THE MERCK VETERINARY MANUAL
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Enzyme Activity in Hepatic Disease in Small Animals

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Liver disease is often first suspected based on increased liver enzyme activity. However, abnormally increased liver enzyme activity is considerably more common than the prevalence of liver disease. A wide spectrum of nonhepatic disorders may influence liver enzyme activity. Liver enzyme measurements are not liver function tests; rather, they reflect hepatocyte membrane integrity, hepatocyte or biliary epithelial necrosis, cholestasis, or induction phenomenon.

The pattern of liver enzyme abnormalities in relation to the signalment, history, total bilirubin concentration, serum bile acid values, and comorbid conditions/medications, provides the first indication of a liver-specific disorder. The full assessment of the liver enzyme aberration considers: 1) the predominant pattern of enzyme change (hepatocellular leakage enzymes vs cholestatic enzymes); 2) the magnitude of increase of enzyme activity above the normal reference range (mild is <5× the upper reference range, moderate is 5–10×, marked is >10×); 3) the rate of change (increase or resolution); and 4) the nature of the course of change (fluctuation vs progressive increase or decrement). Up to 2.5% of “normal” individuals can have borderline abnormal enzyme values.

Recognizing whether enzyme abnormalities are persistent or cyclic helps categorize likely causes. Investigating liver function with paired fasting and postprandial total serum bile acid (TSBA) or urine bile acid/creatinine measurements (urine collected 4–8 hr after meal ingestion) may expedite a decision to pursue liver biopsy when clinical signs remain vague and enzymes are only mildly increased. Imaging studies assist in detecting primary underlying disorders that have secondarily influenced the liver causing increased enzyme activity.

Age-appropriate reference ranges for serum liver enzyme activity are essential in puppies and kittens. Plasma enzyme activities of AP and GGT in neonatal dogs and cats are remarkably higher than those of adults. Differences reflect physiologic adaptations during the transition from fetal and neonatal life stages, colostrum ingestion, maturation of metabolic pathways, growth effects, differences in volume of distribution and body composition, and nutrition. Serum activity of AP, AST, CK, and LDH in neonates usually increase greatly during the first 24 hr. In kittens, serum activity of AP, CK, and LDH exceed adult values through 8 wk of age. Serum AP increases remarkably in day-old puppies and kittens after colostrum ingestion; this phenomenon is also seen in neonatal calves, lambs, pigs, foals, and human infants.

AST and ALT are commonly measured for detection of liver injury; both are present in high concentrations in liver but are also in other tissues. AST activity is higher in kidney, heart, and skeletal muscle than in liver, while ALT activity is highest in liver. Because hepatic ALT activity is 10,000-fold greater than plasma enzyme activity in healthy animals, it has high diagnostic utility for “liver lesions.” The cytosolic location of transaminases allows their immediate release with even minor change in hepatocellular membranes. Unfortunately, indiscriminant leakage limits their diagnostic utility. Nonetheless, duration and magnitude of transaminase activity measured serially can predict disease activity and severity and the number of involved cells.

Hepatic transaminases increase with muscle injury as well as vigorous physical activity in dogs. Persistence of transaminases in plasma contributes to their sustained high activities in certain disorders. Because catabolism of transaminases occurs by absorptive endocytosis at the hepatocyte sinusoidal border, slow enzyme clearance may sustain plasma enzyme activity in hepatic insufficiency (portosystemic shunting, nodular regeneration, hepatic fibrosis).

Alanine Aminotransferase:

The largest increases in ALT develop with hepatocellular necrosis and inflammation. After acute severe hepatocyte necrosis, serum ALT activity increases sharply within 24–48 hr to values >100-fold normal, peaking during the first 5 days after injury. If the injurious event resolves, ALT activity gradually declines to normal over 2–3 wk. While this pattern is considered classic, some severe hepatotoxins are not associated with increased ALT activity because of inhibited gene transcription or other interference with ALT biosynthesis (eg, aflatoxin B1 hepatotoxicity, microcystin hepatotoxicity). A declining ALT also may represent a paucity of viable hepatocytes in end-stage chronic hepatitis.

