Myoglobin

Myoglobin is a small monomeric protein approximately 17.8 kDa in mass¹. It has a heme prosthetic group and can bind oxygen analogously to an individual subunit of hemoglobin. However, hemoglobin is a tetramer and so displays cooperative binding kinetics which are absent in monomeric myoglobin. Hemoglobin is therefore an ideal molecule for transporting oxygen (as well as other gases) whereas myoglobin is much more suited to an oxygen storage role². As such it is an abundant protein in sea mammals that must rely on oxygen reserves to stay submerged. It is an important constituent of muscle tissue in land mammals as well, including humans. Myoglobin is primarily found in skeletal and cardiac muscle tissue and though it is generally not considered a major protein of smooth muscle, it is present³.

There is no difference in primary structure between the myoglobin found in skeletal muscle and the myoglobin found in cardiac muscle tissue. Myoglobin would therefore seem to be an unlikely candidate for a cardiac marker since skeletal muscle damage would release myoglobin into the serum that would be identical to that released by injured cardiac muscle⁴. It is certainly true that an increased serum myoglobin level by itself does not demonstrate acute myocardial infarction (AMI). However serum myoglobin is still an analyte on the complete cardiac panel due to its excellent sensitivity and because it appears to be released earlier than the other major cardiac markers. Thus it is still considered by many to be a valuable tool to aid in the diagnosis (rule-out) and even in the prognosis of AMI^5-10. However the true value of myoglobin in diagnosing the cause of chest pain has been called into question by numerous reports^11-14, especially as more sensitive TnI assays continue to gain ground^15,16.

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. Mathews C.K. and van Holde, K.E. Biochemistry. Benjamin/Cumming Publishing Co., 1990.
3. 3. Qiu, Y., et. al. Identification of Myoglobin in Human Smooth Muscle. Journal of Biological Chemistry, 273, 23426-23432, 1998.
4. 4. Romero-Herrera, A.E. and Lehman, H. The Amino Acid Sequence of Human Myoglobin and its Minor Fractions. Proc R Soc Lond, 186, 249-279, 1974.
5. 5. Gibler, W.B., et. al. Myoglobin as an Early Indicator of Acute Myocardial Infarction. Annals of Emergency Medicine, 16, 851-856, 1987.
6. 6. De Winter, R.J., et. al. Value of Myoglobin, Troponin T, and CK-MBmass in Ruling Out Acute Myocardial Infarction in the Emergency Room. Circulation, 92, 3401-3407, 1995.
7. 7. Tucker, J.F., et. al. Early Diagnostic Efficiency of Cardiac Troponin I and Troponin T for Acute Myocardial Infarction. Academic Emergency Medicine, 4, 13-21, 1997.
8. 8. McCord, J., et. al. Ninety-Minute Exclusion of Acute Myocardial Infarction by Use of Quantitative Point-of-Care Testing of Myoglobin and Troponin I. Circulation, 104, 1483-1488, 2001.
9. 9. McCord, J., et. al. The Prognostic Significance of Serial Myoglobin, Troponin I, and Creatine Kinase-MB Measurements in Patients Evaluated in the Emergency Department for Acute Coronary Syndrome. Annals of Emergency Medicine, 42, 343-350. 2003.
10. 10. Sallach, S.M., et. al. A Change in Serum Myoglobin to Detect Acute Myocardial Infarction in Patients with Normal Troponin I levels. American Journal of Cardiology, 94, 864-867, 2004.
11. 11. Mair, J., et. al. Equivalent Early Sensitivities of Myoglobin, Creatine Kinase MB Mass, Creatine Kinase Isoform Ratios, and Cardiac Troponins I and T for Acute Myocardial Infarction. Clinical Chemistry, 41, 1266-1272, 1995.
12. 12. Collinson, P.O., et. al. Multicentre Evaluation of the Diagnostic Value of Cardiac Troponin T, CK-MB Mass, and Myoglobin for Assessing Patients with Suspected Acute Coronary Syndromes in Routine Clinical Practice. Heart, 89, 280-286, 2003.
13. 13. Fesmire, F.M., et. al. Delta Creatine Kinase-MB Outperforms Myoglobin at Two Hours During Emergency Department Identification and Exclusion of Troponin Positive Non-ST-Segment Elevation in Acute Coronary Syndromes. Annals of Emergency Medicine, 44, 12-19, 2004.
14. 14. Eggers, K.M., et. al., Diagnostic Value of Serial Measurement of Cardiac Markers in Patients with Chest Pain: Limited Value of Adding Myoglobin to Troponin I for Exclusion of Myocardial Infarction. American Heart Journal, 148, 574-581, 2004.
15. 15. Wu, A.H.B. and Jaffe, A.S. The Clinical Need for High-Sensitivity Cardiac Troponin Assays for Acute Coronary Syndromes and the Role of Serial Testing. American Heart Journal, 155, 208-214, 2008.
16. 16. Jaffe, A.S. and Ordonez-Llanos, J. High Sensitivity Troponin in Chest Pain and Acute Coronary Syndromes. A Step Forward? Rev. Esp. Cardiol., 63, 763-769, 2010.
17. 17. Burtis, C.A. and Ashwood, E.R. Tietz Textbook of Clinical Chemistry 2nd Edition, W.B. Saunders Company, 1994.