Significance & Application of Serum Enzymes of Farm Animals in Disease Diagnosis
The analysis of various blood parameters is a routine part of clinical evaluation of animals. Blood or serum tests reveal the health status of the animal and provide a true reflection of the functional or metabolic status of the tissues underneath. Tissue damage causes the enzymes to leak out of their tissue cells into the blood, often resulting in a remarkable spurt in their plasma levels which can be used for diagnostic purposes. Their rate of appearance and disappearance from plasma can, thus, help in anticipating the future course of disease and can be used to dictate timely therapeutic interventions. In the light of such significance, routine assessment of these tissue specific enzymes becomes crucial and indispensable to a well-rounded and effective clinical approach. Species differences have been reported in enzyme activity and distribution in different tissues. Genetic variation has been reported in cattle, sheep, human beings and other species which is influenced by ecological conditions. Given below are the serum enzymes which constitute an important diagnostic palette for routine clinical assessment. The biochemical parameters used to assess liver pathology may be divided into two classes: the enzymes that reflect liver damage and/or cholestasis (discussed below) and the indicators of liver function (bile acids, ammonia, albumin etc. covered in a separate article). Serum enzyme screening for hepatobiliary disease is common in veterinary practice. Clinical signs associated with liver disease are wide-ranging and often non-specific, and consequently laboratory profiles are often run in patients that have a constellation of clinical signs that includes one or more of those seen in liver disease. More frequently, patients that are just “not well”, or those that are clinically normal (in for a “check up” often young patients coming in for vaccination/deworming or geriatric patients), or because surgery is being contemplated, have test “panels” conducted that almost invariably include liver enzymes. It is against this background that one should assess the usefulness of laboratory tests. It is just too simplistic to evaluate liver enzymes by comparing the serum activity in patients with liver disease with those that are healthy. For a test to be really useful, it must be abnormal in most, patients with liver disease most of the time and normal in most sick patients, with signs that could represent liver disease as well as the latter “healthy” groups, most of the time.
As patients may be presented some time after the damage first occurred, a one-off serum activity is difficult to interpret in terms of severity. Furthermore, pathophysiological processes in diseases that are not primarily hepatic may generate fairly substantial serum enzyme activity (secondary liver disease or induced enzyme activity). Even those primary diseases, such as parenchymal damage, cholangitis, cholangiohepatitis, chronic hepatitis and diffuse neoplasia, may be accompanied by negligible or no increases in serum enzyme activity. Enzymes play a key role in the metabolic activities of all organisms, whether human, animal, plant or microbial, enzyme has a wide range of applications in microbial biotechnology and its diagnosis process. The abnormality of enzyme metabolism system leads to many metabolic diseases. Studies have shown that many diseases are related to many components of enzyme metabolism system, and have been widely used in clinical examination as a specific marker of disease. Researchers in the last two decades have concentrated more on enzymes such as creatine kinase MB, alanine transaminase, aspartate transaminase, acid phosphatase, alkaline phosphatase, etc. for clinical applications. Enzymes are the preferred markers in various disease states such as myocardial infarction, jaundice, pancreatitis, cancer, neurodegenerative disorders, etc. They provide insight into the disease process by diagnosis, prognosis, and assessment of response therapy.
Aminotransferases
Aspartate Amino Transaminase (AST) or Serum Glutamate Oxaloacetate Transaminase (SGOT ) and Alanine Amino Transaminase (ALT) or Serum Glutamate Pyruvate Transaminase (SGPT) are the most requested enzymes for disease diagnosis. The presence of these enzymes in diverse tissues such as heart, liver, skeletal muscle, kidney and erythrocytes renders the discretion of clinician in differential disease diagnosis extremely important (Kaneko et al., 2008)
SGOT or AST
Clinical significance in Large Animals
Hepatocytes of mature horses, cattle, sheep and pigs contain significant levels of AST (GOT), in the cytosol as well as mitochondria. Discussed below are the causes of increased AST: 1. In horses: Increasing age, excessive training, paralytic myoglobinuria 2. In sheep: myodegeneration, ingestion of toxic plants and carbon tetrachloride poisoning, white muscle disease in lambs 3. In poultry: Hereditary muscular dystrophy in chickens 4. Other reasons include in-vitro haemolysis, severe haemolytic anaemia, after surgery, circulatory failure with shock and hypoxia, acute viral or toxic hepatitis, cirrhosis (Hoffmann & Philip, 2008) Note: In large animals, elevation of AST is more specific than that of ALT in evaluating hepatic disorders.
SGPT or ALT
The presence of high concentrations of ALT in the cytoplasm of hepatocytes of dogs, primates and some other small animal species, imparts it specificity in those species for liver disorders.Minimal ALT activity is noted in large domestic animals and hence, it is not used for the evaluation of hepatic diseases in horses, cows, sheep, goats and pigs Causes of increased plasma concentrations include: 1. Hepatocellular damage/necrosis, hepatocyte proliferation, hepatocellular degeneration 2. Corticosteroid treatment 3. Muscle damage or degeneration, alongside increased CK.
