CPD Article | EQUINE
*Only for in-house use where analyses can be performed
immediately as this enzyme is highly labile.
Glutamate Dehydrogenase (GLDH)
GLDH as a cytosolic enzyme with elevations seen in the
presence of acute hepatocellular damage. If hepatic
damage persists for any period, the GLDH will return to
normal. This is a mitochondrial enzyme found mainly in
liver, with lesser amounts documented in heart muscle,
kidney and intestine. The GLDH activity in these non-
hepatic tissues is relatively small compared to that
found in liver. It is a relatively stable enzyme and is a
suitable liver function test replacement for sorbitol
dehydrogenase (SDH), in samples transported to a
diagnostic laboratory.
In horses, increases in serum GLDH activity are
considered liver specific. Its relatively short serum
half-life of 14 hours indicates that increased levels are
associated with acute active hepatic damage (ischemia,
hepatic toxicity). Non-specific increases of GLDH
are documented in young foals, and so it should be
interpreted with caution in this age group.
The sensitivity of GLDH activity for detection of hepatic
necrosis, hepatic lipidosis, and hepatic cirrhosis has
been reported at 78%, 86%, and 44%, respectively. The
sensitivity is considered higher than that of SDH and
comparable to AST.
Gamma Glutamyl Transferase (GGT)
GGT is predominantly associated with the brush border
or microvilli on the canalicular surfaces of hepatocytes,
biliary epithelial cells, renal tubular epithelial cells,
pancreatic acinar cells and mammary gland epithelial
cells. Increased GGT activity in serum is due to enzyme
induction involving hepatocytes or biliary epithelium,
increases of serum GGT activity are not generally
associated with primary injury to the pancreas or kidney.
Elevations in serum levels are seen in the presence of
acute hepatitis, chronic liver cirrhosis and in very rare
cases of pancreatitis. Following injury, the mean average
half live of equine GGT is 3 days.
Although mild to moderate increases in serum GGT are of
limited diagnostic or prognostic value, it is nevertheless
very unusual to find significant hepatopathy in horses
in the absence of increased serum GGT. Additionally,
marked increases in serum GGT concentration are
associated with a poor prognosis.
Alkaline Phosphatase (ALP)
Elevations in this brush border enzyme are most
commonly observed with chronic biliary obstruction
(cholestasis). High levels are also seen with abnormalities
of bone metabolism and intestinal malfunction.
Although many tissues or cell types have some ALP
activity, cells from liver, bone, kidney, intestinal mucosa,
and placenta have the greatest ALP activity. These
various isoenzymes can be differentiated by certain
reference laboratories.
Cholestasis causes significant increases in serum of ALP
in horses and frequently precedes the development of
hyperbilirubinemia. ALP increases within 48 hours of
liver damage and the highest increases are observed
with cholangitis, biliary cirrhosis and extra bile duct
obstruction. Increases in ALP are also reported
following administration of some drugs (phenobarbital,
corticosteroids and others). Care needs to be taken
when interpreting ALP levels in pregnant mares, due
to placental origin sources of this enzyme. Caution
should also be applied when evaluating ALP levels in
growing animals, where normal values are 2 or 3 times
higher than the reference values of adults, because
of increased bone turnover associated with physical
growth. Increases in bone ALP can also be observed in
• Volume 22 Issue 01 | March 2020 •
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