Creatine kinase (CK) is an enzyme that’s found in your skeletal muscle, heart muscle, and brain. When any of these tissues are damaged, they leak creatine kinase into your bloodstream. Elevated CK levels may indicate muscle injury or disease.
What is Creatine Kinase?
Creatine Kinase (CK) also called creatine Phosphokinase (CPK) or phosphocreatine kinase is an enzyme mainly Found in your Heart Muscle, Skeletal muscle, and a small amount in your Brain. Creatine Kinase is released by these muscles when they are damaged.
CK is a type of protein that catalyzes or encourages the biochemical reaction that leads to the conversion of creatine and uses ATP to create Phosphocreatine and ADP. This enzyme reaction is reversible thus ATP can be generated from Phosphocreatine and ADP.
In our cells, The Normal function of the Creatine Kinase is to add a phosphate group to creatine, which turned it into the high-energy molecule Phosphocreatine. This molecule is used by our cells as a quick source of energy.
The Small amount of Creatine kinase that is normally present in your blood mainly comes from skeletal muscle. Increased amounts of CK are released into the blood when there is muscle damage such as a heart attack, skeletal muscle injuries, certain muscle disorders, or strenuous exercise. Thus, a rise in blood CK levels is usually a marker for muscle injury, disease, or inflammation.
A CK level may also rise when excessive alcohol, cocaine, and certain medications are taken, such as a statin for elevated cholesterol.
Structure of Creatine Kinase
Creatine Kinase is a polypeptide chain that is made up of a specific sequence of amino acids. This Polypeptide chain is taking 3D (Three- dimensional) Form when folded properly, which can interact with certain molecules.
The structure of the creatine kinase provides a strong interaction with the molecule Creatine and Phosphocreatine, and also provide another binding site on the enzyme that is dedicated to the interaction with ATP and ADP respectively.
As both groups are attached to the binding site on the enzyme it will catalyze their respective conversions when the ATP and Creatine are bound it will take the phosphate group from the ATP and add it to the creatine and get phosphocreatine and ADP or If ADP and phosphocreatine are binding it will take Phosphate group from the phosphocreatine and add it to the ADP and get creatine and ATP. So, the end result is either the creation or use of ATP.
There are several types of creatine kinase encoded by different genes. While these forms of creatine kinase differ in their amino acid structure, their function remains similar. However, slight changes in their structure and function allow creatine kinase to work in different environments.
Types of Creatine Kinase
Creatine kinase is a dimer (it consists of two subunits that combine to form a functional enzyme). There are two different forms of these subunits, B-type, and M-type. The B type stands for “brain type” because this subunit is most commonly associated with the brain. The M type stands for “muscle type” as this subunit is most commonly associated with skeletal muscle.
Therefore, there are three types of CK enzymes This form is a result of different combinations of these subunits:
- CK-MM, the enzyme with two Muscle subunits, found mainly in skeletal muscle
- CK-BB, the enzyme with two brain subunits, mainly found in brain tissue
- CK-MB, the enzyme with one of each, found mainly in the heart muscle or Cardiac muscle
These enzymes are located in the cytoplasm of cells and are localized in a tissue-specific manner. There are also two forms of the enzyme that are unique to the mitochondria (CK-Mt). Ubiquitous CK-Mt is non-sarcomeric and is found in various tissues such as the brain, smooth muscle, and sperm. Sarcomere Mt-CK is only expressed in cardiac and skeletal muscle.
The different forms of creatine kinase all perform the same function but under different conditions. These different shapes are necessary to manage the energy reservoir in many different cell types. In most cells, this phosphocreatine reservoir is maintained at a concentration much higher than that of ATP. This allows you to do many strenuous tasks.
The function of Creatine Kinase
Creatine kinase’s main function is to add a phosphate group, a group of natural chemicals, to creatine, a substance in your muscle cells that helps your muscles produce energy. When CK adds phosphates to creatine, it converts the creatine into the high-energy molecule phosphocreatine, which your body uses for energy.
CK enters your bloodstream when your muscles, heart, or brain suffer acute damage or chronic degeneration. When your muscles are damaged, muscle cells break down and their contents, including creatine kinase, enter your bloodstream.
What is a creatine kinase (CK) test?
