Creatine Kinase, MB Form (CKMB/CPK-MB/CK-2)

EC 2.7.3.2

Creatine kinase is also sometimes referred to as creatine phosphokinase, creatine phosphotransferase, CPK, or just CK. All isoenzymes of CK catalyze the phosphorylation of creatine to form phosphocreatine. Very high levels of CK are found in skeletal muscle, primarily the MM form. Phosphocreatine is used as an energy storage system in muscle tissue with CK serving to catalyze the reverse reaction; The rapid formation of ATP from phosphocreatine and ADP when ATP is needed by the tissue¹. CKMB is primarily found in the specialized muscle tissue of the heart.

CK is a dimeric enzyme. There are two common gene products, one coding for the subunit (so named because of its predominance in muscle) and the other for the B subunit (so named because of its predominance in brain tissue). The three common forms of active CK include two homodimers and one heterodimer. The first homodimer (CK-1) consists of two B subunits and is referred to as CKBB. The other has two M subunits and is referred to as CKMM (CK-3). The heterodimer has one of each subunit and is referred to as CKMB (CK-2)². The specificity of CKMB for cardiac tissue is what has made it such a powerful diagnostic tool for the diagnosis of acute myocardial infarction (AMI). There is a third gene product which results in the mitochondrial form of CK.

Along with CKMB, significant levels of CKMM activity are found in cardiac muscle and therefore a large increase in total CK was once used as a tool in the diagnosis of AMI³. Once the CK isoenzymes were elucidated and isoenzyme tests became available, CKMB quickly became the marker of choice for AMI^4-7, and remained so for many years, only to be eventually surpassed by the cardiac troponins. However even today CKMB is frequently tested alongside troponin I (and/or troponin T) and sometimes with myoglobin to get the most definitive picture possible as to what may be causing a patient’s symptoms⁸.

The techniques used to measure serum CKMB in the clinical laboratory have changed significantly over the years. One of the earliest tests for measuring CKMB was ion-exchange chromatography, a very time consuming and laborious technique. When electrophoretic methods became more practical this became a common technique, using a chromogenic CK activity stain. Subsequently immunoinhibition methods became popular for their speed and their ability to be automated. In this technique an anti-M subunit antibody is used to inhibit any CKMM present along with the M subunit of any CKMB that might be present. The only remaining activity would be from the B-subunit, usually from CKMB. The disadvantage of this technique is that it would not distinguish CKMB from CKBB, and although the presence of CKBB is rare in practice, it has always been a concern^9,10. A traditional immunoassay that could distinguish CKMB from CKBB and CKMM was a little longer in coming due to the fact that CKMB has no unique subunits compared to these other forms. When CKMB-specific antibodies directed against regions of the M-B subunit interface were finally developed¹¹, commercial CKMB immunoassays quickly became popular. They are the preferred and most common assays used to measure CKMB to this day, though immunoinhibition and electrophoresis are still performed.

REFERENCES

1. 1. Burtis, C.A., Ashwood, E.R. and Bruns, E.R. Tietz Textbook of Clinical Chemistry, 4th Edition, W.B. Saunders Company, 2006.
2. 2. Eppenberger, H.M., et. al. The Ontogeny of Creatine Kinase Isoenzymes. Developmental Biology, 10, 1-16, 1964.
3. 3. Smith, A. Diagnostic Value of Serum Creatine Kinase in a Coronary-Care Unit. The Lancet, 290, 178-182, 1967.
4. 4. Fiolet, J.W.T., et. al. Determination of Creatine Kinase Isoenzyme MB (CK-MB): Comparison of Methods and Clinical Evaluation. Clinica Chimica Acta, 80, 23-25, 1977.
5. 5. Marmor, A., et. al. The MB Isoenzyme of Creatine Kinase as an Indicator of Severity of Myocardial Ischaemia. The Lancet, 2(8094), 812-814, 1978.
6. 6. Marmor, et. al. Creatine Kinase Isoenzyme MB (CK-MB) in Acute Coronary Ischemia. American Heart Journal, 97, 574-577, 1979.
7. 7. Baur, H.R., et. al. Serum Myocardial Creatine Kinase (CK-MB) After Coronary Arterial Bypass Surgery. American Journal of Cardiology, 44, 679-686, 1979.
8. 8. Morrow, D.A., et. al. National Academy of Clinical Biochemistry Laboratory Medicine Practice Guidelines: Clinical Characteristics and Utilization of Biochemical Markers in Acute Coronary Syndromes. Clinical Chemistry, 53, 552-574, 2007.
9. 9. Obzansky, D. and Lott, J.A. Clinical Evaluation of an Immunoinhibition Procedure for Creatine Kinase-MB. Clinical Chemistry, 26, 150-152, 1980.
10. 10. Wu., A. and Bowers Jr.,G.N. Evaluation and Comparison of Immunoinhibition and Immunoprecipitation Methods for differentiating MB and BB from macro forms of Creatine Kinase in Patients and Healthy Individuals. Clinical Chemistry, 28, 2017-2021, 1982.
11. 11. Vaidya, H.C., et. al. Direct Measurement of Creatine Kinase-MB Activity in Serum After Extraction with a Monoclonal Antibody Specific to the MB Isoenzyme. Clinical Chemistry, 32, 657-663, 1986.