Veterinary Clinical Biochemistry and Laboratory Diagnosis-II

REGULATION, SYNTHESIS AND SECRETIONS

Clinically Important Serum Enzymes

Alanine aminotransferase (ALT):

• It is an enzyme necessary for energy production. It is present in a number of tissues, including the liver, heart, and skeletal muscles, but is found in the highest concentration in the liver.
• Because of this, it is used in conjunction with other liver enzymes to detect liver disease, especially hepatitis or cirrhosis without jaundice.
• Additionally, in conjunction with the aspartate aminotransferase test (AST), it helps to distinguish between heart damage and liver tissue damage.

Aspartate aminotransferase (AST) :

• It also known as SGOT. Elevated in liver and heart disease. In liver disease, the AST increase is usually less than the ALT increase. However, in liver disease caused by alcohol use, the AST increase may be two or three times greater than the ALT increase.

Alkaline phosphatase (ALP)

• Its levels usually include two similar enzymes (isoenzymes) that mainly come from the liver and bone and from the placenta in pregnant women.
• Increased in liver disorders bone disorders. 

Gamma-glutamyl transferase (GGT):

• Sometimes called gamma-glutamyl transpeptidase (GGPT)
• It is an enzyme that is compared with ALP levels to distinguish between skeletal disease and liver disease.
• Because GGT is not increased in bone disorders, as is ALP, a normal GGT with an elevated ALP would indicate bone disease.
• Conversely, because the GGT is more specifically related to the liver, an elevated GGT with an elevated ALP would strengthen the diagnosis of liver or bile-duct disease.

Bilirubin

• Bilirubin, a breakdown product of hemoglobin, is the predominant pigment in a substance produced by the liver called bile. Excess bilirubin causes yellowing of body tissues (jaundice).
• There are two tests for bilirubin: direct-reacting (conjugated) and indirect-reacting (unconjugated). Differentiating between the two is important diagnostically.
• Elevated levels of indirect bilirubin are usually caused by liver cell dysfunction (e.g. hepatitis), While elevations of direct bilirubin typically result from obstructive jaundice. The obstruction may be either within the liver (intrahepatic) or a source outside the liver (e.g. gallstones or a tumor blocking the bile ducts).
• Bilirubin measurements are valuable in newborns, as highly elevated levels of unconjugated bilirubin can accumulate in the brain, causing irreparable damage.

Ammonia

• Analysis of blood ammonia aids in the diagnosis of severe liver diseases and helps to monitor the course of these diseases.
• Ammonia levels are also helpful in the diagnosis and treatment of hepatic encephalopathy, a serious brain condition caused by the accumulated toxins that result from liver disease and liver failure.

Glucose

• The liver plays a major role in maintaining concentrations of glucose, by storing or releasing glucose as needed.

Proteins

• Most blood proteins (except for antibodies) are synthesized and secreted by the liver.
• One of the most abundant serum proteins is albumin.
• Impaired liver function that results in decreased amounts of serum albumin may lead to edema, swelling due to fluid accumulation in the tissues.
• The liver also produces most of the proteins responsible for blood clotting, called coagulation or clotting factors.
• If the blood cannot clot normally due to a decrease in the production of these factors, excessive bleeding may result.

Bile

• Bile is a greenish fluid synthesized by hepatocytes and secreted into biliary ducts.
• It then leaves the liver to be temporarily stored in the gallbladder before emptying into the small intestine.
• The major components of bile include cholesterol, phospholipids, bilirubin (a metabolite of red blood cell hemoglobin), and bile salts.
• Importantly, bile salts act as "detergents" that aid in the digestion and absorption of dietary fats.
• Liver damage or obstruction of a bile duct (e.g., gallstone) can lead to cholestasis, steatorrhea (Fat in faeces) and jaundice.

Lipids

• Cholesterol, a type of lipid, is a substance found in cell membranes that helps maintain the physical integrity of cells.
• The liver synthesizes cholesterol, which is then packaged and distributed to the body to be sued or excreted into bile for removal from the body.
• Increased cholesterol concentrations in bile may predispose to gallstone formation.
• The liver also synthesizes lipoproteins, which are made up of cholesterol, triglycerides (containing fatty acids), phospholipids, and proteins.
• Lipoproteins circulate in the blood and shuttle cholesterol and fatty acids (an energy source) between the liver and body tissues.
• Most liver diseases do not significantly affect serum lipid levels, with the exception of cholestatic diseases, which may be associated with increased levels.