (GGT) are found in serum.

• GGT IN PANCREATITIS: Enzyme activity may be 5 to 15 times the upper limit of normal.– Especially if associated with Hepatobiliary

obstruction.

• GGT IN PROSTATIC MALIGNANCY: Elevated levels of GGT are observed. The activity of GGT in sera of males is approximately 50% higher than in sera from females.

• ELEVATED GGT IN: Primary & Secondary metastatic neoplasms.– Alpha 1-antitrypsin deficiency.– Tumors.– Anticonvulsant drugs.– Acute uroneal infection.

DECREASED LEVELS OF GGT:• Chronic renal failure• Hypothyroidism• Hypothalamic malfunctions

ADVANTAGES:• Relatively high sensitivity and specificity

because of their measurement is easy and in expensive.

• Elevated earlier in liver diseases.• Early detection of chronic alcohol misuse.• Enzyme level found correlate with the duration

of the drug action.

SUMMARY: GGT is the most sensitive enzyme indicator of

the Hepatobiliary disease. Normal values are rarely found in the presence

of liver disease. GGT is of little value in attempting to

discrimination between kinds of liver diseases.

Thank You