Examples of classic necrotizing hepatotoxins are carbon tetrachloride, acetaminophen, and nitrosamine. A single exposure to carbon tetrachloride causes an acute sharp increase in ALT that resolves over the ensuing week. Hepatotoxicity induced by acetaminophen causes a marked increase in ALT and AST within 24 hr that may decline within 72 hr to near normal values. This toxin is highly dose dependent in dogs and cats. Cats are exceedingly susceptible, with hematologic signs dominating after as little as 125 mg. In dogs, a dosage of 200 mg/kg may be life-threatening. Hepatocellular necrosis induced by nitrosamines increases plasma ALT activity, but not significantly, until after 1 wk of intermittent chronic exposure. ALT activity persists for weeks until the necrosis resolves. Low-grade hepatocellular degeneration, observed in some dogs with congenital portosystemic shunts, reflects delayed enzyme clearance and low-grade hepatocyte dropout; most of these dogs lack histologic features consistent with ALT release.

Acute hepatic necrosis caused by infectious canine hepatitis increases plasma ALT activity by 30-fold, peaking within 4 days. Thereafter, chronic sustained ALT activity persists as chronic hepatitis develops in dogs unable to clear the virus. Hepatic injury induced by toxins usually causes plasma ALT activity to increase, peak, and return to normal sooner than it does in infectious viral hepatitis. Chronic hepatitis, a persistent necroinflammatory disorder in dogs, is associated with varying severities of necrosis and fibrosis and cyclic disease activity associated with plasma enzyme “flares.” At times, plasma ALT activity is >10-fold normal. Enzyme fluctuations contrast with profiles associated with single injurious events. In dogs with hepatitis, serum ALT activity declines as injury resolves, but serum ALP activity may increase as a result of regenerative processes. Dogs treated with glucocorticoids may develop mildly increased ALT activity that resolves within several weeks of glucocorticoid withdrawal.

Despite high sensitivity of ALT for identifying liver disorders, its lack of specificity for differentiating clinically significant liver disease, specific histologic abnormalities, or hepatic dysfunction requires that it be interpreted in conjunction with other diagnostic tests.

Aspartate Aminotransferase:

AST is present in substantial concentrations in a wide variety of tissues. Increased AST activity can reflect reversible or irreversible changes in hepatocellular membrane permeability, cell necrosis, hepatic inflammation, and in dogs, microsomal enzyme induction. After acute diffuse severe hepatic necrosis, serum AST sharply increases during the first 3 days to values 10- to 30-fold above normal in dogs and up to 50-fold above normal in cats. If necrosis resolves, AST activity gradually declines over 2–3 wk. In most cases, AST parallels changes in ALT activity.

While increased AST activity in the absence of abnormal ALT activity implicates an extrahepatic enzyme source (notably muscle injury), there are clinical exceptions that may relate to severity and zonal location of hepatic damage. In some cats with liver disease, AST is a more sensitive marker of liver injury than ALT (eg, hepatic necrosis, cholangiohepatitis, myeloproliferative disease, hepatic infiltrative lymphoma, and EHBDO). A similar trend is evident in some dogs. Dogs treated with glucocorticoids may develop mildly increased AST activity that resolves within several weeks of glucocorticoid withdrawal.

Increased AP activity in dogs is the most common biochemical abnormality on routine biochemical testing; its high sensitivity and low specificity can defy diagnostic interpretation without a liver biopsy. AP activity in dogs has the lowest specificity of routinely used liver enzymes as a result of complexity associated with induction of different isozymes.

In dogs and cats, tissues containing highest AP activity (in descending order) are intestine, kidney (cortex), placenta (dogs only), liver, and bone. Distinct serum AP isozymes can be extracted from some of these tissues in each species; eg, bone (B-AP), liver (L-AP), and glucocorticoid-induced (G-AP) isoenzymes in canine serum. In dogs, L-AP and G-AP are primarily responsible for high serum AP activity, whereas L-AP is primarily responsible in cats. Increased AP activity develops in up to 75% of hyperthyroid cats, depending on the chronicity of the condition with B-AP substantially contributing.

