Enzymes are proteins that function to break down other molecules and speed up chemical reactions in the body. Each type of enzyme is highly specific, functioning in a particular way or acting on a certain molecule. Liver and muscle enzymes are used as indicators for normal or abnormal organ and metabolic function. This is because damaged cells tend to leak extra chemicals, including enzymes, into the bloodstream.
The Enzymes
Alanine transaminase, also known as ALT, and aspartate transaminase, AST, are indicator enzymes used to help determine injury or death of liver tissue. ALT is a more specific indicator for problems with the liver as it is most prevalent in the liver cytosol. AST is found in the heart, muscle, kidneys, brain, pancreas and lungs, in addition to the liver. The common muscle enzyme markers are creatine phosphokinase, lactate dehydrogenase and serum glutamic oxaloacetic transaminase, which is also called aspartate transaminase, AST. All three of these enzymes are cardiac muscle enzymes, although creatine phosphokinase is also associated with skeletal muscle.
Elevated Levels
ALT and AST levels can become very elevated, in excess of 2,000 units per liter; normal levels are less than 250 U/L. Some common causes of elevated ALT and AST levels include alcohol, cirrhosis, hepatitis, medications, toxins, ischemia, hemolysis, myopathy, hyperthyroidism and even strenuous exercise. Elevated cardiac muscle enzyme levels can indicate cardiac damage and even provide evidence of a heart attack. High creatine phosphokinase levels indicate damage or tearing of skeletal muscle fibers. Normal creatine phosphokinase levels are between 60 and 400 U/L. Normal lactate dehydrogenase levels range from 140 to 280 U/L. Normal serum glutamic oxaloacetic transaminase levels are the same as AST.
Testing
A simple blood test is necessary to determine ALT and AST levels. The test is most often used to establish if is there is damage to the liver. In most cases, elevated ALT and AST levels are only temporary and may not indicate a serious liver problem. Blood tests are also performed to determine cardiac muscle enzyme levels. Electrophoresis and radioimmunoassay tests are then used to pinpoint the location of the damage.
Treatments
The ALT/AST blood test does not lead directly to a definitive diagnosis. Physicians take into account health history, physical examination and the results of other tests to determine a diagnosis. Lifestyle modifications such as weight loss and the discontinuation of alcohol or certain medications are often recommended. Additional tests may also be necessary if cardiac muscle enzyme levels indicate a heart problem; a physician will determine specific treatments.
References
- "American Family Physician"; Mildly Elevated Liver Transaminase Levels in the Asymptomatic Patient; Paul T. Giboney, MD; March 2005
- MayoClinic.com; Elevated Liver Enzymes; May 2011
- Medline Plus; ALT; David C. Dugdale, MD; February 2011
- "Exercise Physiology: Energy, Nutrition, and Human Performance"; William D. McArdle, et al.; 2001