Serum Alkaline Phosphatase (ALKP)
Clinical significance in Animals
The various isoenzymes of alkaline phosphatase are present in high concentrations in liver, bone (osteoblast), placenta and intestinal epithelium in dogs, cats, horses and ruminants (Divers & Barton, 2018). However, consistent values are obtained for dogs and cats and thus, it is used as a marker of liver and bone disorders in these species. The normal serum alkaline phosphatase activity varies considerably in ruminants and its utility as a diagnostic enzyme is limited. Increase in ALKP levels can be observed under physiological as well as pathological conditions. Physiological increase seen during: Bone growth, pregnancy,lactation,aging, after fatty meal Pathological increase associated with: 1. Hepatic: Acute hepatitis (viral and toxic; 2-5 fold increase), cirrhosis,cholestatic lesion (stones and tumors), granulomatous inflammation, abscesses. (Kataria et al., 2011) 2. Osteoblastic: Bone metastasis, rickets, bone fracture, acromegaly, osteomalacia, Paget’s disease of bone) 3. Other causes include: Renal origin, hyper-parathyroidism, malnutrition, induced by various drugs.
γ-Glutamyl Transferase or GGT
The predominant levels of the serum enzyme, GGT, are derived from the biliary tract (bile duct epithelium). Low levels of this enzyme are also associated with kidney (renal brush border) and pancreas. Thus, mainly it is useful in the diagnosis of hepatobiliary disorders, especially which are of cholestatic nature .Ruminant serum generally has higher levels as compared to dogs and cats. High plasma activity is due to: 1. Cholestatic liver disorders (sensitive and specific marker of cholestasis and bile duct proliferation in horse, cattle, sheep and pigs) 2. Induction by anticonvulsant drugs (phenytoin), glucocorticoids (prednisolone).
Note: Generally, liver diseases without cholestasis will not have marked elevation in GGT levels, although ALT and AST may be elevated.
Glutamate Dehydrogenase (GDH) GDH, a mitochondrial enzyme, is used as a marker of liver diseases in cattle. Although, the level of transaminases increase significantly during liver injury as compared to GDH, it could be useful in differential diagnosis as former could be elevated in other disorders.
Sorbitol Dehydrogenase (SDH)
Sorbitol dehydrogenase, also called as L-iditoldehydrogense, is a reliable indicator of acute liver cell lesions in ruminants while its serum levels in dogs and horses are lower than in other domestic animals. Its levels rise significantly within 6 h of the acute hepatic intoxication. The half-life is higher in sheep cattle and goats as compared to horses.
Arginase
The activity of arginase in liver is higher than in other organs and elevated serum arginase levels are observed in acute liver injury for horses, cattle, sheep and goats. There is a rapid rate of increase and decrease in plasma following hepatocellular injury; prolonged rise is indicative of a grave prognosis. (DeNotta & Divers, 2020)
Creatine Phosphokinase or Creatine Kinase (CPK or CK)
Clinical significance The three isoenzymes of CK are composed of 2 different subunits: M, muscle and B, brain. Theyare localised in different tissues (CK-MM in skeletal muscles, CK-MB in myocardial muscle and CK-BB in brain). Plasma levels are mostly associated with CK- MM which is of muscular origin. Elevated CK is seen in: 1. Myocardial infarction (CK-MB): 30 fold rise within 24 hours of chest pain 2. In mypopathy, nutritional myopathy (5- 10 fold rise along with increased LDH and aldolase levels), muscle anoxia, prolonged recumbency, myositis 3. Excessive exercise 4. Muscle necrosis after intramuscular injections 5. CNS damage (elevated CK-BB) The half-life of CK is very short and levels decrease rapidly. Elevated CK values indicate active or recent muscle damage.
Lactate Dehydrogenase (LDH)
The five different isoenzymes of LDH, each with four polypeptide chains (H, heart and M, muscle) (HHHH (LD1), HHHM (LD2), HHMM (LD3), HMMM (LD4), and MMMM (LD5) occur in a variety of tissues, hence, it is not a tissue specific enzyme and assessment of its levels in isolation can be misleading. Clinical significance Elevation in LDH is noted in the following conditions: 1. After myocardial infarction (LD1 and LD2) 2. Acute leukemia (LD2 and LD3) 3. Liver or skeletal muscle damage (LD5) 4. Chronic glomerulonephritis 5. Systemic lupus erythematosus 6. Diabetic nephrosclerosis 7. Bladder and kidney malignancies 8. Haemolysis (increased levels RBC’s)
Aldolase:
Aldolase is present most significantly in skeletal and heart muscle. Increased levels are observed in skeletal muscle damage (progressive muscular dystrophy).
α-Amylase
α-Amylaseispresent in high concentrations in pancreatic juice and in salivain the digestive systems of humans and many other mammals. It catalyses the breakdown of starch and glycogen to maltose. Its high levels are associated with acute pancreatitis.
Lipase
Lipase is found in multiple tissues like gastric, pulmonary and intestinal mucosa but the highest concentration is seen in pancreas. Lipase levels tend to increase in pancreatitis and are preferred over amylase for the diagnosis of pancreatic dysfunction as it is absent in the saliva. Biochemical parameters are underutilized for evaluation of diseases in large animal practice. Appropriate usage of these enzymes is imperative for determining organ function tests and rational diagnosis. These enzymes could be employed in preclinical diagnosis, assessing progression of diseases, prognosis of metabolic and infectious diseases and monitoring response to therapy.
Compiled & Shared by- Team, LITD (Livestock Institute of Training & Development)
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