As a muscle that expends energy, the heart requires its own supply of blood, just like the tissues to which it pumps blood. Blood carries oxygen, which is used to make ATP in cells. The ATP is an energy source required for muscular contraction. If an artery of the heart becomes blocked, the tissue downstream from the blockage will be deprived of oxygen. This is called a heart attack, or myocardial infarction. In a short time, the cells of the tissue will die, releasing cardiac enzymes into the blood, and that portion of the heart will no longer be able to contract. The muscle will be replaced with scar tissue.
Cardiac enzymes are proteins normally found in the muscle cells of the heart, where they perform specific functions such as helping muscular contraction or energy metabolism. When the cells are damaged, they break open, or lyse, releasing their contents including the enzymes into the blood. Some of the cardiac enzymes measured in the blood include troponin T, troponin I, creatine kinase, lactate dehydrogenase and glycogen phosphorylase isoenzyme BB.
Troponins control the binding between the proteins actin and myosin -- these proteins bind together using ATP as an energy source to contract the heart muscle cells. Troponins are found in all muscle cells, but troponin T -- normally less than 0.2 mcg/L -- and I -- normally less than 0.35 mcg/L -- are more specific to heart muscle. Creatine kinase -- normally 0 to 3 mcg/L -- helps to regenerate used ATP. Lactate dehydrogenase helps in energy metabolism. Glycogen phosphorylase isoenzyme BB also helps in energy metabolism.
After a heart attack, enzymes typically rise within 2 to 4 hours. A blood test can measure the presence of these markers and, along with other data from the medical history and an ECG, can help to determine the likelihood that a person is having a heart attack.
Some of these enzymes are not as specific for cardiac muscle. CK-MB is found in muscles other than the heart, so damage to other types of muscle, such as skeletal muscle, can release CK-MB. However, measuring enzyme levels such as these are still useful. Troponin I and T are specific to the heart, and remain elevated for a long time after a heart attack occurs. Since CK-MB rises and falls rapidly, it can be used to determine if a second heart attack is occurring while troponin I and T are still elevated from the first heart attack. In addition, troponins tend to be elevated in some forms of kidney disease, so troponins are not as useful in this situation, whereas CK-MB can be.