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

II. Hepatotoxicity
823
Therefore, ALT is not helpful in evaluating chronic liver
disease. ALT levels also may be elevated by corticosteroid
treatment and some anticonvulsant medications. ALT
is not useful in evaluating hepatic disease in the horse and
cow because of relatively low levels of ALT in hepatocytes
relative to muscle ( Lassen, 2004 ).
Aspartate aminotransferase (AST) is found in several
different cell types including hepatocytes, erythrocytes, and
cardiac and skeletal myocytes. However, it is most useful
in evaluating hepatocellular and muscular injury because
of its high activity in the cells of these tissues. AST is commonly
used as a marker of hepatocyte damage in the equine
and bovine, though muscle damage and hemolysis can also
increase serum activity. AST is less specific for hepatocyte
damage in the dog as compared to ALT ( Stockham
and Scott, 2002 ). As with ALT, AST may not be useful in
detecting chronic liver disease ( Lassen, 2004 ).
Alkaline phosphatase (ALP) activity is found in the
cell membranes of many tissues including liver, bone,
intestine, kidney, and placenta. Serum ALP is primarily of
hepatic origin in the dog, cat, and horse, though bone isoenzyme
can also be detected. Also dogs can have measurable
levels of corticosteroid induced ALP in serum when
the animal is subjected to increased levels of endogenous
corticosteroids or with administration of glucocorticoids
( Stockham and Scott, 2002 ). Hepatic isoenzymes have a
longer half-life (days) than intestinal, renal, or placental
(minutes) isoenzymes. Cholestasis induces hepatic ALP.
An increase in serum ALP can precede hyperbilirubinemia.
Corticosteroids, phenobarbital, dieldrin, and other compounds
may induce hepatic ALP. Increased osteoblastic
activity in hyperparathyroidism, bone healing, or osteosarcoma
may elevate ALP. Horses and ruminants have wide
reference intervals for ALP; therefore, this enzyme has
decreased sensitivity for the detection of cholestatic disease
in these animals ( Lassen, 2004 ; Stockham and Scott,
2002 ).
Gamma glutamyltransferase (GGT) is found in many
cells, but specifically the renal tubular epithelium, canalicular
surfaces of hepatocytes, pancreas, and bile duct
epithelium. The mammary gland is another source of GGT
in cattle, sheep, and dogs, which can result in high serum
levels in neonates of these species after nursing ( Lassen,
2004 ). In renal disease, GGT is excreted in the urine (see
nephrotoxicity). Serum GGT is generally of hepatic origin
and is elevated by cholestasis. GGT has narrower reference
intervals than ALP in horses and ruminants, which makes
it more useful for detecting cholestatic disease in these species
( Lassen, 2004 ; Stockham and Scott, 2002 ). Increased
serum GGT activity proved to be a sensitive and longlived
indicator of liver insult in cattle exposed to moldy
hay ( Casteel et al. , 1995 ). Like alkaline phosphatase, GGT
appears in serum as a result of increased synthesis, rather
than as a result of leakage from cells ( Pearson, 1990 ). In
dogs, the increase of GGT tends to parallel that of ALP.
A high activity of sorbitol dehydrogenase (SDH) is
found in hepatocellular cytoplasm of dogs, cats, horses,
and ruminants ( Lassen, 2004 ). The plasma half-life of this
enzyme is very short, and serum activities may return to
normal within 5 days of hepatocellular insult. Though this
enzyme is more specific for hepatocellular damage than
other enzymes in the horse and ruminant, the relatively low
in vitro stability makes it less commonly used in the dog
and cat compared to ALT.
Lactate dehydrogenase (LDH) is a tetrameric enzyme
with five isoenzymes that catalyze the reversible conversion
of L-lactate to pyruvate in all tissues. All LDH isoenzymes
are found in varying concentrations in all tissues.
LDH 1 is the principal isoenzyme in cardiac muscle and
kidney of most species. It is also found in the liver of cattle
and sheep. Unlike the other isoenzymes, it is heat stable at
65°C for 30 min. LDH 5 is the principal isoenzyme in skeletal
muscle and erythrocytes. Serum LDH activity is tissue
nonspecific; however, necroses of muscle, liver, and hemolysis
are the major causes sources of elevations. Isoenzyme
analysis would improve the specificity of LDH analysis
for hepatocellular damage, but this is not commonly performed
in most veterinary laboratories ( Lassen, 2004 ;
Stockham and Scott, 2002 ).
Bilirubin is derived from destruction of damaged or
senescent erythrocytes by macrophages of the spleen, liver,
and bone marrow. It is noteworthy that bilirubin at physiological
levels is an antioxidant ( Stocker et al. , 1987 ).
Bilirubin is transported in plasma bound to proteins (albumin,
globulin). Hepatic uptake and glucuronide conjugation
render it water soluble. Conjugated bilirubin is secreted
into bile canaliculi and transported to the intestine where
the majority is transformed into urobilinogen by intestinal
flora and excreted. Direct diazo assay for bilirubin detects
conjugated bilirubin. Total bilirubin is measured after addition
of alcohol, which allows additional color development.
Unconjugated bilirubin is determined by the difference in
direct and total bilirubin. Cholestasis results in conjugated
hyperbilirubinemia. Bilirubinuria may occur as a result
of “ regurgitation ” of conjugated bilirubin. Increased ALP
or GGT can precede hyperbilirubinemia in most species.
Hemolysis may result in unconjugated hyperbilirubinemia
and elevations of LDH 5 .
Sulfobromophthalein (BSP) injected intravenously is
removed rapidly from the blood, conjugated by hepatocytes,
and excreted in bile. The rate of hepatic blood flow,
functional hepatic mass, and patency of the biliary system
affect the hepatic clearance of this compound. Altered
blood flow secondary to cardiotoxicity discussed later
may increase BSP retention. The use of this test is limited
as BSP is no longer commercially available, and similar
information can be obtained by assessment of bile acids
and total bilirubin ( Stockham and Scott, 2002 ).
Bile acids are secreted from the liver into the bile and
are subsequently reabsorbed in the intestine. The portal