This test measures the amount of an enzyme called creatine kinase (CK) in your blood. CK is a type of protein. The muscle cells in your body need CK to function. CK levels can rise after a heart attack, skeletal muscle injury, or strenuous exercise. They can also increase after drinking excessive amounts of alcohol or taking certain medications or supplements.
CK consists of 3 enzyme forms. These are CK-MB, CK-MM, and CK-BB. CK-MB is the substance that increases when your heart muscle is damaged. CK-MM increases with other muscle damage. CK-BB is mainly found in the brain. If this test shows your CK levels are high, you may have muscle or heart damage.
What are creatine kinase (CK) tests used for?
A CK test is most commonly used to diagnose and monitor muscle injuries and disorders. These diseases include
Muscular dystrophy is a rare inherited disorder that causes weakness, breakdown, and loss of function of skeletal muscles. It mainly occurs in men.
Rhabdomyolis, is a rapid breakdown of muscle tissue. It can be caused by a serious injury, muscle disease, or other disorder.
The test can be used to diagnose a heart attack, although not very often. CK tests used to be a common test for heart attacks. However, another test called troponin has been found to be better at detecting heart damage.
Why do I need a creatine kinase (CK) test?
Your doctor may order a creatine kinase (CK) test if you have symptoms of muscle disease, including:
- Muscle pain and/or cramps.
- Muscle weakness.
- Balance problems.
- Numbness or tingling.
- Dark urine (pee).
Your provider may also order this test if you’ve had a muscle injury or stroke. CK levels may not peak for two days after certain injuries, so you may need to do the test more than once.
How do I prepare for a creatine kinase (CK) test?
In most cases, you do not need any special preparation for a CK test.
If your healthcare provider has ordered a CK test to diagnose a possible muscle disorder, you should limit your exercise to normal activities before the test because the muscle strain caused by exercise can temporarily increase the levels of CK in your blood.
What do the results of a creatine kinase (CK) test mean?
If your results show you have a higher-than-normal level of CK, it may mean you have an injury or disease of the muscles, heart, or brain. To get more information, your provider may order tests to check the levels of specific CK enzymes:
- If you have higher than normal CK-MM enzymes, it may mean you have a muscle injury or disease, such as muscular dystrophy or rhabdomyolis.
- If you have higher than normal CK-MB enzymes, it may mean you have an inflammation of the heart muscle or are having or recently had a heart attack.
- If you have higher than normal CK-BB enzymes, it may mean you have had a stroke or brain injury.
Other conditions that can cause higher than normal CK levels include:
- Blood clots
- Hormonal disorders, including disorders of the thyroid and adrenal glands
- Lengthy surgery
- Certain medicines
- Strenuous exercise
If you have questions about your results, talk to your healthcare provider.
What Are Normal Creatine Kinase (CK) Levels?
Normal CK levels can vary slightly from lab to lab. CK levels vary by gender, race, age, muscle mass, and physical activity. In general, a healthy range is 30 to 145 U/L for women and 55 to 170 U/L for men. While men tend to have higher CK levels than women, African Americans tend to have even higher levels, while age-related CK levels have been observed in older men but not in women.
In cases where CK levels are slightly elevated, a doctor may decide to repeat the test after a period of time to see if CK levels are rising or falling.
Additional tests will likely be ordered for more worrying levels. For example, a doctor may order CK isoenzymes or a CK-MB as follow-up tests to distinguish between the three types (isoenzymes) of CK
What Does A High Level Of Creatine Kinase (CK) Mean?
A high level of creatine kinase (CK) or an increase in values in subsequent CK tests generally indicates that you have recently suffered muscle damage. A CK test cannot indicate which muscle(s) has been damaged or the cause of the damage.
Healthcare providers often have you do multiple CK tests to check your progress. If you have several test results that peak and then start to fall, it usually means your muscle damage has decreased. If your CK levels rise or remain at persistently high levels, it may indicate ongoing muscle damage or muscle degeneration.
If your results show that you have higher than normal CK levels, your doctor may order tests to check levels of certain CK enzymes to determine the type of muscles affected, including:
- CK-MM enzymes: High CK-MM levels may mean you have a muscle injury or diseases such as muscular dystrophy or rhabdomyolysis.
- CK-MB enzymes: High levels of CK-MB can mean you have inflammation in your heart muscle or you are having or have recently had a heart attack.
- CK-BB enzymes: High levels of CK-MB can mean you’ve had a stroke or brain injury. Other causes of increased creatine kinase (CK) levels