The analysis of enzymes in blood plasma plays a central role in the diagnosis of several disease processes. Many enzymes are functional constituents of blood. Examples include pseudocholinesterase, lipoprotein lipase, and components of the cascades that trigger blood clotting, clot dissolution, and opsonization of invading microbes. Several enzymes are released into plasma following cell death or injury. While these latter enzymes perform no physiologic function in plasma, they can serve as biomarkers, molecules whose appearance or levels can assist in the diagnosis and prognosis of diseases and injuries affecting specific tissues. The plasma concentration of an enzyme or other protein released consequent to injury may rise early or late, and may decline rapidly or slowly. Cytoplasmic proteins tend to appear more rapidly than those from sub cellular organelles.

Quantitative analysis of the activity of released enzymes or other proteins, typically in plasma or serum but also in urine or various cells, provides information concerning diagnosis, prognosis, and response to treatment. Assays of enzyme activity typically employ standard kinetic assays of initial reaction rates. Table 7–1 lists several enzymes of value in clinical diagnosis. Note that these enzymes are not absolutely specific for the indicated disease. For example, elevated blood levels of prostatic acid phosphatase are associated typically with prostate cancer, but also may occur with certain other cancers and noncancerous conditions. Interpretation of enzyme assay data must make due allowance for the sensitivity and the diagnostic specificity of the enzyme test, together with other factors elicited through a comprehensive clinical examination that includes patient’s age, sex, prior history, and possible drug use.

Analysis of Serum Enzymes Following Tissue Injury

 An enzyme useful for diagnostic enzymology should be relatively specific for the tissue or organ under study, and should appear in the plasma or other fluid at a time useful for diagnosis (the “diagnostic window”). In the case of a myocardial infarction (MI), detection must be possible within a few hours or less of a preliminary diagnosis to permit initiation of appropriate therapy. The first enzymes used to diagnose MI were aspartate aminotransferase (AST), alanine aminotransferase (ALT), and lactate dehydrogenase (LDH). Diagnosis using LDH exploits the tissue-specific variations in its quaternary structure (Figure 1). However, it is released relatively slowly following injury. Creatine kinase (CK) has three tissue specific isozymes: CK-MM (skeletal muscle), CK-BB (brain), and CK-MB (heart and skeletal muscle), along with a more optimal diagnostic window. As with LDH, individual CK isozymes are separable by electrophoresis. Today, assays of plasma CK levels are primarily used to assess skeletal muscle disorders such as Duchene muscular dystrophy.

Fig1. Normal and pathologic patterns of lactate dehydrogenase (LDH) isozymes in human serum. Samples of serum were separated by electrophoresis. LDH isozymes were then visualized using a dye-coupled reaction-specific for LDH. Pattern A is serum from a patient with a myocardial infarct; B is normal serum; and C is serum from a patient with liver disease. Arabic numerals identify LDH isozymes 1 through 5. Electrophoresis and a specific detection technique thus can be used to visualize isozymes of enzymes other than LDH.

Plasma Troponin Constitutes the Currently Preferred Diagnostic Marker for an MI

Troponinis a complex of three proteins present in the contractile apparatus of skeletal and cardiac muscle but not in smooth muscle . Troponin levels in plasma typically rise for 2 to 6 hours after an MI, and remain elevated for 4 to 10 days. Immunological measurements of plasma levels of cardiac troponins I and T thus provide sensitive and specific indicators of damage to heart muscle. Since other sources of heart muscle damage also elevate serum troponin levels, cardiac troponins provide a general marker of cardiac injury.

Additional Clinical Uses of Enzymes

Enzymes are employed in the clinical laboratory to determine the presence and the concentration of critical metabolites. For example, glucose oxidase frequently is utilized to measure plasma glucose concentration. Enzymes also are employed with increasing frequency for the treatment of injury and disease. Examples include tissue plasminogen activator (tPA) or streptokinase for treatment of acute MI, and trypsin for treatment of cystic fibrosis. Intravenous infusion of recombinantly produced glycosylases can be used to treat lysosomal storage syndromes such as Gaucher disease (β-glucosidase), Pompe disease (α-glucosidase), Fabry disease (α-galactosidase A), Sly disease (β-glucuronidase), and mucopolysaccharidosis types I, II, and VI—also known as Hurler syndrome (α-L-iduronidase), Hunter syndrome (iduronate 2-sulfatase), and Maroteaux-Lamy syndrome (arylsulfatase B), respectively.