Muscle function at the cellular level relies on biochemical reactions, and these reactions in turn rely on proteins called enzymes in order to proceed rapidly and efficiently. Because exercise results in some degree of muscle breakdown, enzymes normally confined to muscle cells can sometimes spill into the blood. As a result, measuring the blood levels of certain muscle enzymes can be helpful in assessing certain physical processes and in determining the extent to which muscle damage has occurred.
Creatine Kinase
Creatine kinase, abbreviated CK or CPK, is the enzyme for which an elevated blood level is most commonly associated with intense exercise. In particular, exercises involving eccentric muscle contractions -- those in which the muscle lengthens even as its fibers are contracting, as with downhill running or the extension phase of biceps curls -- produce markedly elevated CK measurements. Clinicians need to rule out exercise as a cause of elevated CK in patients with suspected rhabdomyolysis, a sometimes painless but often very serious condition resulting from skeletal muscle breakdown.
Lactate Dehydrogenase
Lactate dehydrogenase, or LDH, is, like CK, an enzyme used as a marker of tissue damage in various organs. It exists in the form of specific isoenzymes that originate in distinct locations, thereby helping medical professionals pinpoint the origins of LDH increases. Heart attacks, strokes, and kidney and liver damage are common causes, but strenuous exercise can cause temporary increases, and aerobic exercise, such as marathon training, can cause chronic changes in the proportion of isoenzymes in muscle.
Transaminases
Alanine aminotransferase, or ALT, and aspartate aminotransferase, or AST, are enzymes found predominantly in the liver, heart and skeletal muscle, as well as red blood cells. In a clinical setting, they are most often used to assess patients with suspected liver damage, as with acute or chronic hepatitis or alcohol use. However, strenuous exercise can cause levels of both of these to rise; because ALT is more strongly confined to liver than AST, when the latter rises out of proportion to the former, non-liver causes should be suspected, and clinicians should, as always, question patients about recent physical activity.
Oxidative Enzymes
Various enzymes found in skeletal muscle show increases in activity over time with physical training, as would be expected in someone becoming more aerobically fit and thus more efficient at utilizing oxygen in target tissues. These enzymes include succinate dehydrogenase, phosphofructokinase and various others involved in the citrus acid cycle, often called the Krebs cycle.
References
- "Journal of Family Practice"; How much can exercise raise creatine kinase level---and does it matter?; J. Latham; August 2008
- American Academy of Family Physicians: Mildly Elevated Liver Transaminase Levels in the Asymptomatic Patient
- "Journal of Applied Physiology"; Effect of training on enzyme activity and fiber composition of human skeletal muscle; P.D. Gollnick; January 1973
- RNCEUS: Alanine aminotransferase (ALT) and Aspartate aminotransferase (AST)
- "British Journal of Sports Medicine"; Serum lactate dehydrogenase and creatine kinase during marathon training; A. G. Rumley; September 1985
