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

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Chapter | 28 Avian Clinical Biochemistry
VI . HEPATOBILIARY DISEASE
A . Clinical Enzymology
Clinical enzymology is described more fully in a separate
chapter in this book, so only a brief synopsis is given here.
Enzymes occur normally in the cytoplasm (e.g., aspartate
aminotransferase [AST], alanine aminotransferase [ALT],
lactic dehydrogenase [LDH]), mitochondria (glutamate
dehydrogenase [GLDH] and AST), nucleus, or membranes
(alkaline phosphatase [AP], gamma glutamyl transferase
[GGT, γ GT]) of body cells, where they catalyze specific
reactions. The distribution of various enzymes is markedly
different among organs and animal species, which explains
the variation in organ and tissue specificities among animal
species. Generally, increased plasma enzyme concentrations
indicate recent organ damage rather then decreased
organ function. Increased enzyme production has been
reported in cholestatic liver disease in mammals (AP and
GGT). Sometimes a decreased activity is of diagnostic
value (e.g., decreased cholinesterase activity in organophosphate
toxicity). Baseline activity of an enzyme in
plasma is generally a reflection of the amount and turnover
of the tissue that contains this enzyme. For example, the
creatine kinase (CK) activity in plasma increases in direct
proportion to the increase of skeletal muscle mass as a
result of training. Increases in CK may also be observed
simply as a result of the muscle cellular damage associated
with capture and restraint. Conversely, in chronic liver
diseases with severe fibrosis and a reduction in the number
of functional hepatocytes, plasma activities of liver
enzymes may be within normal limits. The increase of a
particular enzyme also depends on factors such as its rate
of release, rate of production, and rate of clearance from
plasma. Cytoplasmatic enzymes will be released early in
cell degeneration, whereas mitochondrial enzymes will be
released after advanced cell damage (necrosis). Enzymes
with high tissue concentrations but with short elimination
half-lives are of limited value in clinical enzymology
because of their rapid disappearance from plasma.
Generally, EDTA samples are not appropriate for
enzyme assays, because this anticoagulant may chelate
metal ions, which are required for maximal enzyme activity.
Plasma and cells should be separated immediately
after sampling to prevent leakage of intracellular enzymes
into the plasma. Even if the cellular elements are separated
from the plasma, freezing/thawing and refrigeration
of plasma samples for several days may severely decrease
enzyme activity and therefore should be avoided unless the
effects of the storage procedure used is known.
B . Enzyme Activities in Avian Tissues
Enzyme profiles of the various organs have been studied
in chickens, mallards, turkeys, racing pigeons, budgerigars
and African grey parrots ( Lumeij 1994d ; Figs. 28-13
through 28-15 ).
C . Clearance of Enzymes from Plasma
The half-life of an enzyme is defined as the time required
for its concentration to be reduced by half. When an
enzyme is injected into plasma, its clearance from the
plasma generally follows a biphasic exponential decline.
Initially, there is a rapid decline, which is the primary mixing
or distribution phase, followed by a slower secondary
decline phase, which is the actual clearance of the enzyme
from the plasma. During this secondary phase, a constant
fraction of enzyme present is cleared per unit of time;
hence, the decline is linear on a semilogarithmic scale.
The (t½ β)half-life of the enzyme can be calculated from
the regression function of the secondary linear phase of the
semilogarithmic concentration-time curve and is independent
of plasma enzyme activity.
Clearance half-lives of various enzymes considered to
be of use for the differential diagnosis of liver and muscle
disease in pigeons have been established by studying
the disappearance rates of enzymes from plasma after IV
injections of supernatants of homologous liver and muscle
homogenates ( Lumeij et al ., 1988a ). For AST, ALT, and
LD, the half-lives of the respective enzymes from liver and
muscle were compared, whereas for GLDH and CK, only
liver and muscle were used, respectively ( Table 28-1 ).
D . Experimentally Induced Liver and
Muscle Disease
Plasma enzyme profiles after experimentally or spontaneously
occurring liver disease have been studied in a
number of avian species. The results of studies in racing
pigeons ( Lumeij et al ., 1988a, 1988b ) with two different
types of liver disease were compared to plasma chemistry
changes after muscle injury. Liver disease was induced by
ethylene glycol or D-galactosamine, and muscle injury was
induced by an intramuscular injection of doxycycline in
three groups of six pigeons each. Plasma chemical changes
were correlated with histological findings from organ samples
taken just after the last blood collection ( Fig. 28-16 ,
Table 28-2 ).
Plasma AST activity and bile acid (BA) concentration
were the most sensitive indicators of liver disease in the
racing pigeon, followed by ALT, GGT, and LD. Although
all pigeons with histological proven ethylene glycol- or
galactosamine-induced liver damage had increased AST
activity and BA concentrations in their plasmas, these constituents
were not raised at every sampling time. Increased
plasma GLDH activities were associated with large
necrotic areas in the liver. Moderate necrosis of liver cells
resulted in slightly elevated GLDH activities. Degeneration