The skeletal muscle and other specialized muscle tissues, namely smooth and cardiac, contain various protein-based substances that help the muscle fibers function at the cellular level. These protein-based substances, also known as enzymes, catalyze or speed up reactions that involve how the muscle cell uses its energy sources. When muscle cells becomes damaged due to trauma, an inflammatory process, infection or even exercise, the contents of the cells, including the enzymes, spill out into the surrounding fluid and become detectable in the blood.
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Blood plasma is synonymous to the fluid that surrounds cells. The movement and exchange of these fluids can expose detectable levels of cellular contents, like enzymes, when cells become damaged. Myositis, for example, is a condition of extreme muscle fatigue and weakness caused by muscle inflammation. The Myositis Association website notes that the isolated plasma portion of the blood may be tested for muscle enzymes such as aldolase, creatine phosphokinase (CPK), aspartate aminotransferase (AST) and lactate dehydrogenase (LDH). When such enzymes are found to be abnormally high or low, they indicate a process of muscle tissue degradation.
Aldolase
Aldolase appears to be specific to muscle cells. The extent of muscle damage is directly correlated to the amount of aldolase measured in the blood, according to the Myositis Association. However, a chronic or long-standing condition that results in muscle damage can cause aldolase levels to become depleted, as the muscle itself becomes less pronounced over time. Aldolase, like other muscle enzymes found in humans, is also common to other species of animals.
Creatine Phosphokinase
Creatine Phosphokinase, or CPK, is an enzyme that helps the body turn the protein, creatine, into a quick source of energy for muscle cells. Skeletal muscle cells are the primary storage source of creatine, and the brain and heart store the remainder. CPK levels are thus very specific to damage to skeletal muscle. A particular subtype of creatine, which indicates its use by the heart tissue, is used to measure heart tissue damage after a heart attack has occurred. Lab Tests Online notes that the subfractions of CPK isolate the area of muscle that has been damaged. CK-MM is found in the skeletal and heart muscle, while CK-MB and CK-BB are specific to the heart and brain, respectively. CPK tends to be higher in people who have more muscle mass, in African Americans and in those who play contact sports or do weight lifting.
Aspartate Aminotransferase
Aspartate Aminotransferase, or AST, is another enzyme found in high proportions in muscle cells, however, it is also specific to the liver. Liver problems are generally the reason why this test is performed, but Lab Tests Online notes that damage to heart and skeletal muscle can cause their levels to increase in the blood. The Myositis Association notes that another name for AST is serum glutamic-oxaloacetic transaminase, or SGOT. Either name or both names can be found on a blood test result sheet, depending on the lab that was used.
Lactate Dehydrogenase
Lactate Dehydrogenase, or LDH, is another muscle enzyme whose levels can increase significantly when muscle damage or inflammation has occurred. Lab Tests Online notes that elevated LDH levels can be seen with certain blood disorders, infections, kidney and liver disease and reactions to certain medications. It can also be an indicator for stroke. LDH is not as specific for muscle damage as some of the other markers, but when combined with other muscle enzyme markers, can confirm muscle damage.
