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

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Chapter | 12 Diagnostic Enzymology of Domestic Animals
anticipated to be low or nonexistent. First, the serum lipase
activity of pancreatectomized dogs remains near 50% of the
presurgery activity, pointing to the presence of nonpancreatic
lipase and strongly suggesting that the reference interval
of serum lipase activity is broadened by nonpancreatic
sources ( Simpson et al. , 1991 ). Second, in a study of dogs
with exocrine pancreatic insufficiency (EPI), there was
no difference in the mean serum lipase activity between
the dogs with EPI and the control dogs, suggesting again
that a significant portion of normal serum lipase is of nonpancreatic
origin ( Steiner et al. , 2006 ). Lipase assays that
are likely specific for pancreatic lipase include a canine
immunoreactive pancreatic lipase assay (cPLI) ( Steiner
et al. , 2003 ). Using immunohistochemistry, it was demonstrated
that test antibody recognizes lipase in pancreatic
acinar cells and no other tissues ( Steiner et al. , 2002 ).
Although there are nonpancreatic lipases and esterases
in serum, there is apparently only one gene for canine
pancreatic lipase ( Mickel et al. , 1989 ). Likewise, only one
protein with a molecular weight of approximately 50.7 kDa
was recovered from dog pancreas by affinity purification
of tissue lipase ( Steiner and Williams, 2002 ), suggesting
that isoenzymes of pancreatic lipase do not exist.
The blood half-life of lipase activity from pancreatic
juice or extracts in dogs is 1 to 3 hours ( Hayakawa et al. ,
1992 ; Hudson and Strombeck, 1978 ). The half-life was
found to increase from 2 to 11 hours following nephrectomy,
suggesting that there is renal clearance or inactivation of
lipase ( Hudson and Strombeck, 1978 ). Only a small amount
of lipase activity is identified in urine, and it is assumed the
renal tubular cells metabolize the filtered lipase.
Activity assays for serum lipase have been used classically
for the diagnosis of acute pancreatitis in dogs.
Typically, an approximately 25-fold increase in serum lipase
activity is observed within 2 to 5 days following experimentally
induced pancreatitis ( Brobst et al. , 1970 ; Mia et al. ,
1978 ; Simpson et al. , 1989 ). This increase tends to parallel
increased serum amylase activity, but it may persist a few
days longer. The sensitivity of serum lipase for the diagnosis
of pancreatitis ranges from approximately 60% to
75% ( Cook et al. , 1993 ; Graca et al. , 2005 ; Mansfield and
Jones, 2000 ). The low sensitivity of some assays is likely
associated with the broad reference range usually seen for
serum lipase (influenced by nonpancreatic lipase activity)
and possibly influenced by the relatively short half-life
of the enzyme in blood. However, in a study using 1,2-odilaryl-rac-glycero-3-glutaric
acid-(6 -methylresorufin) ester
as substrate, a sensitivity of 93% was achieved suggesting
this substrate may be more appropriate for identifying pancreatic
lipase ( Graca et al. , 2005 ) . The specificity of serum
lipase is generally low, ranging from approximately 50% to
75%. These reports point to the variable results associated
with assay methodology. The low specificity may be due
to the wide range of conditions that can lead to increased
serum lipase activity, as well as the release of nonpancreatic
lipase into serum ( Cook et al. , 1993 ; Polzin et al. , 1983 ;
Rallis et al. , 1996 ; Strombeck et al. , 1981 ). Renal failure,
enteritis and gastroenteritis, some hepatic disorders, bile
duct carcinoma, and lymphosarcoma of the gastrointestinal
tract among others are examples of nonpancreatic disease
conditions that have been associated with increased serum
lipase activity. In addition, treatment with glucocorticoids
is routinely associated with increased serum lipase activity.
Dexamethasone-treated dogs had serum lipase increase fourfold
from the baseline activity without histological evidence
of pancreatitis (Fittscen and Bellamy, 1984; Parent, 1982 ).
Whether this increase in serum lipase activity is truly pancreatic
lipase has not been determined.
Serum lipase activity may also be useful in differentiating
severe from mild pancreatitis, with values of greater
than 4010 U/l having a sensitivity of 64% but a specificity
of 100% ( Mansfield et al. , 2003 ). Some of the most
dramatic increases of serum lipase in dogs have been
associated with pancreatic or hepatic neoplasia. In three
pancreatic carcinomas, two endocrine carcinomas involving
liver, and one hepatic carcinoma of unknown origin,
serum lipase activity increased from 11 to 93 times the
upper limit of the reference interval, whereas serum amylase
remained normal ( Quigley et al. , 2001 ).
Six cats with experimentally induced acute pancreatitis
had increases in serum lipase activity of two- to six-fold
within 4 days (Kitchel et al. , 1986). However, there were no
differences in serum lipase activity found between 12 cats
with severe, naturally occurring pancreatitis, and 43 healthy
cats or 31 cats with other diseases ( Parent et al. , 1995 ).
Hence, serum lipase activity assays have little use in the
diagnosis of pancreatitis in cats. However, in a study using
a radioimmunoassay for determining feline pancreatic lipase
immunoreactivity (fPLI), a sensitivity of 100% was demonstrated
for cats with moderate to severe pancreatitis and a
sensitivity of 54% for cats with mild pancreatitis, resulting
in an overall sensitivity of 67% ( Forman et al. , 2004 ). In this
study, a specificity of 100% was observed for healthy cats.
Serum lipase activity has been of little use in the diagnosis
of exocrine pancreatic insufficiency, likely because
of the presence of nonpancreatic lipase masking the loss
of pancreatic lipase activity in serum. In a study of dogs
with EPI and healthy dogs, the range of serum lipase activity
was essentially the same in the two groups ( Steiner
et al. , 2006 ). However, using a cPLI assay, all of the 25 dogs
with EPI in the study had cPLI values below the reference
range, allowing for 100% diagnostic sensitivity. There was
some overlap of the lowest cPLI values in healthy dogs and
the highest values in EPI dogs, resulting in the possibility
of false-positive test results.
H . Amylase
Alpha-amylase (EC 3.2.1.1) is a low-molecular-weight enzyme
(approximately 45 kDa) that cleaves the alpha-D-(1–4)