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

IV. Specific Enzymes
363
Studies of the blood half-lives of the various alkaline
phosphatase isoenzymes are of interest in that they help to
explain the magnitude and time of increase of serum ALP
activity following insult to an organ. Unlike in most species,
IALP is always identified in rat serum and is often identified
in human serum. It has not been observed in serum
from dogs, cats, horses, and cattle ( Hoffmann and Dorner,
1975 ; Hoffmann et al. , 1983b ) . The clearance of intravenously
injected IALP in dogs has a half-life of shorter than
5.4 min, whereas the half-life of phase 2 of rat IALP is 54.1
to 68.3 min ( Hoffmann and Dorner, 1977 ; Young et al. ,
1984 ). The rate of clearance of IALP from dog blood can be
inhibited by simultaneous injection of galactose-terminated
bovine serum albumin, supporting the theory that IALP is
cleared by hepatocyte asialoglycoprotein or galactose receptors
( Kuhlenschmidt et al. , 1991 ). However, the clearance
of IALP from rat blood is not blocked by asialofetuin but
is blocked by glucosamine-terminated bovine serum albumin,
suggesting that IALP in rats is cleared by mannose/
N-acetylglucosamine-specific receptors on hepatic reticuloendothelial
cells ( Young et al. , 1984 ). The presence of
IALP in rat serum and not dog serum may be explained in
part by the differences in IALP serum half-lives between the
two species. However, the description in rats of a surfactantlike
secretory particle containing ALP may offer a second
mechanism for increased IALP in the blood of rats ( Eliakim
et al. , 1991 ).
The half-life of IALP in cats following intravenous
injection is approximately 4min, and in horses it is approximately
8min, which likely explains in part the absence of
IALP in the serum of these two species ( Hoffmann et al. ,
1977, 1983b ) .
The half-life of intravenously injected LALP and CALP
into dogs is approximately 3 days ( Hoffmann and Dorner,
1977 ). The removal of the ALP under normal conditions
is not known but may involve the slow hydrolysis of
sialic acid from the carbohydrate portion of the molecule
allowing recognition and uptake by the galactose receptor
on hepatocytes. The half-life of intravenously injected
cat LALP is approximately 5.8 h (Hoffmann et al. , 1977 ) .
Likewise canine LALP intravenously injected into cats has
a half-life of approximately 5 h. Therefore, the difference
in half-life of dog LALP and cat LALP (3 days versus 5 h)
is likely a species difference in removal of the enzyme and
not a difference in the enzyme. The mechanism of removal
of ALP from the blood of cats is not known.
The blood half-life of BALP is not known, but has an
electrophoretic migration near that of LALP from dogs,
cats, and horses, suggesting that a full compliment of sialic
acid is present resulting in a half-life similar to LALP.
The half-life of renal and placental ALP from dogs is
less than 6 min ( Hoffmann and Dorner, 1977 ). They have
minimal anodal migration on electrophoresis, suggesting that
they have little or no sialic acid and are likely removed by
the asialoglycoprotein or galactose receptor on hepatocytes.
Approximate percentages of BALP and LALP in adult
dogs are 30% and 70%, respectively, with an increasing
percentage of LALP in older age, whereas in adult horses,
there is approximately 20% BALP and 80% LALP ( Allen
et al. , 2000 ; Hank et al. , 1993 ).
Cholestasis is the most common cause of significant
increases in serum LALP in most species. Experimentally
induced cholestasis in dogs, generally by bile duct ligation,
results in marked increases in serum LALP activity beginning
after approximately 24 h and reaching a maximum of
30 to 40 times normal serum ALP activity at 4 to 7 days
( Guelfi et al. , 1982 ; Shull and Hornbuckle, 1979 ). A similar
response time of ALP increase was seen in two cats following
bile duct ligation, but the magnitude of the increase
was approximately 10% of that seen in the dog ( Hoffmann
et al. , 1978 ). This marked difference in the magnitude
of increase between the two species is in part due to the
12-fold shorter serum half-life of LALP in cats as compared
to dogs. The marked increase of serum LALP in
cholestasis is paralleled by a marked increase in LALP
activity in the liver as well. Numerous studies in rats have
shown that this increase results from increased synthesis,
regulated at the level of transcription or translation ( Kaplan
et al. , 1983 ; Schlaeger, 1975 ).
For many years, the mechanism of increased LALP
in blood was thought to involve regurgitation from bile
through tight junctions into blood. Although there is evidence
of disruptive changes within tight junctions during
cholestasis ( Boyer, 1983 ), it is doubtful if these alterations
permit passage of macromolecules the size of ALP
( Debroe et al. , 1985 ). Studies using a chloledochocaval
shunt model show that within 12 h of shunting of bile or
taurocholic acid into blood, there is a marked induction of
ALP synthesis and appearance of ALP on the basolateral
membranes and a parallel increase in serum ALP ( Ogawa
et al. , 1990 ). This occurs in the absence of increased biliary
pressure and any evidence of alterations in tight junctions
(Toyota et al. , 1983). Bile acids appear to participate
in both induction and release of ALP into serum.
A second model to study the mechanism of release of
ALP from liver into serum utilizes the observation that dogs
acutely treated with high doses of prednisone first induce
LALP synthesis in the absence of cholestasis, as evidenced
by a lack of increase of serum or hepatic tissue bile acids
(Solter et al. , 1994, 1997 ). This model results in the appearance
of easily identified LALP activity on the basolateral
surface and marked increases of serum LALP activity. The
appearance of LALP activity on the basolateral membrane
is likely a transient appearance of the protein before it is
sorted to the apical or bile canalicular membrane as part of
the normal trafficking of bile canalicular membrane proteins
(Bartles et al. , 1987 ; Maurice et al. , 1994 ). The ALP on the
sinusoidal or basolateral membrane is susceptible to release
into blood or hepatic lymph. However, in contrast to the choledochocaval
shunt model, where bile acid concentrations