The comparably small magnitudes of AP activity in cats with liver disease (2- to 3-fold normal) relative to dogs (usually >4- to 5-fold) reflect the lower specific activity of AP in feline liver and its shorter half-life. Nevertheless, AP activity remains clinically useful in the diagnosis of feline liver disease when the species-appropriate perspective is maintained.

The utility of serum AP activity as a diagnostic indicator in dogs is complicated by the common accumulation of L-AP and G-AP isozymes, which can both be induced by steroidogenic hormones.

Because the B-AP isozyme increases secondary to osteoblast activity, it is detected in young growing animals and in animals with bone tumors, secondary renal hyperparathyroidism, and osteomyelitis. However, the minor contribution of B-AP to total serum AP activity usually does not lead to an erroneous diagnosis of cholestatic liver disease. Bone remodeling secondary to neoplasia may not substantially affect serum AP activity or may cause only a trivial increase (2- to 3-fold) in dogs. In young growing cats, increased B-AP activity may simulate enzyme activity seen in hepatobiliary disease.

While ALT is immediately released from the hepatocellular cytosol in acute hepatic necrosis, the small quantities of membrane-bound AP are not. It takes several days for induction of membrane-associated enzyme to “gear up” and spill into the systemic circulation. Increased serum AP reflects enhanced de novo hepatic synthesis, canalicular injury, cholestasis, and solubilization of its membrane anchor (by bile salts). The largest increases in serum AP activity (L-AP and/or G-AP ≥100-fold normal) develop in dogs with diffuse or focal cholestatic disorders, massive hepatocellular carcinoma, bile duct carcinoma, and those exposed to steroidogenic hormones.

While serum activity of AP may be normal or only modestly increased in dogs with metastatic neoplasia involving the liver, it may be increased dramatically in mammary neoplasia. High serum AP activity develop in ~55% of dogs with malignant and 47% with benign mammary tumors; highest AP activity is observed in dogs with malignant mixed tumors. Nevertheless, serum AP has no value as a diagnostic or prognostic marker in mammary cancer; it remains unclear whether disease remission (surgical, chemotherapy) is followed by a regression in serum AP activity or whether serum AP activity functions as a paraneoplastic marker.

After acute severe hepatic necrosis, AP activity increases 2- to 5-fold in dogs and cats, stabilizes, and then gradually declines over 2–3 wk. Sustained AP activity usually correlates with biliary epithelial hyperplasia. In cats, EHBDO results in a 2-fold increase in AP within 2 days, as much as a 4-fold increase within 1 wk, and up to a 9-fold increase within 2–3 wk. Thereafter, activity stabilizes and gradually declines but usually not into the normal range; the declining enzyme activity coordinates with developing biliary cirrhosis. Inflammatory disorders involving biliary or canalicular structures or disorders compromising bile flow increase serum AP activity secondary to membrane inflammation/disruption and local bile acid accumulation. In both dogs and cats, similar increases in serum AP activity develop in spontaneous intrahepatic cholestasis or obstruction involving the extrahepatic biliary structures. Consequently, AP activity cannot differentiate between intra- and extrahepatic cholestatic disorders.

Many extrahepatic and primary hepatic conditions are associated with increased L-AP. In cats, HL (see Feline Hepatic Lipidosis) is associated with profound increases in total AP activity and marked jaundice. The increased AP appears to reflect canalicular dysfunction or compression. While AP in cats is rarely affected by anticonvulsants or glucocorticoids, it can increase with diabetes mellitus, hyperthyroidism, and pancreatitis.

In dogs, primary hepatic inflammation as well as systemic infection or inflammation and exposure to steroidogenic hormones may induce a vacuolar hepatopathy (VH). When severe, VH has a cholestatic effect causing canalicular compression. While VH was initially characterized as a glucocorticoid-initiated lesion, it is now established that nearly 50% of dogs with VH lack overt exposure to steroidogenic substances. Chronically ill dogs may produce the G-AP isozymes secondary to stress-induced endogenous glucocorticoid release. Chronically ill dogs with VH (lacking exogenous glucocorticoid exposure) may demonstrate normal dexamethasone suppression and adrenocorticotropic (ACTH) response tests. In some dogs, high AP associated with VH signals the presence of atypical adrenal hyperplasia associated with abnormal sex hormone production. There is no consistent relationship between the magnitude of serum AP activity, the presence of high G-AP activity, and the histologic lesion. Unfortunately, G-AP is not useful for syndrome characterization because it can become the predominant enzyme in dogs treated with glucocorticoids, with spontaneous or iatrogenic hyperadrenocorticism, with hepatic or nonhepatic neoplasia, with inflammation, and with many different chronic illnesses, including primary liver disease.

