Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

IV. Specific Enzymes
359
GGT is a membrane-bound enzyme on the external surface
of cells and is bound to the cell membrane via a hydrophobic
transmembrane peptide. The cellular location of GGT
is of interest as it affects the organ specificity when used as
a diagnostic test in serum. GGT is located on the luminal
surface of the proximal tubular cells of the kidney where
it is shed into urine during tubular injury. In the pancreas,
GGT is located on the luminal surface of cells lining the
acini and pancreatic ducts. Increases of serum GGT activity
are not generally associated with injury to the pancreas
or kidney. The location of GGT in liver is of considerable
interest because the liver likely contributes most if not all
of the serum GGT activity. In liver, GGT activity is primarily
associated with the biliary epithelial cells. Early reports
suggested GGT was not present on hepatocyte membranes;
however, when GSH was used in the fixative solution to
prevent fixation-induced inhibition of GGT activity, GGT
activity was identified on both the canalicular and sinusoidal
surfaces of hepatocytes of rats, although to a considerably
lesser extent than on biliary epithelial cells ( Lanca and Israel,
1991 ). GGT mRNA has been detected in hepatocytes of normal
rats and rats depleted of glutathione, supporting the presence
of GGT on hepatocytes ( Moriya et al. , 1994 ). Whether
GGT activity is present on the hepatocyte membranes
of domestic animals is not documented to our knowledge.
Efforts have been made to determine the presence of
isoenzymes of GGT in laboratory animals and humans, but
it is likely that only one form of GGT exists. Variations in
sialic acid content have been reported and likely explain
the ability to separate different fractions of GGT by various
means, such as isoelectric focusing ( Mortensen and
Huseby, 1997 ). Only one band of GGT activity was found
with cellulose acetate electrophoresis of serum from dogs
( Milne and Doxey, 1985 ).
Removal of GGT activity from blood likely involves
endocytosis by the asialoglycoprotein or galactose receptor,
as purified human liver GGT that has been fractionated
by ion exchange chromatography and infused into rats has
shown that the slowest rate of clearance is associated with
the most sialated forms (Morensen and Huseby, 1997).
Clearance can be blocked with asialofetuin, also suggesting
that clearance of GGT is via the asialoglycoprotein or galactose
receptor on hepatocytes. GGT is apparently eliminated
from blood without prior desialylation via exposed galactose
units binding with low affinity to the receptor. The rate of
removal of each molecule may be related to the number of
available galactose molecules not blocked by sialic acid.
The half-life of GGT activity in blood is not known,
as there have been no definitive studies done in domestic
animals to our knowledge. However, in dogs, serum GGT
and ALP activity increase and decrease in parallel during
cholestasis, suggesting that in dogs the half-life of GGT
may be similar to the approximate 3-day half-life of liver
ALP. A half-life of 3 days is also suggested for GGT activity
in horses ( Barton and Morris, 1998 ). In any event, the
half-life of liver GGT activity is of sufficient length that
increases are maintained throughout the disease process
and diagnostic sensitivity is not lost because of a rapid
return of the GGT activity to reference ranges.
Conditions such as hepatic necrosis and reversible hepatocellular
injury induced by CCl 4 , dibromobenzene, chloroform
toxicity, and trauma result in minimal or no changes in
serum GGT activity in dogs ( Barakat and Ford, 1988 ; Guelfi
et al. , 1982 ; Noonan and Meyer, 1979 ). However, some
increase in serum GGT activity is observed with chloroform
toxicity in horses, although no increase is observed in calves
and sheep ( Barakat and Ford, 1988 ). Following administration
of CCl 4 to ponies, a four-fold increase in serum GGT
activity was observed, which persisted up to 10 days with no
increase in serum ALP activity ( Hoffmann et al. , 1987 ). The
minimal increase in GGT activity in serum following hepatocellular
injury is likely due to the fact that GGT in liver
is primarily associated with biliary epithelial cells and the
absence of significant increase of bile acids to release the
hydrophobic transmembrane attachment to the cell membrane.
Increases in serum GGT are most often observed with
cholestasis and conditions resulting in biliary hyperplasia in
all species, as observed in experimentally induced cholestasis
(DeNovo and Prasse, 1983; Hoffmann et al. , 1987 ; Shull
and Hornbuckle, 1979 ; Spano et al. , 1983 ).
Serum GGT activity is an especially useful clinical indicator
of cholestasis in horses and cattle because of relatively
high liver GGT activity compared to dogs and cats. Serum
GGT activity in horses and cattle has relatively higher sensitivity
for the identification of cholestatic disorders than serum
ALP activity. Only a two-fold increase of serum ALP activity
was observed with cholestasis, whereas serum GGT activity
rose nine-fold in horses ( Hoffmann et al. , 1987 ). Although
the magnitude of increase is greatest in cholestasis, serum
GGT activity can be used in large animals as a screening
test for generalized hepatic disease as well. In a retrospective
study of 50 cases of hepatic disease in horses, of the serum
chemistry parameters evaluated only serum GGT activity
increased in all cases ( McGorum et al. , 1999 ). Similarly,
serum GGT activity showed 75% sensitivity and 90% specificity
for detecting subclinical liver disease in horses exposed
to pyrrolizidine alkaloids, whereas serum ALP activity
showed only 58% sensitivity ( Curran et al. , 1996 ). Serum
GGT activity was shown to be the most sensitive serum
enzyme for detection of hepatic injury secondary to proximal
enteritis in horses ( Davis et al. , 2003 ). Increases in serum
GGT activity in numerous cases of plant-related hepatotoxicity
in both cattle and horses have also been reported ( Craig
et al. , 1991 ; Curran et al. , 1996 ; Mendel et al. , 1988 ) .
Serum GGT activity in cats and dogs is often interpreted
in conjunction with serum ALP activity. The suggested
advantage of serum GGT activity determination
over serum ALP activity in dogs is increased specificity, as
GGT activity is derived solely from liver whereas serum
ALP activity is derived from bone and liver as well as the