Isoenzymes and myocardial infarction

The blood plasma of a healthy individual typically contains 60-80 g/L of protein. This protein can be separated into five classes designated a through "y. The separation is based on the overall surface charge on each of the types of protein. (See A Clinical Perspective Enzymes, Isoenzymes, and Myocardial Infarction in Chapter 20 for a discussion of the separation of proteins based on surface charge.)... 

A Clinical Perspective Enzymes, Isoenzymes, and Myocardial Infarction... 

The role of LDH and other enzymes in disease diagnosis is discussed in A Clinical Perspective Enzymes, Isoenzymes, and Myocardial Infarction. 

The only other condition which causes (MB) to appear in serum is muscular dystrophy. The MB isoenzyme appears in the serum of the patient with acute myocardial infarction within 6 hours and remains elevated for approximately 48-60 hours. 

LD isoenzyme abnormality does not precede the appearance of the (mb) in the serum following myocardial infarction. The reappearance of (mb) is interpreted as diagnostic of reinfarction, and correlates with new electrocardiographic changes and an altered clinical course. 

Wagner et. al (46) studied 376 patients to evaluate the importance of identification of the myocardial-specific MB isoenzyme in the diagnosis of acute myocardial infarction. An attempt was made to determine the incidence of falsely positive (mb). No acute infarction was diagnosed in all patients in whom neither total CK nor the isoenzymes of LD indicated myocardial necrosis. Incidence of falsely negative (MB) was zero in 33 patients. They concluded that determination of the isoenzymes of CK provides both a sensitive and specific indication of acute myocardial infarction. 

Ronttinen, A. and Somer, H. Specificity of seriim creatine kinse isoenzymes in diagnosis of acute myocardial infarction. Br. Med. J. (1973), 1, 386-389. 

Correct answer = D. The CK isoenzyme pattern at admission showed elevated MB isozyme, indicating that the patient had experienced a myocardial infarction in the previous 12 to 24 hours. [Note 48 to 64 hours after an infarction, the MB isozyme would have returned to normal values.] On day 2, 12 hours after the cardioconversions, the MB isozyme had decreased, indicating no further damage to the heart. However, the patient showed an increased MM isozyme after cardo-conversion. This suggests damage to muscle, probably a result of the convulsive muscle contractions caused by repeated cardioconversion. Angina is typically the result of transient spasms in the vasculature of the heart, and would not be expected to lead to tissue death that results in elevation in serum creatine kinase. 

Isoenzymes Isoenzymes are different forms of an enzyme which catalyze the same reaction, but which exhibit different physical or kinetic properties. The isoenzymes of lactate dehydrogenase (LDH) can be separated electrophoretically and can be used clinically to diagnose a myocardial infarction. 

An example of an enzyme which has different isoenzyme forms is lactate dehydrogenase (LDH) which catalyzes the reversible conversion of pyruvate into lactate in the presence of the coenzyme NADH (see above). LDH is a tetramer of two different types of subunits, called H and M, which have small differences in amino acid sequence. The two subunits can combine randomly with each other, forming five isoenzymes that have the compositions H4, H3M, H2M2, HM3 and M4. The five isoenzymes can be resolved electrophoretically (see Topic B8). M subunits predominate in skeletal muscle and liver, whereas H subunits predominate in the heart. H4 and H3M isoenzymes are found predominantly in the heart and red blood cells H2M2 is found predominantly in the brain and kidney while HM3 and M4 are found predominantly in the liver and skeletal muscle. Thus, the isoenzyme pattern is characteristic of a particular tissue, a factor which is of immense diagnostic importance in medicine. Myocardial infarction, infectious hepatitis and muscle diseases involve cell death of the affected tissue, with release of the cell contents into the blood. As LDH is a soluble, cytosolic protein it is readily released in these conditions. Under normal circumstances there is little LDH in the blood. Therefore the pattern of LDH isoenzymes in the blood is indicative of the tissue that released the isoenzymes and so can be used to diagnose a condition, such as a myocardial infarction, and to monitor the progress of treatment. 

The patient, a 63-year-old Caucasian female, was hospitalized on 4 April 2002 though 10 April 2002 for a non-ST segment elevation myocardial infarction (non-Q-wave MI per chart documentation). She had a negative adenosine stress test after the initial event. Her serum cardiac-specific troponin I (cTnl) concentration 24 hours after her onset of chest pain was 1.4 pg/L (upper limit of normal is 0.3 ng/mL), and her creatine kinase (CK) MB level was 12.5 pg/L (upper limit of normal 6.0 ng/mL). Three days post-event her cTnl level was 0.5 pg/L and her CK-MB level was 4.5 pg/L (Fig. 5-1). MB refers to one of the isoenzyme forms of CK found in serum. The form of the enzyme that occurs in brain (BB) does not usually get past the blood-brain barrier and therefore is not normally present in the serum. The MM and MB forms account for almost all of the CK in serum. Skeletal muscle contains mainly MM, with less than 2% of its CK in the MB form. MM is also the predominant myocardial creatine kinase and MB accounts for 10%-20% of creatine kinase in heart muscle. 

Jaffe AS, Landt Y, Parvin CA,et al. Comparative sensitivity of cardiac troponin I and lactate dehydrogenase isoenzymes for diagnosing acute myocardial infarction. Clin Chem 42 1770-1776,1996. 

