Correspondence

More on Subforms of Creatine Kinase MB

N Engl J Med 1995; 333:390-391August 10, 1995

Article

To the Editor:

Puleo et al. (Sept. 1 issue)1 described the use of a rapid test of subforms of creatine kinase MB (CK-MB) to diagnose myocardial infarction earlier than was previously thought to be possible. This report is of obvious importance, since the authors claim a potential reduction of about 70 percent in admissions to coronary care units. The assay is based on measuring the ratio of CK-MB₂ (the subform containing C-terminal lysine) to CK-MB₁ (the des-lysine subform), for which the normal cutoff value is 1.5. CK-MB₂ released from the damaged myocardium is converted to CK-MB₁ in the blood by carboxypeptidase N (lysine carboxypeptidase). This enzyme, discovered in our laboratory, is an important inactivator of peptide hormones such as bradykinin and anaphylatoxins.2,3 Carboxypeptidase N is synthesized in the liver and secreted into the blood (concentration, approximately 30 μg per milliliter) as a 280-kd tetramer consisting of two 50-kd catalytic subunits and two glycosylated 83-kd subunits coupled noncovalently.3 The 83-kd regulatory subunits carry and protect the active subunits in the circulation.3 Although no person has been known to lack the enzyme completely, plasma levels of carboxypeptidase N can vary considerably. For example, enzyme levels are elevated during pregnancy and in patients with certain types of cancer and are low in patients with cirrhosis of the liver2; there can even be up to a threefold variation in levels among normal persons.4

The variation in carboxypeptidase N activity has obvious relevance for the use of the CK-MB isoform ratio in the diagnosis of myocardial infarction. In patients with myocardial infarction who have an abnormally high level of carboxypeptidase N and more rapid conversion of CK-MB₂ to CK-MB₁, the test may possibly yield a false negative result, whereas in normal persons, a low carboxypeptidase N level may result in a ratio of CK-MB₂ to CK-MB₁ that is higher than 1.5, leading to a false positive result. Thus, the determination of carboxypeptidase N levels can potentially increase the specificity and sensitivity of this improved test. A similar point was made by Abendschein and coworkers.4

Some issues with the test remain to be settled. For example, the rate of cleavage can depend on whether only one or both of the active subunits in the carboxypeptidase N tetramer are simultaneously functional with a large substrate such as creatine kinase. In addition, the new test is based on the assumption that only the M subunit in CK-MB is cleaved, resulting in two subforms, although a previous report5 showed that the C-terminal lysine of the B subunit of CK is released first, followed by hydrolysis of the M subunit, yielding a total of three CK-MB subforms not easily separated by electrophoresis.

Ervin G. Erdös, M.D.
Randal A. Skidgel, Ph.D.
University of Illinois College of Medicine, Chicago, IL 60612

5 References

1. 1

   Puleo PR, Meyer D, Wathen C, et al. Use of a rapid assay of subforms of creatine kinase MB to diagnose or rule out acute myocardial infarction. N Engl J Med 1994;331:561-566
   Full Text | Web of Science | Medline

2. 2

   Erdos EG, Wohler IM, Levine MI, Westerman MP. Carboxypeptidase in blood and other fluids: values in human blood in normal and pathological conditions. Clin Chim Acta 1965;11:39-43
   CrossRef | Web of Science | Medline

3. 3

   Levin Y, Skidgel RA, Erdos EG. Isolation and characterization of the subunits of human plasma carboxypeptidase N (kininase i). Proc Natl Acad Sci U S A 1982;79:4618-4622
   CrossRef | Web of Science | Medline

4. 4

   Abendschein DR, Serota H, Plummer TH Jr, et al. Conversion of MM creatine kinase isoforms in human plasma by carboxypeptidase N. J Lab Clin Med 1987;110:798-806
   Medline

5. 5

   Prager NA, Suzuki T, Jaffe AS, Sobel BE, Abendschein DR. Nature and time course of generation of isoforms of creatine kinase, MB fraction in vivo. J Am Coll Cardiol 1992;20:414-419
   CrossRef | Web of Science | Medline

To the Editor:

In their reply to the letters about their article, Roberts and Puleo state, “the [glass bead] assay is not used as widely as the most popular assay, electrophoresis, or mass assays” (March 2 issue).1 I disagree that electrophoresis is the most popular (i.e., most frequently used) assay for measuring serum CK-MB concentrations.

According to data from approximately 3500 laboratories participating in external proficiency surveys administered by the College of American Pathologists (the Electrophoresis/Chromatography Survey2 and the Chemistry Series 3, 4, 5, 6, and 7 Surveys3), immunoassay and immunochemical methods are more popular than electrophoresis by a margin of more than 3 to 1 (Table 1Table 1Quantitative Methods Used to Measure CK-MB Isoenzymes by Laboratories Participating in the Electrophoresis/Chromatography and Chemistry Surveys Conducted by the College of American Pathologists.). In addition, immunoassay methods for the measurement of CK-MB are more popular than either immunochemical or electrophoretic methods.

F.H. Wians, Jr., Ph.D
University of Texas Southwestern Medical Center, Dallas, TX 75235-9072

3 References

1. 1

   Roberts R, Puleo P. Subforms of creatine kinase MB in the diagnosis of myocardial infarction. N Engl J Med 1995;332:609-610
   Web of Science

2. 2

   Participant summary, electrophoresis/chromatography survey, set EC-C. Northfield, Ill.: College of American Pathologists, 1994.
   

3. 3

   Participant summary, chemistry — series 3, series 4, series 5, series 6, series 7 surveys, set C3, C4, C5, C6, C7-C. Northfield, Ill.: College of American Pathologists, 1994.
   

Citing Articles (3)

Citing Articles

1. 1

   Randal A. Skidgel, Ervin G. Erdös. (2007) Structure and function of human plasma carboxypeptidase N, the anaphylatoxin inactivator. International Immunopharmacology 7:14, 1888-1899
   CrossRef

2. 2

   Johannes Mair. (1997) Cardiac troponin I and troponin T: Are enzymes still relevant as cardiac markers?. Clinica Chimica Acta 257:1, 99-115
   CrossRef

3. 3

   Martina Zaninotto, Sara Altinier, Mattia Lachin, Mario Plebani. (1997) Carboxypeptidase N and Creatine Kinase-MB Isoforms in Acute Myocardial Infarction. Clinical Chemistry and Laboratory Medicine 35:4, 291-296
   CrossRef