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

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Chapter | 13 Hepatic Function
using the canine reference range will lead to erroneous
conclusions. Additionally, because of the comparatively
low serum GGT activity of cats, assay sensitivity may be a
problem and low GGT activity may be undetectable.
Remarkable elevations of serum GGT have been
observed in dogs and cats with primary hepatic or pancreatic
neoplasia. Although a unique GGT isozyme is associated
with hepatocellular carcinoma in humans, it has not
been determined that a similar phenomenon occurs in dogs
or cats. In humans, GGT also is used in surveillance for
hepatic metastasis but is not suitable for this application in
either the dog or the cat.
Neonatal animals of several species develop elevated
levels of serum GGT activity following the ingestion
of colostrum. In neonatal calves, the direct relationship
between serum GGT activity and immune globulin levels
allows serum GGT activity to serve as a surrogate for successful
passive immune transfer ( Parish et al., 1997 ; Perino
et al., 1993 ). Similar transient neonatal elevations of
serum GGT are observed following ingestion of colostrum
by neonatal lambs ( Maden et al., 2003 ; Tessman et al.,
1997 ), crias ( Johnston et al., 1997 ), and pups ( Center et al.,
1991b ) but apparently not kittens ( Crawford et al., 2006 ;
Levy et al., 2006 ).
B . Serum Bilirubin
Bilirubin in serum is measured by the van den Bergh or
“ diazo ” reaction in which bilirubin is coupled with diazotized
sulfanilic acid. Azo pigments produced by this
reaction are dipyrroles that are stable, and this characteristic
has been useful in studies of the structure of bilirubin
conjugates. Conjugated bilirubin, which is water
soluble, reacts promptly with diazotized sulfanilic acid in
aqueous solution (the van den Bergh “ direct reaction ” ),
but unconjugated bilirubin reacts slowly. Only after addition
of an accelerator such as methanol or ethanol to the
aqueous solution can the diazo reaction with unconjugated
bilirubin be completed ( “ the indirect reaction ” ). It is said
that approximately 10% of the unconjugated bilirubin
in plasma can react with the diazo reagent giving a false
“ direct ” reaction.
The requirement of an organic solvent for the diazo
reaction with unconjugated bilirubin to occur suggests the
delay was related to water insolubility. There is evidence,
however, that intramolecular hydrogen bonding may be
more important than aqueous solubility in determining the
reaction of unconjugated bilirubin with the diazo reagent
( Fog and Jellum, 1963 ; Nichol and Morrell, 1969 ). The
two propionic acid side chains of bilirubin that are esterified
with glucuronic acid or other carbohydrates disrupt
intramolecular hydrogen bonding ( Fog and Jellum, 1963 )
and allow the direct diazo reaction to occur. Accelerators
of the van den Bergh reaction may have a similar effect
FIGURE 13-3 Normal formation, excretion, and enterohepatic circulation
of bilirubin and other bile pigments.
on the intramolecular hydrogen bonds of unconjugated
bilirubin.
The following is a discussion of the physiologic mechanisms
of bilirubin conjugation and interpretation of the Van
den Bergh reaction. There are limitations in clinical application,
however, because of differences that exist in duration
and severity between the spontaneous liver diseases of
domestic animals and experimental liver disease models.
Figure 13-3 summarizes the normal production and
excretion of bilirubin and other bile pigments. Unconjugated
hyperbilirubinemia is observed when there is increased
production of bilirubin (e.g., hemolytic anemia) or when
either hepatic uptake or conjugation of bilirubin is diminished.
Although the unconjugated bilirubin of serum may
be significantly increased in such disorders, essentially
none of the albumin bound unconjugated bilirubin is filtered
by the glomerulus. Consequently, bilirubinuria is not
characteristic in animal patients with unconjugated hyperbilirubinemia.
In hemolytic disease, the amount of bilirubin
excreted by the liver and, therefore, the amount that
reaches the intestine may be remarkably increased. This
results in increased formation and urinary excretion of urobilinogen
( Fig. 13-4 ).
Hyperbilirubinemia of the conjugated type is caused
either by intrahepatic cholestasis ( Fig. 13-5 ) or extrahepatic
bile duct obstruction ( Fig. 13-6 ). When the primary defect
is impaired excretion of bilirubin into bile, hepatic uptake
and conjugation may proceed at a relatively normal rate,
but conjugated bilirubin is effluxed into the plasma. The
plasma concentration of conjugated bilirubin increases, and
the conjugated pigment, which is less avidly bound to albumin,
is readily filtered by the glomerulus, resulting in bilirubinuria
( Fulop et al., 1965 ; Laks et al., 1963 ). Because
bilirubin excretion into the intestine is either significantly
reduced or absent in cholestasis, formation of urobilinogen