The magnitude of AP activity induced by administration of exogenous glucocorticoids depends on the type of drug administered, the dose given, and the individual's response. The production of G-AP does not imply that a dog treated with cortisone has iatrogenic hyperadrenocorticism, a suppressed pituitary adrenal axis, or a clinically important VH. By comparison, the feline liver is relatively insensitive to glucocorticoids.

In dogs, serum total AP activity and L-AP isozyme also may be induced by administration of certain anticonvulsants (phenobarbital, primidone, and phenytoin) and other drugs; AP activity usually increases 2- to 6-fold. In contrast, serum AP and L-AP did not increase in cats after administration of phenobarbital (0.25 grain, bid) for 30 days.

GGT is a membrane-bound glycoprotein that plays a critical role in cellular detoxification and confers resistance against a number of toxins and drugs. Highest tissue concentrations of GGT in dogs and cats are located in the kidney and pancreas, with lesser amounts in the liver, gallbladder, intestines, spleen, heart, lungs, skeletal muscle, and erythrocytes. Serum GGT activity is largely derived from the liver, although there is considerable species variation in its localization within this organ.

Acute, severe, diffuse necrosis is associated with either no change or only mild increases (1- to 3-fold normal) in GGT activity that resolve in ~10 days. In dogs with EHBDO, serum GGT activity increases 1- to 4-fold above normal within 4 days, and 10- to 50-fold within 1–2 wk. Thereafter, values may plateau or continue to increase as high as 100-fold. In cats with EHBDO, serum GGT activity may increase up to 2-fold within 3 days, 2- to 6-fold within 5 days, 3- to 12-fold within 1 wk, and 4- to 16-fold within 2 wk. Glucocorticoids and certain other microsomal enzyme inducers may stimulate GGT production in dogs similar to their influence on AP. Administration of dexamethasone (3 mg/kg, sid) or prednisone (4.4 mg/kg, IM, sid) increases GGT activity within 1 wk to 4- to 7-fold above normal and up to 10-fold within 2 wk. Dogs treated with phenytoin or primidone develop only a modest increase in serum GGT activity (up to 2- to 3-fold), unless they develop anticonvulsant hepatotoxicosis, in which marked enzyme activity is observed.

Cats with advanced necroinflammatory liver disease, EHBDO, or inflammatory intrahepatic cholestasis can develop a larger increase in GGT activity relative to AP. Glucocorticoids and other enzyme inducers in dogs do not clinically influence serum GGT in cats. The normal range for feline serum GGT activity is much narrower and lower than that in dogs; therefore, assays must be sensitive enough to detect low GGT activity.

GGT values can be markedly increased in dogs and cats with primary hepatic or pancreatic neoplasia. However, GGT does not appear to be suitable for surveillance of hepatic metastasis in dogs or cats.

Like AP, GGT lacks specificity in differentiating between parenchymal hepatic disease and occlusive biliary disease. It is not as sensitive in dogs as AP but does have higher specificity. In cats with inflammatory liver disease, it is more sensitive but less specific than AP, and these two enzymes should be interpreted simultaneously. The likelihood that HL has developed secondary to necroinflammatory liver disease, EHBDO, or pancreatic disease can be predicted by examining the relative increase in GGT compared with that of AP. Necroinflammatory disorders involving biliary structures, the portal triad, or pancreas are associated with a greater fold increase in GGT than in AP. With the exclusion of these underlying disorders, cats with HL usually have a higher fold increase in AP relative to GGT; this has important diagnostic utility in discerning the underlying cause of HL.

Neonatal animals of several species, including dogs but not cats, develop high serum GGT activity secondary to colostrum ingestion.

Last full review/revision March 2012 by Sharon A. Center, DVM, DACVIM

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