Creatine kinase (CK) occurs in high concentrations in the brain, cardiac and skeletal muscle and is elevated in the blood with muscle damage. A rise in CK is seen in acute myocardial infarction but also in other conditions. A more specific marker is creatine kinase MB (CK-MB), which is an isoenzyme of creatine kinase that is more specific for cardiac muscle damage. CK or CK-MB will rise approximately 4 hours after an acute cardiac event and will reach a peak after approximately 24 hours and will remain raised for 3-4 days. 

Q4 Cardiac enzymes are released into the blood following heart muscle damage during a heart attack. Creatine kinase, particularly its MB isoenzyme, is one of the most specific of these enzymes, which reaches a peak 24 hours after infarction. It rises and then falls within the first 72 hours of the heart attack. Aspartate transaminase is also released, but levels of this enzyme can be raised in several other conditions, so it is less specific than creatine kinase MB. Troponin T is also specific for myocardial damage and is raised for approximately two weeks following infarction. Finding a high concentration of these enzymes in a patient s blood therefore supports the evidence obtained from the ECG and confirms that the patient has suffered a myocardial infarction. 

LDH is especially diagnostic for myocardial infarction. This enzyme exists in five closely related, but slightly different forms (isoenzymes). The five types and their normal distribution and levels in non-disease/injury are listed below ... 

Sildenafil is contraindicated in patients who are taking organic nitrates, for their metabolism is blocked and severe and acute hypotension result. Patients with recent stroke or myocardial infarction or whose blood pressure is known to be < 90/50 mmHg should not use it. Sildenafil is a substrate for the P450 isoenzyme CYP3A4 (and to a lesser extent CYP2C9) which gives scope for interaction with inhibitors or inducers of this system. The metabolic inhibitors erythromycin, saquinavir and ritonavir (protease inhibitors used for AIDS), and cimetidine, for example, produce substantial rises in the plasma concentration of sildenafil. 

E620 Hadberg, A., Hassager, C., Hildebrandt, P. and Christiansen, C. (1990). Comparison of two immunoinhibitory methods with agarose gel-electrophoresis for measuring the MB isoenzyme of creatine kinase in serum from cases of suspected myocardial infarction. J. Clin. Chem. Clin. Biochem. 28, 453-458. 

E632 Jensen, A.E., Reikvam, A., NordgSrd, Pedersen, K.G. and Asberg, A. (1990). Diagnostic accuracy of Kodak creatinine kinase MB, Stratus creatine kinase MB, and lactate dehyrogenase isoenzyme 1 in serum after acute myocardial infarction. Clin. Chem. 36, 1847-1848. 

ST80 Christenson, R.H., Clemmensen, P., Ohman, E.M., Toffaletti, J., Silverman, L.M., Grande, P., Vollmer, R.T. and Wagner, G.S. (1990). Relative increase in creatine kinase-MB isoenzyme during reperfusion after myocardial infarction is method dependent. Clin. Chem. 36, 1444-1449. 

The changes of serum CK and its MB isoenzyme following a myocardial infarction are discussed in Chapter 44. Other cardiac conditions have been reported to increase serum CK and CK-MB in serum. These conditions include cardioversion, cardiopulmonary bypass and coronary artery bypass surgery, cardiac transplantation, myocarditis, pericarditis, and pulmonary embolism. Despite improvements, in the diagnostic performance and practicality of CK and CK-MB assays, there is no controlled cUnicai impact trial showing that these tests are effective for decisions to send a patient home or to the appropriate level of care of admission for patients with suspected acute cardiac ischemia, either as one-time or serial tests.For diagnosis of acute myocardial infarction, it is now advantageous to use more cardiac-specific nonenzymatic tests, such as cardiac troponin I orT. 

Figure 21-1 A, The electrophoretic separation of serum CK isoenzymes from a healthy adult (left) and from a patient (right) who had a myocardial infarction 24 hours previously. B, A diagrammatical representation of the CK isoenzymes (some of which are only seen in disease) and of some of the reported anomalous forms.

Hemolysis, if sufficiently severe, produces an LD isoenzyme pattern similar to that in myocardial infarction. Megaloblastic anemias, usually resulting from the deficiency of folate or vitamin cause the erythrocyte precursor cell to break down in the bone marrow (ineffective erythropoiesis), resulting in the release of large quantities of LD-1 and LD-2 isoenzymes. Marked elevations of the total LD activity in serum— up to 50 times the upper reference limit—have been observed in the megaloblastic anemias. These elevations rapidly return to normal after appropriate treatment. 

Galen RS, Reiffel JA, Gambino SR. Diagnosis of acute myocardial infarction relative efficiency of serum enzyme and isoenzyme measurements. JAMA 1975 232 145-7. 

Grenadier E, Keidar S, Kahana L, Alpan G, Marmur A, Palant A. The roles of serum myoglobin, total CK, and CK-2 isoenzymes in the acute phase of myocardial infarction. Am Heart J 1983 10 408-16. 

The enzyme CK is a dimer of two subunits, called M (found in skeletal muscle) and B (found in brain). Heart muscle, the myocardium, contains both subunits and the principal isoenzyme in myocardium is MB in contrast to MM, found in skeletal muscle and BB, found in brain. After a myocardial infarction, where part of the myocardium is damaged by low oxygen due to reduced blood flow (ischemia), it is specifically the MB isoenzyme that increases in the blood. 

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