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

IV. Specific Enzymes
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glycan linkage of starch and glycogen. It has been in use as
a diagnostic enzyme longer than any of the other enzymes.
It is found in very high concentration in the pancreas in
dogs and cats. Human beings and rats have substantial
amounts of salivary amylase activity, but in most species
there is very little. Amylase is also produced in the small
intestines and liver, both of which may contribute to normal
serum amylase activity ( Murtaugh and Jacobs, 1985 ;
Nothan and Callow, 1971). The liver secretes its amylase
in a similar fashion to albumin. Amylase mRNA has been
identified by RT-PCR in dog liver, intestine, fallopian tubes,
and the pancreas ( Mocharla et al. , 1990 ). Pancreatectomy
results in up to a 50% decrease in serum amylase, consistent
with the assumption of nonpancreatic sources of some
serum amylase ( Simpson et al. , 1991 ). There have been several
attempts to identify isoenzymes of amylase in serum
and tissue, with the results dependent on the technique used.
There have been four isoenzymes or isoforms of amylase
identified using cellulose acetate and agarose gel electrophoresis
( Jacobs et al. , 1988 ; Murtaugh and Jacobs, 1985 ;
Simpson et al. , 1989 ). Although fraction three was increased
to the greatest extent with experimentally induced pancreatitis
suggesting this is the true pancreatic amylase, pancreatectomy
did not result in a drop in this fraction ( Murtaugh
and Jacobs, 1985 ). Analysis of amylase isoenzymes has not
found a place in diagnostic veterinary medicine.
Estimates of the half-life of serum amylase in normal
dogs ranges from 1 to 5 h ( Hayakawa et al. , 1992 ; Hudson
and Strombeck, 1978 ; Yacoub et al. , 1969 ). Following
nephrectomy, the half-life increases to 14 h ( Hudson and
Strombeck, 1978 ; Nakashima et al. , 1980 ). Although
nephrectomy increases its serum half-life, less than 1%
of pancreatic amylase infused in normal dogs is found in
urine, suggesting amylase is catabolized by the kidney.
The fact that pancreatic amylase infused into nephrectomized
dogs still clears suggests that there are additional
means of clearing amylase from blood besides the kidneys.
This is supported by an increase in amylase in liver, which
could be prevented by blocking the reticuloendothelial system
following infusion of pancreatic amylase ( Hiatt and
Bonorris, 1966 ).
Serum amylase is routinely used as a screening test for
acute pancreatitis. In experimentally induced pancreatitis
in dogs, increases of eight- to twelve-fold and as much as
29-fold were seen at 1 to 3 days, and returned to normal
in 3 to 5 in one study, and 8 days in another study ( Brobst
et al. , 1970 ; Mia et al. , 1978 ; Simpson et al. , 1989 ). An
increase of serum amylase activity of two-fold or greater
above the reference interval, in the absence of renal disease,
is generally considered suggestive of pancreatitis.
However, the sensitivity of serum amylase activity in the
diagnosis of pancreatitis in dogs is low, with reports of
63% and 78% ( Cook et al. , 1993 ; Jacobs et al. , 1988 ). The
specificity of serum amylase activity has been reported as
77%. Increased serum amylase activity in dogs has been
observed with renal disease, diabetes mellitus, lymphosarcoma,
and hemangiosarcoma ( Strombeck et al. , 1981 ).
Increased serum amylase activity is commonly observed
with renal disease in dogs. Ligation of the renal vessel
results in a 60% increase in serum amylase activity in 48 h
( Hudson and Strombeck, 1978 ). A 2.5-fold increase above
the reference interval was observed in dogs with both
induced and spontaneously occurring renal disease ( Polzin
et al. , 1983 ). Dogs with renal insufficiency may have both
increased serum amylase and urinary amylase ( Corazza
et al. , 1994 ). The increased serum amylase may be due
in part to a decreased glomerular filtration rate, but the
authors also detected a macroamylase in 77% of the dogs
with proteinuria. The increase in urinary amylase is likely
due to reduced renal tubular absorption.
Serum amylase activity for the diagnosis of acute pancreatitis
in cats is considered of little value and has been
shown to actually decrease in experimentally induced pancreatitis
( Kitchell et al. , 1986 ).
Treatment of normal dogs with either low or high
doses of dexamethasone results in a statistically significant
decrease in serum amylase activity ( Parent, 1982 ).
Similar observations have been made in dogs treated with
prednisone (Fittscen and Bellamy, 1984). However, the
stress associated with surgery does not affect the normal
serum amylase activity ( Bellah and Bell, 1989 ; Finco and
Stevens, 1969 ), although there was a transient increase in
serum amylase activity following endoscopic retrograde
pancreatography ( Spillmann et al. , 2004 ).
I . Trypsin and Trypsinogen
Trypsin (EC 3.4.21.4) is a serine proteinase enzyme produced
by the pancreas in the form of the proenzyme
trypsinogen. The pancreas secretes trypsinogen into the
intestine where it is converted by enterokinase to trypsin,
the active proteolytic enzyme. Early attempts to evaluate
pancreatic function by utilizing assays for the tryptic activity
in serum were unsuccessful. This is likely because the
enzyme released into the vascular space is trypsinogen and
not trypsin and therefore has no tryptic activity ( Steiner
and Williams, 1999 ). Hence, the development of speciesspecific
immunoassays for trypsin referred to as trypsinlike
immunoreactivity (TLI) ( Steiner et al. , 1996 ; Williams
and Batt, 1983 ). These immunoassays detect both trypsin
and trypsinogen.
Trypsinogen in serum is thought to be derived primarily
from the pancreas. This is supported by the observation
that pancreatectomy of healthy dogs reduced serum canine
TLI (cTLI) from a mean of 6.2 μg/l to 1.2μg/l (Simpson
et al. , 1991 ). The blood half-life of TLI is not reported, but
it is likely relatively short, as these enzymes are rapidly
scavenged by endopeptidases whose role it is to inactivate
enzymes released from the pancreas ( Zoran, 2006 ). cTLI